High-quality AlN Layers Research Articles

High-quality AlN epilayers prepared by atomic layer deposition and large-area rapid electron beam annealing

Low-temperature atomic layer deposition and high-temperature electron beam annealing (EBA) are used to achieve high-quality epitaxial AlN layers. Efficient energy transfer from the single exposure for only 1 min of electron irradiation with a large area contributes to substantial improvement in the film density and crystal quality of AlN thin films, as demonstrated by the X-ray reflectivity and the θ-2θ/ω-scan X-ray diffraction. High-resolution transmission electron microscopy indicates well-arranged lattice fringes in the epitaxial AlN layer on a sapphire substrate. A low surface roughness of the AlN epilayer, as revealed by the atomic force microscopy, suggests that the damage induced by the EBA treatment is insignificant. The outcomes indicate that the large-area rapid EBA is a low-damage and efficient technique for the recrystallization of thin films prepared at low temperatures.

High-Quality AlN Layers Grown on Si(111) Substrates by Metalorganic Chemical Vapor Deposition

The effect of trimethylaluminum preflow time on the crystalline quality of AlN films grown by metalorganic chemical vapor deposition on Si(111) substrates has been investigated. It is found that semipolar (10$$\bar {1}$$1) layers are formed in the AlN film with long preflow times. It is shown that the AlN nucleation can be controlled by choosing an optimal preflow time, which provides a desired film quality. At the optimum preflow time, the FWHM of the rocking curve for the 0002 reflection amounts to 0.59°.

Producing deep UV-LEDs in high-yield MOVPE by improving AlN crystal quality with sputtered AlN nucleation layer * * Project supported by the National Natural Sciences Foundation of China (Nos. 61334009, 61474109, 61306050).

High-quality AlN layers with low-density threading dislocations are indispensable for high-efficiency deep ultraviolet light-emitting diodes (UV-LEDs). In this work, a high-temperature AlN epitaxial layer was grown on sputtered AlN layer (used as nucleation layer, SNL) by a high-yield industrial metalorganic vapor phase epitaxy (MOVPE). The full width half maximum (FWHM) of the rocking curve shows that the AlN epitaxial layer with SNL has good crystal quality. Furthermore, the relationships between the thickness of SNL and the FWHM values of (002) and (102) peaks were also studied. Finally, utilizing an SNL to enhance the quality of the epitaxial layer, deep UV-LEDs at 282 nm were successfully realized on sapphire substrate by the high-yield industrial MOVPE. The light-output power (LOP) of a deep UV-LED reaches 1.65 mW at 20 mA with external quantum efficiency of 1.87%. In addition, the saturation LOP of the deep UV-LED is 4.31 mW at an injection current of 60 mA. Hence, our studies supply a possible process to grow commercial deep UV-LEDs in high throughput industrial MOVPE, which can increase yield, at lower cost.

Structural and optical properties of AlN grown on nanopillar/patterned SiO2 by hydride vapor phase epitaxy

We demonstrate the growth of high-quality AlN layers on an AlN nanopillar structure with a patterned SiO2 layer employing horizontal hydride vapor phase epitaxy (HVPE). The use of the AlN nanopillar structure with the patterned SiO2 resulted in improving the crystalline, and overall material properties of the AlN layer. The full width half maximum (FWHM) of peaks corresponding to (002) and (102) reflections of the AlN layer with the nanopillar structure were significantly decreased from 386 and 576arcsec to 265 and 318arcsec, respectively, as compared with that of the as-grown AlN layer. The laterally overgrown AlN regions consisted of a continuous well-coalesced layer exhibiting a lower dislocation density than that of the templates used owing to the dislocation blocking and dislocation bending effects. Complementary characterization by transmittance, photoluminescence and Raman spectroscope, indicates further the overall good material properties of the AlN layer grown on the nanopillar structure.

Phonon frequencies of a highly strained AlN layer coherently grown on 6H-SiC (0001)

Phonon frequencies of a high-quality AlN layer coherently grown on a 6H-SiC (0001) substrate are investigated by Raman scattering. Owing to the largest strain in our coherent AlN layer among heteroepitaxially grown AlN layers ever reported, phonon frequencies of the E2 (low), E2 (high), and A1 (LO) modes are considerably shifted to 244.5 (−3.3, compared with bulk AlN), 672.1 (+16.3), and 899 (+11)cm−1, respectively. Full widths at half maximum of the phonon modes in the coherent AlN are almost equal to those of high-quality bulk AlN, clearly indicating its high crystalline quality and uniform strain. We discuss accuracy of phonon deformation potentials reported by several other groups thorough comparing our experimental results.

