GaN Templates Research Articles

Polarity control of GaN grown on pulsed-laser-deposited AlN/GaN template by metalorganic vapor phase epitaxy

We report on the polarity control of GaN regrown on pulsed-laser-deposition-grown N-polar AlN on a metalorganic-vapor-phase-epitaxy-grown Ga-polar GaN template. The polarity of the regrown GaN, which was confirmed using aqueous KOH solutions, can be inverted from that of AlN by inserting a low-temperature GaN (LT-GaN) buffer layer. We hypothetically ascribe the Ga-polarity selection of GaN on the LT-GaN buffer layer to the mixed polarity of LT-GaN grains and higher growth rate of the Ga-polar grain, which covers up the N-polar grain during the initial stage of the high-temperature growth. The X-ray rocking curve analysis revealed that the edge-dislocation density in the N-polar regrown GaN is 5 to 8 times smaller than that in the Ga-polar regrown GaN. N-polar GaN grows directly on N-polar AlN at higher temperatures. Therefore, nucleus islands grow larger than those of LT-GaN and the area fraction of coalescence boundaries between islands, where edge dislocations emerge, becomes smaller.

Electrically conducting n-type AlGaN/GaN distributed Bragg reflectors grown by metalorganic chemical vapor deposition

We report an electrically conducting 40-pair silicon doped Al0.12Ga0.88N/GaN distributed Bragg reflector (DBR) grown by metalorganic chemical vapor deposition on a silicon doped n-type GaN template. Due to the relatively small lattice mismatch between AlGaN and GaN, strain managing layers are not required for crack-free n-DBR growth. The DBR demonstrates a peak reflectivity of 91.6% at 368nm with stopband of 11nm. In addition, the 40-pair n-DBR shows the vertical resistance of 5.5Ω, which corresponds to bulk resistivity of 0.52Ωcm, near the maximum measured current of 100mA.

Alternately double‐sided growth of low‐curvature GaN templates on sapphire substrates using hydride vapor phase epitaxy

GaN layers have been grown on double‐side‐polished sapphire substrates using hydride vapor phase epitaxy. The GaN layers on each side of the substrate were alternately grown using a new susceptor with a rotating mechanism. Although a conventional GaN layer grown on one side of a sapphire substrate exhibits a small radius of curvature, the alternately double‐sided‐grown GaN template exhibits a large radius of curvature. The radius of curvature of the alternately double‐sided‐grown GaN template with a thickness of 180 μm was found to be approximately 16.4 m, and was larger than the 0.5 m radius of curvature of the conventional GaN template.

Focused gas beam injection for efficient ammonia-molecular beam epitaxial growth of III-nitride semiconductors

A focused gas beam injection is proposed for high-efficiency ammonia molecular beam epitaxial growth of III-nitride. This new injector design is based on a double, coaxial radial high-conductance geometry, which allows rotation-free growth with fast gas switching. The injection profile is characterized through a mobile ion gauge and is then compared to simulations, where experimental results show that up to 27% of the injected molecules reach the surface of the substrate. The injector is tested for the growth of GaN layers, and high-resolution x-ray diffraction rocking curves of a 1 μm-thick GaN layer grown on a commercial GaN template (1 μm-thick layer of GaN on Si) was measured around the 002 Bragg condition and a full width at half maximum of 594 arc sec was obtained. Low-temperature photoluminescence for the same layer shows intense band edge emission and a low yellow luminescence. Hall measurements of the silicon-doped layers show high carrier concentrations up to 2 × 1019 cm−3 and a corresponding mobility of 204 cm2/V s.

