Low-temperature AlN Research Articles

?Properties of a GaN Layer on a Si(111) Substrate with Low-temperature AlN Interlayers

The properties of a second GaN epitaxial layer grown on a Si(111) substrate with various low-temperature AlN interlayer thicknesses have been investigated. The thickness of the lowtemperature AlN interlayer can effectively counteract the tensile stress usually observed in the second GaN layer deposited on Si(111). For a 10-nm-thick interlayer, a low crack density of 5/cm and a high crystalline quality of 651 arcsec are obtained. These results are due to a low out-plane strain of 9.7 × 10−4. Strong band-edge emission from GaN on Si(111) is observed, and the full width at half maximum of the bound exciton line is as low as 21.1 meV at 13 K. Optimization of the low-temperature AlN interlayer thickness is indispensible if a high-quality GaN layer is to be obtained on a Si(111) substrate.

Electrical properties and deep traps spectra in AlGaN films with nitrogen and gallium polarities prepared by molecular beam epitaxy

Electrical properties and deep traps spectra of Ga-face and N-face AlGaN films with Al mole fraction ranging from 0 to 0.6 were studied by means of current voltage, capacitance–voltage measurements, deep traps transient spectroscopy, admittance spectroscopy, photoluminescence spectroscopy. The films were grown by molecular beam epitaxy using a combined buffer consisting of a low temperature AlN nucleation layer and AlGaN/GaN superlattice. The polarity of the film is determined by whether the low temperature AlN is grown under the nitrogen-rich (promotes N-polarity) or Al-rich (promotes Ga-polarity) conditions. It shown that the Ga-face samples grown in this fashion have highly compensated p-type conductivity with the dominant acceptors being similar to standard acceptor species in GaN. For N-face films the conductivity is n-type and the dominant donors can be attributed to Si. The density of deep traps was found to be much higher for the N-face samples compared to the Ga-face samples.

Electrical properties and deep traps spectra of N-polar and Ga-polar AlGaN films grown by molecular beam epitaxy in a wide composition range

The polarity type, surface morphology, electrical properties, and deep trap spectra were studied for undoped AlxGa1−xN films (x=0–0.6) grown by molecular beam epitaxy on on-axis (0001) sapphire using composite buffers consisting of low temperature (LT) AlN nucleation layer, AlN, and AlN/AlGaN superlattice. It is shown that the films grow with N-polarity if the LT AlN layer is deposited under N-rich conditions and with Ga-polarity for the LT AlN layers deposited under Al-rich conditions. For both polarities the film morphology was acceptable for fabrication of typical GaN-based devices. It is demonstrated that the Ga-polar AlGaN films are heavily compensated p-type, with the dominant acceptors believed to be due to C. N-polar films are n type, with the residual donors pinning the Fermi level being most likely due to Si. N-polar films show a high concentration of deep electron traps.

Fabrication of a GaN lateral polarity junction by metalorganic chemical vapor deposition

A fabrication process for growth of GaN lateral polarity junctions consisting of Ga-polar and N-polar domains grown simultaneously side-by-side on c-plane sapphire was developed using the polarity control scheme. An ammonia-annealing step following deposition and patterning of a thin low-temperature AlN nucleation layer played a crucial role in avoiding mixed-polarity growth of the remaining AlN nucleation layer, as well as in nitriding the bare sapphire surface to facilitate growth of N-polar GaN. The achievement of both polar domains, free from inversion domains within a contiguous domain, led to Ga-polar domain exhibiting featureless morphology with highly resistive characteristics, while N-polar domains exhibited hexagonally faceted morphology and were highly conductive.

Properties of GaN layers deposited on AlN/sapphire template substrates

Gallium nitride thick layers were deposited on template substrates which consisted of low temperature aluminium nitride (LT-AlN) buffer layer or AlN/Al0.2Ga0.8N double-layer buffer grown on (0001) oriented sapphire substrates. Buffers were deposited by using Metal Organic Vapor Phase Epitaxy (MOVPE) in various temperatures and thick GaN layers by Hydride Vapor Phase Epitaxy (HVPE) at 1050°C in two-step growth procedure. Morphology of samples was evaluated by AFM (Atomic Force Microscopy) and Scanning Electron Microscopy (SEM), crystalline quality by High-resolution X-Ray Diffractometry (HRXRD), optical quality by Photoluminescence (PL) spectra, and residual strain by micro-Raman scattering measurements.