All AlGaN epitaxial structure solar-blind avalanche photodiodes with high efficiency and high gain

Solar-blind avalanche photodiodes were fabricated with an all AlGaN-based epitaxial structure on sapphire by metal–organic chemical vapor deposition. The devices demonstrate a maximum responsivity of 114.1 mA/W at 278 nm and zero bias, corresponding to an external quantum efficiency (EQE) of 52.7%. The EQE improves to 64.8% under a bias of −10 V. Avalanche gain higher than 2 × 104 was obtained at a bias of −140 V. The high performance is attributed to the all AlGaN-based p–i–n structure comprised of undoped and Si-doped n-type Al0.4Ga0.6N on a high quality AlN layer and highly conductive p-type AlGaN grown with In-surfactant-assisted Mg-delta doping.

나노구조를 응용한 AlN 성장 방법 및 특성

In this paper, high quality AlN layers were regrown on AlN nanopillar structure with <TEX>$SiO_2$</TEX>-dots by HVPE. Surface morphology of AlN layer regrown exhibited flatter than a conventional AlN template. The laterally overgrown AlN regions would consist of a continuous well coalesced layer with lower dislocation density than in the template because of the dislocation blocking and dislocation bending effects. Moreover, result of Raman spectroscopy suggest that the AlN nanopillar structure with <TEX>$SiO_2$</TEX>-dots relieves the strain in the AlN layer regrown by HVPE.

Growth and characterization of high quality AlN using combined structure of low temperature buffer and superlattices for applications in the deep ultraviolet

This study reports on the growth and characterization of AlN/AlxGa1−xN superlattices (SLs) for reduction of threading dislocations to grow high quality AlN layer. Insertion of optimized SLs is shown to effectively reduce the dislocation density, there by resulting in a high crystal quality 2.5-µm-thick AlN layer. It was found that full width at half-maximum (FWHM) of X-ray rocking curves (XRCs) around both (0002) and diffraction were decreased to 230 and 420, respectively. The results of XRC FWHM and dislocation densities from etch pit density (EPD) by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs are in a good agreement. This high quality and low dislocation density AlN layer is of key importance for high efficiency deep-UV light emitting diodes and power devices.

A new AlON buffer layer for RF-MBE growth of AlN on a sapphire substrate

A 20nm-thick AlON buffer layer consisting of Al2O3, graded AlON, AlN, and thin Al2O3 amorphous films was used to grow AlN on a sapphire substrate by molecular beam epitaxy with radio frequency plasma for nitrogen source (RF-MBE). Mirror-smooth AlN layers were successfully obtained using this new AlON buffer layer. The total threading dislocation densities of AlN layers are comparable to those reported for the high-quality AlN layers grown by RF-MBE using the conventional low-temperature (LT) buffer layer.

INFLUENCE OF CONDITIONS OF GROWTH ON STRUCTURAL PERFECTION OF LAYERS OF ALN RECEIVED BY METHOD MOS-GIDRIDNOY OF AN EPITAXY

In present work the influence of growth conditions on structural quality of AlN layers grown by МОСVD have been investigated. The influence of buffer layers grown with different temperatures and V/III ratio on crystalline quality of AlN were explored. We have reported that the high temperature buffer layer with low V/III ratio is most efficient way to improve the structural quality of AlN. Further improvement was achieved by minimizing the parasitic reactions between NH3 and TMAl. It was carried out by optimization the total flow through the reactor. Applying these methods, it was possible to obtain a high-quality AlN layers (FWHM for (0002), (0004) and (101–3) reflections were 50, 97 and 202 arc seconds respectively) with good root-mean-square roughness of surface 0.7 nm.