Crystal orientations of β‐Ga2O3 thin films formed on c‐plane GaN substrate

A β‐Ga2O3 thin film was formed on a (0001) c‐plane GaN template substrate by gallium evaporation in an oxygen plasma. From X‐ray diffraction measurements using ω–2θ and ϕ scans, the film orientation was found to be () β‐Ga2O3 || (0001) GaN || (0001) sapphire. In addition, the β‐Ga2O3 had a sixfold domain structure in which the (010) planes in each crystal domain were parallel to the {} planes of GaN. This structure was confirmed by cross‐sectional and plan‐view transmission electron microscopy images and selected‐area electron diffraction patterns. The crystal orientation of the β‐Ga2O3 formed on (0001) c‐plane GaN is explained using an atomic‐arrangement model.

Mask-free regrowth of GaN p-i-n structure on selective-area Si-implanted n-GaN template layer

GaN p-i-n homoepitaxial structures are in-situ formed through mask-free selective-area growth (SAG) on Si-implanted GaN (SIG) templates. Selective-area Si implantation on GaN layer creates damaged surface regions that lead to lattice distortion from the neighboring implantation-free regions. GaN homoepitaxial regrowth occurs preferentially on the implantation-free regions when the SIG films serve as growth templates. The optical and electrical characteristics of the GaN p-i-n diodes formed by mask-free SAG are comparable to those of diodes fabricated by conventional technique. The GaN p-i-n photodiodes exhibit typical ultraviolet (UV)-to-visible (360/480 nm) spectral rejection ratios over three orders of magnitude. With forward injection currents of 10–100 mA, the proposed GaN diodes exhibit a UV emission at around 362 nm because of the band–edge transition between shallow donor states and the valence-band edge, rather than defect-related emission.

Single-crystal nanopyramidal BGaN by nanoselective area growth on AlN/Si(111) and GaN templates

We report nano-selective area growth (NSAG) of BGaN by MOCVD on AlN/Si(111) and GaN templates resulting in 150 nm single crystalline nanopyramids. This is in contrast to unmasked or micro-selective area growth, which results in a multi-crystalline structure on both substrates. Various characterization techniques were used to evaluate NSAG as a viable technique to improve BGaN material quality on AlN/Si(111) using results of GaN NSAG and unmasked BGaN growth for comparison. Evaluation of BGaN nanopyramid quality, shape and size uniformity revealed that the growth mechanism is the same on both the templates. Further STEM analysis of BGaN nanopyramids on AlN/Si (111) templates confirmed that these are single-crystalline structures without any dislocations, likely due to single nucleation occurring in the 80 nm mask opening. CL results correspond to boron content between 1.7% and 2.0% in the nanopyramids. We conclude that NSAG is promising for growth of high-quality BGaN nanostructures and complex nano-heterostructures, especially for low-cost silicon substrates.

The impact of RF-plasma power in carrier relaxation dynamics of unintentional doped GaN epitaxial layers grown by MBE

In this work, unintentionally doped GaN samples were prepared on GaN template by radio frequency (RF)-plasma MBE technique using two different RF-plasma powers. Photoluminescence (PL), steady state photoconductivity (PC) and ultrafast optical pump-probe spectroscopy measurements have been carried out to characterize the samples. The effect of RF-plasma power towards unintentional doping and giving rise to yellow luminescence (YL) is discussed. Our PC measurements show relatively faster decay for sample grown with higher RF-plasma power. In addition, the ultrafast optical pump-probe spectroscopy results show the presence of various defect levels with different relaxation times. A faster ultrafast relaxation time from the conduction band to the closest defect level and conduction band to the next defect level was observed for the sample grown with higher plasma power. A comparatively low defect density and faster carrier relaxation observed in higher RF-plasma power grown samples is caused by lower impurities and gallium vacancies. The results imply that RF-plasma power is very important parameter for the growth of epitaxial GaN films and undesirable impurities and gallium vacancies might get incorporated in the epitaxial GaN films.

Spatially resolved and orientation dependent Raman mapping of epitaxial lateral overgrowth nonpolar a-plane GaN on r-plane sapphire.