Analysis of the leakage current in GaN-based heterostructure buffer layer

The relations between the buffer leakage current and the characteristics of nucleation layer of AlGaN/GaN heterostructure have been obtained by comparative study of depletion capacitance of C-V characteristics in GaN-based heterostructure. The results showed that, the heterostructure material based on a sapphire substrate with a low-temperature GaN nucleation layer and SiC substrate with a high-temperature AlN nucleation layer have smaller buffer leakage current and lower carrier concentration of the background GaN buffer layer than that based on a sapphire substrate with a low-temperature AlN nucleation layer. A thorough analysis shows that, there is an n-type GaN conductive layer near the GaN/substrate interface in buffer layer based on the thinner nucleation layers. Contrarily, high resistivity GaN buffer layers appear in heterostructure materials based on thicker nucleation layer. One of the main causes of device leakage is the n-type GaN conductive layer. Appropriately improving the quality and thickness of nucleation layer can effectively reduce the carrier concentration, raise the resistivity and suppress the leakage current of the GaN buffer layer.

Effect of the MgO substrate on the growth of GaN

We investigate the effect of the MgO substrate on the growth of GaN by molecular beam epitaxy with radio frequency plasma source. MgO substrate is advantageous for its closer lattice constant to GaN. However, epitaxial GaN layer grown on MgO (1 1 1) substrate is often inclined about 2 ∘ toward a particular a-axis of the GaN. This inclination is found to be caused by the domain structure of the MgO substrates. On the tilted domains of the substrate, the strain is effectively relaxed through the inclination. Another difficulty of MgO substrate is the diffusion of Mg atoms into the GaN layer. This Mg diffusion from the MgO substrate has successfully been suppressed by introducing the thin low-temperature grown AlN buffer layer. The AlN buffer also improves the crystalline quality of the GaN layer.

Epitaxial growth of GaN films grown on single crystal Fe substrates

GaN films have been grown on single crystalline Fe(110), Fe(100), and Fe(111) substrates with low-temperature AlN buffer layers by the use of pulsed laser deposition. AlN films with 30° rotated domains and polycrystalline AlN films grow on Fe(100) and Fe(111), respectively. On the other hand, AlN(0001) films grow epitaxially on Fe(110) substrates. X-ray reflectivity measurements have revealed that the heterointerface of AlN∕Fe(110) is quite abrupt and that its abruptness remains unchanged, even after annealing at 700°C. GaN epitaxial films have been grown on AlN∕Fe(110) structures with an in-plane relationship of GaN[11−20]∥AlN[11−20]∥Fe[001]. The first-principles calculations have revealed that this in-plane epitaxial relationship is determined by the adsorption process of nitrogen atoms on the Fe surfaces at the initial stage of the growth.

Growth behaviors of GaN/Si heteroepitaxy with various terrace widths grown by the LEPS method

Epilayers of GaN were grown on patterned Si (111) substrates of various terrace widths by means of metal organic chemical vapor deposition. The technique of lateral epitaxy on a patterned substrate used the growth of GaN epilayers from the periodic and parallel stripes that form as a result of the substrate etching. Silicon substrates were patterned for various terrace widths of 3 μm, 8 μm, and 18 μm. A low temperature AlN was used as a seed layer for the growth of a 1.5 μm thick GaN epilayer. The as-grown samples were characterized by using double-crystal X-ray diffractometry (DCXRD), photoluminescence and atomic force microscopy (AFM). From the DCXRD spectra, the full width at half maximum (FWHM) of the samples was found to decrease as the terrace width decreased. This behavior indicates that there is an improvement in the crystalline quality of the GaN epilayers as the terrace width decreases. The photoluminescence spectra reveal a decrease in the FWHM and an increase in the peak intensity as the terrace width decreases. This behavior indicates that there is an improvement in the optical quality of the GaN epilayer as the terrace width decreases. The atomic force micrographs reveal a dislocation-free homogeneous surface in the trench region compared to the terrace region with defects such as pits and dislocations. The results clearly show that GaN epilayers grown on a patterned Si substrate with a terrace width 3 μm have a good crystalline quality with minimal threading dislocation and excellent band edge emission.