Growth of high‐quality AlN layers on sapphire substrates at relatively low temperatures by metalorganic chemical vapor deposition

We report a three‐step method to grow high‐quality AlN heteroepitaxial layers on sapphire substrates at relatively low temperatures by metalorganic chemical vapor deposition (MOCVD) without the use of epitaxial lateral overgrowth (ELO) or pulse atomic layer epitaxy (PALE) method. The three‐layer AlN structure comprises a 15‐nm thick buffer layer, a 50‐nm thick intermediate layer, and a 3.4‐µm thick template layer grown at 930, 1130, and 1100 °C sequentially on the c‐plane sapphire substrate. The resulting AlN layer had smooth surface with well‐defined terraces and low root‐mean square (RMS) roughnesses of 0.50 and 0.07 nm for 20 × 20 and 1 × 1 µm2 atomic force microscopy (AFM) scans. Band‐edge emission was observed at 208 nm by room temperature (RT) photoluminescence (PL) measurements. The total threading dislocation density was 2.5 × 109/cm2 as determined by transmission electron microscopy (TEM), which is comparable to those of some AlN layers recently grown at significantly higher temperatures. Growth evolution was studied and correlated to the TEM results. The residual impurity concentrations were comparable to those of AlN layers grown at higher temperatures, i.e., 1200–1600 °C. This study demonstrates the high quality AlN layers on sapphire substrates can be grown at achievable temperatures for most of the modern MOCVD systems.

High-quality AlN growth on 6H-SiC substrate using three dimensional nucleation by low-pressure hydride vapor phase epitaxy

There is a method of controlling nucleation and lateral growth using the three-dimensional (3D) and two-dimensional (2D) growth modes to reduce the dislocation density. We performed 3D–2D-AlN growth on 6H-SiC substrates to obtain high-quality and crack-free AlN layers by low-pressure hydride vapor phase epitaxy (LP-HVPE). First, we performed 3D-AlN growth directly on a 6H-SiC substrate. With increasing V/III ratio, the AlN island density decreased and the grain size increased. Second, 3D–2D-AlN layers were grown directly on a 6H-SiC substrate. With increasing the V/III ratio of 3D-AlN, the crystalline qualities of the 3D–2D-AlN layer were improved. Third, we performed 3D–2D-AlN growth on a trench-patterned 6H-SiC substrate. The crack density was reduced to relax the stress by voids. We also evaluated the threading dislocation density by using molten KOH/NaOH etching. As a result, the estimated edge dislocation density of the 3D–2D-AlN sample was 3.9 × 108 cm−2.

Pulsed modulation doping of AlxGa1‐xN (x&gt;0.6) AlGaN epilayers for deep UV optoelectronic devices

AbstractWe report a new approach of pulsed modulation doping (PMD) to achieve n‐type doping in AlxGa1‐xN (x&gt;0.6) layers. In this approach silane flow is cyclically modulated during n‐AlGaN growth which enable us to grow low dislocation density n‐AlGaN templates for UV (λ ∼ 280 nm) LEDs. We observed that dislocation density for un‐doped AlGaN layers on high quality AlN layers is 3.3×108cm‐2. This dislocation density was increased to 2.3×109 cm–2 for conventionally grown n‐AlGaN layers on AlGaN templates. We were able to reduced dislocation density in n‐AlGaN layers to 3.5×108cm–2 using PMD of n‐AlGaN layers. The dislocation densities in these samples were studies using etch pit density (EPD) and Williamson and Hall studies. A comparative study of LED structures on conventional and PMD doped n‐AlGaN showed improved performance for LED emitting at 280 nm for LED structure on PMD n‐doped AlGaN layers. In this paper the details of our growth procedure, the epi‐structure X‐ray, AFM, and other characterizations will be presented. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Two-step method for the deposition of AlN by radio frequency sputtering

This paper presents a detailed study of the influence of deposition conditions on structural and morphological properties of AlN thin films synthesized on c-sapphire substrates by radio frequency (RF) reactive sputtering. After the optimization of deposition parameters such as RF power and substrate temperature, the substrate bias has been identified as a critical variable to improve the structural properties of the AlN layers. The use of negative bias leads to a decrease of the full-width at half-maximum (FWHM) of the rocking curve of the AlN­(0002) x-ray reflection and an increase of the grain size. However, 2θ/ω x-ray scans of layers grown under negative bias reveal lattice disorder at the AlN/sapphire interface, which is attributed to the highly accelerated positive ions (Al+, N+, N2+) arriving to the substrate at the initial stages of the deposition process. In order to prevent this interface degradation, we propose a two­step deposition method which consists of starting the growth with an unbiased AlN buffer layer, at least 30nm thick, followed by AlN deposition under negative bias. This procedure results in high-quality AlN layers with FWHM of the rocking curve of the (0002) reflection of 1.63°, grain size of ~40nm and root-mean-square surface roughness of 0.4nm.