Uncoalesced a-plane GaN epitaxial lateral overgrowth (ELO) structures have been synthesized along two mask stripe orientations on a-plane GaN template by MOCVD. The morphology of two ELO GaN structures is performed by Scanning electronic microscopy. The anisotropy of crystalline quality and stress are investigated by micro-Raman spectroscopy. According to the Raman mapping spectra, the variations on the intensity, peak shift and the full width at half maximum (FWHM) of GaN E2 (high) peak indicate that the crystalline quality improvement occurs in the window region of the GaN stripes along [0001], which is caused by the dislocations bending towards the sidewalls. Conversely, the wing regions have better quality with less stress as the dislocations propagated upwards when the GaN stripes are along []. Spatial cathodoluminescence mapping results further support the explanation for the different dislocation growth mechanisms in the ELO processes with two different mask stripe orientations.

Growth parametric study of N-polar InGaN films by metalorganic chemical vapor deposition

We report the growth of N-polar InGaN alloy films on high-quality N-polar GaN templates with smooth surface by metal organic chemical vapor deposition. In this work, we systematically investigate the influences of growth parameters, such as growth temperature, pressure, V/III source ratio and trimethylindium input flow on the crystalline and optical properties of N-polar InGaN films. It is found that the relatively high growth pressure can promote the incorporation of In atoms into InGaN films and improve the crystalline quality by enhancing the epitaxial lateral overgrowth without mask simultaneously. Also, relatively low growth temperature, small V/III ratio and large trimethylindium input flow are more effective ways to increase In content in the films. Besides, we find that a high growth temperature is beneficial to improve the optical properties of InGaN films. Finally, N-polar InGaN film with green-emitting at 530 nm is obtained.

Growth and characterization of nonpolar (10-10) ZnO transparent conductive oxide on semipolar (11–22) GaN-based light-emitting diodes

We have grown thin films of nonpolar m-plane (10-10) ZnO on a semipolar (11–22) GaN template by atomic layer deposition (ALD) at low growth temperatures (<200 °C). The surface morphology of the ZnO film is found to be an arrowhead-like structure, which is a typical surface structure of the semipolar (11–22) GaN films. On increasing the growth temperature of the ZnO films, the concentration and mobility of the charge carriers in the ZnO film are increased. However, the optical transmittance decreases with an increase in the growth temperature. Based on these results, we have fabricated semipolar (11–22) GaN-based light-emitting diodes (LEDs) with nonpolar m-plane ZnO film as a transparent conductive oxide (TCO) to improve the light extraction efficiency. In spite of a decrease in the optical transmittance, the operation voltage of semipolar (11–22) GaN-based LEDs is found to decrease with an increase in the growth temperature, which might be due to the improvements in the electrical properties and current spreading effect, resulting in an increase in the optical output power.

Influence of Point Defects on the Properties of Undoped and Ga-Doped ZnO Films Grown by Plasma-Assisted Molecular Beam Epitaxy in an O-Rich Environment

Undoped and Ga-doped ZnO films are grown on GaN templates by plasma-assisted molecular beam epitaxy for systematic investigation of the influence of native defects on their electrical properties. The distinct electrical properties of the undoped and Ga-doped ZnO films are observed after thermal treatment in a nitrogen and an oxygen ambient. It is found that the undoped ZnO films show improved characteristics when annealed in an oxygen ambient, and the Ga-doped ZnO films exhibit stable characteristics when annealed in a nitrogen ambient. The variation in electrical properties revealed by the photoluminescence measurements can be attributed to the generation and annihilation of native defects, which are dependent on the ambient of treatments. The properties of the annealed undoped ZnO films grown in an O-rich environment are affected mainly by the oxygen vacancies, antisite oxygen and oxygen interstitials, while the annealed Ga-doped ZnO films are dominated by oxygen interstitials.

Quality-enhanced GaN epitaxial films on Si(111) substrates by in situ deposition of SiN on a three-dimensional GaN template

High-quality crack-free GaN epitaxial films were successfully grown on Si(111) substrates using metal–organic chemical vapor deposition by in situ depositing SiN on a 3-dimensional (3D) GaN template.