Dislocation reduction in GaN with double Mg xN y/AlN buffer layer by metal organic chemical vapor deposition

Unintentionally doped GaN with conventional single low-temperature (LT) AlN buffer layer and with double Mg x N y /AlN buffer layers both were prepared. It was found that we could reduce defect density and thus improve crystal quality of the GaN by using double Mg x N y /AlN buffer layers. GaN with double Mg x N y /AlN buffer layers reveals an asymmetrical reflection (1 0 2) and (0 0 2) with a smaller full-width at half-maximum (FWHM), and a higher mobility, lower background concentration and lower etching pit density (EPD) than the GaN with the LT-AlN buffer layer.

Visible-Blind Metal–Semiconductor–Metal Photodetectors by Capping an In Situ Low-Temperature AlN Layer

We present the characteristics of a nitride-based UV metal–semiconductor–metal photodetector (MSM PD) with an in situ low-temperature (LT) grown AlN cap layer. From atomic-force microscopy scan images, we could clearly see that surface pits on the sample surface are almost invisible with an LT AlN cap layer but can be observed in a conventional cap layer. Compared with conventional MSM PDs, it was found that we achieved smaller dark current and larger UV-to-visible rejection ratio for the PDs with an LT AlN cap layer. With a applied bias, the UV-to-visible rejection ratio between 360 and was for the MSM PDs with an LT AlN cap layer. The calculated noise equivalent power and normalized detectivity biased at for the MSM PDs with an LT AlN cap layer was and , respectively.

Reduction of Mg segregation in a metalorganic vapor phase epitaxial grown GaN layer by a low-temperature AlN interlayer

The redistribution behavior of Mg in a sequentially regrown GaN epilayer on a p-type doped GaN template was studied. All samples in this study were regrown by metalorganic vapor phase epitaxy on the sapphire substrates. A high density and a slow tail of Mg concentration were observed in a nominally undoped layer due to the surface segregation. We found that the insertion of a low-temperature (LT) AlN interlayer was effective to suppress the Mg redistribution in the GaN regrown layer. Analyzing the temperature dependence of the surface segregation, the activation energy of the Mg segregation was estimated to be 0.63eV in GaN and 2.47eV in a LT-AlN layer, respectively.

Morphology and microstructure evolution of AlxGa1−xN epilayers grown on GaN/sapphire templates with AlN interlayers observed by transmission electron microscopy

Morphology and microstructure evolution of Al0.3Ga0.7N epilayers grown on GaN/sapphire templates with low-temperature (LT) AlN interlayers (IL) by means of metal organic chemical vapor deposition have been investigated by transmission electron microscopy and atomic force microscopy. It is found that the IL improves the surface morphology and suppresses edge-type threading dislocations (TDs). When the IL thickness is 20nm, there is the lowest density of the edge-type TD with 8.7×108cm−2. However, the edge-type TD density increases somewhat as IL thickness increases to 40nm. It is believed that two mechanisms determine the microstructure evolution of the AlxGa1−xN epilayers. One is the TD suppression effect of LT-AlN ILs that ILs can provide an interface for edge-type TD termination. Another is the TD introduction effect of ILs that new edge-type TDs are produced. Due to the lattice mismatch between AlN, GaN, and AlxGa1−xN, the strain in AlxGa1−xN epilayers is modified by inserting the AlN IL, and thus changes the formation of the edge-type TDs.

Three-Step Growth Optimization of AlN Epilayers by MOCVD

A three-step growth process is developed for depositing high-quality aluminium-nitride (AlN) epilayers on (001) sapphire by low pressure metalorganic chemical vapour deposition (LP-MOCVD). We adopt a low temperature (LT) AlN nucleation layer (NL), and two high temperature (HT) AlN layers with different V/III ratios. Our results reveal that the optimal NL temperature is 840–880°C, and there exists a proper growth switching from low to high V/III ratio for further reducing threading dislocations (TDs). The screw-type TD density of the optimized AlN film is just 7.86 × 106cm−2, about three orders lower than its edge-type one of 2 × 109cm−2 estimated by high-resolution x-ray diffraction (HRXRD) and cross-sectional transmission electron microscopy (TEM).