Early Observations between Magnet and Film Properties for AlN Deposition by Reactive Magnetron Sputtering

This paper reports the findings taken from early experimentation towards the development of high quality AlN layers deposited using physical vapor deposition (PVD)-production tool. In these experiments, the magnetron was equipped with a rotating magnet to enable full-face target erosion. Four magnet designs were tested with various nitrogen (N2) ratios in the gas mixture with argon (Ar). These results show the thickness non-uniformity for AlN film trending across a range of N2 ratios differs with magnet design. Similarly, it has been verified that the N2 ratio is an effective knob for stress control of an AlN film and that the measured in-film stress trends linearly with N2 ratio for all four magnet designs. Using an optimized magnet design and finely tuned process conditions, these early experiments have shown highly uniform AlN films on 200 mm Si wafer with a strong c-axis orientation and tunable stress, which help the learning and development of AlN film for applications in MEMS and BAW devices.

Effects of Si doping in high-quality AlN grown by MOVPE on trench-patterned template

The effects of Si doping on the structural properties of a high-quality AlN layer grown on a trench-patterned AlN/sapphire by metalorganic vapor phase epitaxy (MOVPE) were systematically studied. With a high Si doping concentration, void formation became much slower than that in undoped AlN film. This phenomenon was attributed to the anti-surfactant effect of Si in the growth process. Moreover, as the Si doping concentration increased, compressive stress was significantly relaxed in Si-doped AlN films. A high Si doping concentration of 1×1018cm−3 was successfully achieved for AlN grown at 1500°C.

High-quality AlN layers grown by hot-wall MOCVD at reduced temperatures

We report on a growth of AlN at reduced temperatures of 1100 °C and 1200 °C in a horizontal-tube hot-wall metalorganic chemical vapor deposition reactor configured for operation at temperatures of up to 1500–1600 °C and using a joint delivery of precursors. We present a simple route—as viewed in the context of the elaborate multilayer growth approaches with pulsed ammonia supply—for the AlN growth process on SiC substrates at the reduced temperature of 1200 °C. The established growth conditions in conjunction with the particular in-situ intervening SiC substrate treatment are considered pertinent to the accomplishment of crystalline, relatively thin, ∼700 nm, single AlN layers of high-quality. The feedback is obtained from surface morphology, cathodoluminescence and secondary ion mass spectrometry characterization.

Fabrication of conducting AFM cantilevers with AlN-based piezoelectric actuators

This paper presents a new fabrication technology for conducting AFM cantilevers with integrated AlN-based piezoelectric actuators. A major effort has been done for the process integration of a high quality AlN layer onto the Si cantilever together with a high wear resistance, PtSi-based conducting tip. Functional AFM devices have been successfully tested in tapping mode imaging experiments. Such type of cantilevers are of great interest for dynamic AFM applications especially for tapping or dithering mode AFMs as well as for using the piezoelectric structures as an integrated position sensor.

Photoluminescence of highly excited AlN: Biexcitons and exciton-exciton scattering

Low-temperature photoluminescence spectra of nominally undoped high quality AlN layers on SiC and Al2O3 substrates are reported. Under high excitation conditions, we observe several bands that increase superlinearly with the excitation density. Based on temperature and excitation level dependences recorded on different samples, we identify a band 36 meV below the free A-exciton transition as due to exciton-exciton scattering (P2 band) and a second band down-shifted from the A-exciton transition by 27 meV as due to biexciton recombination. The combined data yield an exciton binding energy of 48 meV.

High quality AlN grown on SiC by metal organic chemical vapor deposition

Growth conditions for AlN in two dimensional (2D) and three dimensional (3D) growth modes were explored on SiC using metal organic chemical vapor deposition. High quality AlN layers were obtained by alternating between 3D and 2D growth modes, referred to as modulation growth (MG). Long parallel atomic terraces without step terminations were observed in atomic force microscopy (AFM) scans of MG AlN, indicating a reduced dislocation density. X-ray diffraction rocking curves yielded full widths at half maximum (FWHM) of 86 and 363arcsec for the (002) and (102) reflections, respectively, giving further evidence of low dislocation density in the film. 3D-2D MG also releases some of the tensile strain in the AlN film, enabling the growth of thick, crack-free AlN on SiC substrates.