Comparative Study About the Structural Properties of ZnO Films Grown on Si, GaAs, And Al2O3 Substrates and a GaN Template

We report on the structural properties of ZnO films grown on (001) Si, (001) GaAs, (0001) Al 2 O 3 substrates and a (0001) GaN template by RF-sputtering. It is observed that all the ZnO films have textured structures with a preferred orientation of the c-axis, irrespective growth conditions, in terms of atomic-force microscope and wide-range X-ray diffractometer. However, it is found that the ZnO films in the ZnO-on-Si and ZnO-on-GaAs are consisted of various domains with different orientations, while the ZnO films in the ZnO-on-Al 2 O 3 and ZnO-on-GaN are consisted of a single domain with the same orientation in terms of the φ-scan of X-ray diffractometer. Moreover, it is found that the ZnO-on-GaN has smaller edge dislocation density and screw dislocation density than the ZnO-on-Al 2 O 3 , which seriously depends on substrate temperature, in terms of the ω-scan of X-ray diffractometer.

Site controlled red-yellow-green light emitting InGaN quantum discs on nano-tipped GaN rods.

We report a method of growing site controlled InGaN multiple quantum discs (QDs) at uniform wafer scale on coalescence free ultra-high density (>80%) nanorod templates by metal organic chemical vapour deposition (MOCVD). The dislocation and coalescence free nature of the GaN space filling nanorod arrays eliminates the well-known emission problems seen in InGaN based visible light sources that these types of crystallographic defects cause. Correlative scanning transmission electron microscopy (STEM), energy-dispersive X-ray (EDX) mapping and cathodoluminescence (CL) hyperspectral imaging illustrates the controlled site selection of the red, yellow and green (RYG) emission at these nano tips. This article reveals that the nanorod tips' broad emission in the RYG visible range is in fact achieved by manipulating the InGaN QD's confinement dimensions, rather than significantly increasing the In%. This article details the easily controlled method of manipulating the QDs dimensions producing high crystal quality InGaN without complicated growth conditions needed for strain relaxation and alloy compositional changes seen for bulk planar GaN templates.

Structural properties of GaN layers grown on Al2O3 (0001) and GaN/Al2O3 template by reactive radio-frequency magnetron sputter epitaxy

GaN single-crystalline layers were grown by reactive radio-frequency magnetron sputter epitaxy using N2/Ar ambient gas and Ga target. The GaN layers were grown directly on Al2O3 (0001) substrates and GaN/Al2O3 templates. The crystalline quality and the surface morphology of GaN layer depending on substrate temperature and N2 composition ratio, were measured. The crystalline quality of GaN layer was improved by increasing substrate temperature and also decreasing N2 composition ratio. The full-width at half-maximum (FWHM) value of X-ray rocking curve (XRC) of GaN layer grown on Al2O3 at 900 °C and 6% N2, was 27 and 338 arcsec for highly c-axis oriented columnar domains and disordered structure inside the GaN layer, respectively, for (0002) plane. The FWHM values of XRCs of the GaN layers on GaN/Al2O3 for both (0002) and (101̅2) planes, decreased with the increase of substrate temperature, and at above 890 °C they were similar to those of the metal-organic vapor phase epitaxy-grown GaN template, which were approximately 300 and 310 arcsec, respectively. The threading dislocation densities were also estimated using the FWHM values of XRCs, and those of GaN layers on GaN/Al2O3 were considerably decreased as compared with those on Al2O3 at substrate temperatures more than 890 °C.