MOCVD Growth of InN on Si(111) with Various Buffer Layers

We report the growth of InN by metalorganic chemical vapor deposition on Si(111) substrates. It was found that the sharpest InN(002) x-ray diffraction peak could be achieved from the sample prepared on a complex buffer layer that consists of a low-temperature AlN, a graded Al x Ga1−x N (x = 1 → 0), and a high-temperature GaN. The resultant mobility of 275 cm2/V s thus obtained was 75% larger than that of the InN prepared on a single LT-AlN buffer layer only.

The effect of low temperature AlN interlayers on the growth of GaN epilayer on Si (111) by MOCVD

Low temperature (LT) AlN interlayers were used to effectively reduce the tension stress and micro-cracks on the surface of the GaN epilayer grown on Si (111) substrate. Optical Microscopy (OM), Atomic Force Microscopy (AFM), Surface Electron Microscopy (SEM) and X-Ray Diffraction (XRD) were employed to characterize these samples grown by metal-organic chemical vapor deposition (MOCVD). In addition, wet etching method was used to evaluate the defect of the GaN epilayer. The results demonstrate that the morphology and crystalline properties of the GaN epilayer strongly depend on the thickness, interlayer number and growth temperature of the LT AlN interlayer. With the optimized LT AlN interlayer structures, high quality GaN epilayers with a low crack density can be obtained.

Growth and fabrication of AlGaN/GaN HEMT based on Si(1 1 1) substrates by MOCVD

AlGaN/GaN high electron mobility transistor (HEMT) hetero-structures were grown on the 2-in Si (111) substrate using metal-organic chemical vapor deposition (MOCVD). Low-temperature (LT) AlN layers were inserted to relieve the tension stress during the growth of GaN epilayers. The grown AlGaN/GaN HEMT samples exhibited a maximum crack-free area of 8mm×5mm, XRD GaN (0002) full-width at half-maximum (FWHM) of 661arcsec and surface roughness of 0.377nm. The device with a gate length of 1.4μm and a gate width of 60μm demonstrated maximum drain current density of 304mA/mm, transconductance of 124mS/mm and reverse gate leakage current of 0.76μA/mm at the gate voltage of −10V.

Effect of double AlN buffer layer on the qualities of GaN films grown by radio-frequency molecular beam epitaxy

This paper reports that the GaN thin films with Ga-polarity and high quality were grown by radio-frequency molecular beam epitaxy on sapphire (0001) substrate with a double AlN buffer layer. The buffer layer consists of a high-temperature (HT) AlN layer and a low-temperature (LT) AlN layer grown at 800°C and 600°C, respectively. It is demonstrated that the HT-AlN layer can result in the growth of GaN epilayer in Ga-polarity and the LT-AlN layer is helpful for the improvement of the epilayer quality. It is observed that the carrier mobility of the GaN epilayer increases from 458 to 858cm2/V·s at room temperature when the thickness of LT-AlN layer varies from 0 to 20 nm. The full width at half maximum of x-ray rocking curves also demonstrates a substantial improvement in the quality of GaN epilayers by the utilization of LT-AlN layer.

Influence of AlN buffer layer thickness on the properties of GaN epilayer on Si(1 1 1) by MOCVD

The effect of thickness of the high-temperature (HT) AlN buffer layer on the properties of GaN grown on Si(111) has been investigated. Optical microscopy (OM), atomic force microscopy (AFM) and X-ray diffraction (XRD) are employed to characterize these samples grown by metal-organic chemical vapor deposition (MOCVD). The results demonstrate that the morphology and crystalline properties of the GaN epilayer strongly depend on the thickness of HT AlN buffer layer, and the optimized thickness of the HT AlN buffer layer is about 110nm. Together with the low-temperature (LT) AlN interlayer, high-quality GaN epilayer with low crack density can be obtained.

M -plane GaN grown on m-sapphire by metalorganic vapor phase epitaxy

GaN layers have been grown on m-plane sapphire by metal organic vapor phase epitaxy using low-temperature AlN nucleation layers. Depending on substrate nitridation and annealing treatments prior to depositing the nucleation layer, the crystal orientation of the resulting GaN layer may be either (11−22) or (1−100) (m plane). For suitably controlled conditions, GaN epilayers with a single m-plane orientation are reproducibly obtained as confirmed by x-ray diffraction. There is a 90° in-plane rotation of the epilayer such that the GaN a axis is parallel to the sapphire c axis.