Toward defect‐free semi‐polar GaN templates on pre‐structured sapphire

The microstructure of semi‐polar (11–22) GaN templates grown on pre‐structured r‐plane sapphire by MOVPE has been characterized by TEM. Cross‐sectional observations indicate that defects are generated in three regions of the layers: threading dislocations at the inclined GaN/sapphire interface, basal plane stacking faults (BSFs) at the c−‐wing, BSFs and threading dislocations at the coalescence between neighboring GaN stripes. An in situ SiN interlayer deposited at an early stage of the growth is shown to be effective in blocking the propagation of dislocations, which is mainly attributed to SiN formed on the c‐plane rather than on the (11–22) plane. Si‐doped marker layers have been used to study the evolution of the growth front before coalescence as a function of temperature. A high growth temperature is associated with the formation of highly faceted GaN stripes. Dislocations originally running along the c‐direction are bent to the [11–20] direction driven by a progressing (11–22) facet. An efficient defect reduction is realized as a result of terminating these dislocations at voids partially defined by the (11–20) facet.

Defect reduced selectively grown GaN pyramids as template for green InGaN quantum wells

AbstractAbstractauthoren We report the growth of green emitting InGaN quantum wells (QWs) by metal‐organic vapor‐phase epitaxy (MOVPE) on three‐dimensional GaN templates. The facets of GaN pyramids, fabricated by selective‐area growth (SAG), reduce the influence of the quantum‐confined Stark effect (QCSE) on the emission properties of the QW. The luminescence properties of a QW are a good indicator for the crystalline quality of the GaN layer beneath. Especially the presence of voids inside the pyramids, as well as stacking faults (SFs) and threading dislocations (TDs) in the wing region strongly influence the strain situation and the incorporation of In into the overlying InGaN layer. Thus, the crystal quality of the GaN pyramid has a strong influence on the efficiency and the emission properties of the active region. Therefore, a low‐temperature nucleation on the GaN buffer in conjunction with a decreasing Ga‐flux taking the decreasing c‐plane growth surface of the pyramid into account was introduced. The low‐temperature photoluminescence (PL) properties of the InGaN QW reveal the differences between the standard formation of the pyramid and this modified growth. Cathodoluminescence (CL) and transmission electron microscopy (TEM) measurements confirm the differences between the two growth modes in the crystal quality of the inner part of the pyramid.

Green emission from semipolar InGaN quantum wells grown on low‐defect () GaN templates fabricated on patterned r‐sapphire

In this article, InGaN/GaN multiple quantum wells (MQWs) grown on low‐defect () GaN templates are investigated by photoluminescence (PL), cathodoluminescence (CL), and transmission electron microscopy (TEM). The emission wavelength is tuned from 450 nm (blue) to 550 nm (green) by varying the TMGa flux in the QWs, while keeping the temperature constant. The In content in the QWs is found to increase with increasing TMGa flux. CL measurements show that the first QW and often the second one emit systematically at wavelengths shorter than the following QWs, while TEM measurements indicate that these first QWs are slightly thinner and display less In content than the rest of the stack. Both observations might be explained by considering that these first QWs grow under larger compressive strain than the subsequent QWs. Furthermore, since TEM shows that misfit dislocations oriented along the [] direction are mainly located at the lower MQW stack interface, i.e., between the first QW and the underlying template, the correlation between TEM and CL points toward a plastic relaxation occurring after the stacking of a sufficiently large number of QWs.

Origins of hillock defects on GaN templates grown on Si(111)

The origin of surface hillocks (also known as pancake defects) on GaN-on-Si wafers grown by MOVPE has been investigated. FIB/TEM observations confirmed that the appearance of the hillocks is due to the formation of Ga-rich precipitates within the AlGaN buffer layer. XRD (002) FWHM measurements also show that the surface hillocks are associated with a high degree of crystal tilt in the AlN nucleation layer. Two factors are considered to be the cause of such a phase separation: (1) a high density of surface steps associated with the regions of large crystal tilt which act as nucleation centers and (2) a lower mobility of Al adatoms at the growth surface compared with Ga, leading to a preferential incorporation of Ga in the precipitates. The impact of these precipitates on the wafer bow of the structures is considered.