Low-temperature AlN Research Articles

Modulated Ammonia Flow - Low Temperature AlN Buffer LP-MOVPE Growth for High Quality AlGaN Layers

Modulated Ammonia Flow - Low Temperature AlN Buffer LP-MOVPE Growth for High Quality AlGaN Layers

Epitaxial growth and characterization of GaN thin films on graphene/sapphire substrate by embedding a hybrid-AlN buffer layer

This study investigates that high-quality GaN thin films can be grown on a few-layer graphene (FLG)/sapphire substrate by embedding a hybrid AlN buffer layer (BL). The hybrid AlN BL is constructed by low-temperature AlN nucleation layer (LT-AlN NL) and high-temperature AlN BL grown respectively by sputtering and metal organic chemical vapor deposition (MOCVD). The high density of edge-type threading dislocation (TD) in the GaN sample without hybrid AlN BL provide current leakage paths, resulting in a symmetric and temperature-independent I-V characteristic curve for a Ni-based Schottky contact. The excellent adhesion and uniform coverage of LT-AlN NL on the FLG layer by sputtering can overcome the nucleation issue and prevent the thermal etching effect of graphene during MOCVD epitaxial process. The edge-type TD density and carbon concentration of the GaN thin films grown on the hybrid AlN BL/FLG/sapphire substrate can be reduced significantly, resulting in a lower intensity of blue, green, and orange luminescences on a 17-K photoluminescence spectrum. The Ni-based Schottky contact with a barrier height of 0.69 eV and leakage current density of 4.38 × 10−6 A/cm2 is obtained, which demonstrates that a high-quality GaN thin films can be grown onto an FLG substrate by embedding a hybrid AlN BL.

AlxGa1−xN GROWTH BY Si/N TREATMENT OF SAPPHIRE SUBSTRATE: COMPARISON WITH OTHERS GROWTH TECHNIQUES

AlxGa[Formula: see text]N films were grown on Si/N-treated sapphire substrate by atmospheric pressure metalorganic vapor phase epitaxy (AP-MOVPE) in a home-made vertical reactor. This process can be considered as randomly in situ epitaxial lateral overgrowth (ELO) technology. The growth firstly begins by three-dimensional (3D) mode and is completed in two-dimensional (2D) growth mode as shown by real time in situ laser [Formula: see text][Formula: see text]nm[Formula: see text] reflectometry measurements and confirmed by scanning electron microscopy (SEM) images. Secondary ion mass spectroscopy (SIMS) measurements evidence Al composition pulling effect in the AlxGa[Formula: see text]N layer. The Si/N treatment technique is compared to conventional AlxGa[Formula: see text]N growth techniques. The results of high-resolution X-ray diffraction (HRXRD), photoluminescence (PL) measurements and SEM images agree well on the fact that the Si/N treatment produces AlxGa[Formula: see text]N layers with comparable qualities of AlxGa[Formula: see text]N layers grown on high temperature GaN template but with much higher qualities than AlxGa[Formula: see text]N layers grown on low temperature AlN nucleation layer. Moreover, the Si/N treatment technique permits the growth of high quality AlxGa[Formula: see text]N layers with appreciable thicknesses with respect to the others techniques.

A stress sensor based on a silicon field effect transistor comprising a piezoelectric AlN gate dielectric

Piezoelectric materials have been introduced to transistor gate stacks to improve MOSFET behaviour and develop sensor applications. In this work, we present an approach to a partly industrial field effect transistor, with a gate stack based upon low temperature AlN. Using the piezoelectric effect of the nitrogen-polar AlN, we are able to drive the transistor by inducing strain across the device. To ensure maximum sensitivity, the piezoelectric material is placed as closely to the transistor channel as possible and the transistor is operated in the most sensitive part of the sub-threshold regime. This allows the detection of different magnitudes of force applied to the device and to easily distinguish between them. The created sensor was analysed using XRD, current–voltage and specific force application measurements. Furthermore, the continuous response to periodic low frequency stimulation is investigated. Therefore, we introduce a highly scalable device with a wide range of application possibilities, ranging from varying sensor systems to energy harvesting.

Characterization and optimization of AlN nucleation layer for nonpolar a-plane GaN grown on r-plane sapphire substrate

Abstract Nonpolar (11–20) a-plane GaN films with AlN nucleation layer were grown on (10–12) r-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD). The crystalline and surface qualities of a-plane GaN were found to closely depend on the growth conditions of AlN nucleation layer. With decreasing AlN growth temperature, the AlN grains became larger and sparser, which significantly reduced the defects density of a-plane GaN films. The growth time of the low temperature AlN layer was further optimized, and a-plane GaN films with reduced anisotropy in the crystalline quality, surface morphology and in-plane strains were achieved. It was found that the lateral growth lengths along different directions of GaN could be modulated by the growth time of AlN nucleation layer, thus changing the anisotropy of a-plane GaN films.

Correlation of donor-acceptor pair emission on the performance of GaN-based UV photodetector

High responsivity UV photodetector has been realized by fabricating single crystalline epitaxial GaN-based devices grown on silicon substrate by using molecular beam epitaxy system. The influence of trap states existing within the forbidden gap on GaN-based optoelectronic devices has been investigated. The quality and performance of the fabricated GaN based UV photodetector with distinct AlN buffer layer grown at low and high substrate temperature are substantiated. A detailed analysis reveals that high temperature buffer can yield improved crystalline quality of GaN with similar morphological properties as compared to buffer layer grown at low temperature. However, the distinct buffer layer influence the optical properties as the photoluminescence analysis explains that both the films possess superior band-to-band edge emission where GaN grown with high temperature AlN buffer consists of dominant acceptor defect states in comparison to GaN with low temperature AlN buffer layer. The existence of acceptor states has been accredited by the presence of Ga vacancy related states in the band gap. The fabricated devices yield high photoresponsivity of 2.1 and 1.5 A/W at 1 V from GaN films bearing low and high accepter defect states which explain a clear correlation of device performance with trap states within the band-gap region.

MOVPE growth conditions optimization for AlGaN/GaN/Si heterostructures with SiN and LT-AlN interlayers designed for HEMT applications

In this work we present the influence of in situ deposited non-continous SiN layer and the chemical precursors III/V mole ratio change during GaN buffer growth for AlGaN/GaN/Si(111) heterostructures with low temperature AlN interlayer on their crystalline quality and electrical mobility of two dimensional electron gas (2DEG). We show, that application of SiN layer resulted in a decrease of (0002) full width at half maximum of diffraction peak below 400 arcsec and build-in stress in 2 µm thick heterostructures below 200 MPa without any relaxation visible on the surface. In optimized AlGaN/GaN heterostructures, by altering V/III mole ratio during growth of GaN subbuffer, the maximum 2DEG mobility of 2057 cm2 V−1 s−1, measured by impedance spectroscopy method, was obtained.

The influence of V/III ratio on GaN grown on patterned sapphire substrate with low temperature AlN buffer layer by hydride vapor phase epitaxy

In this paper, 2–4 in. GaN wafers with high crystal quality were obtained with optimized V/III ratio of source gases on the patterned sapphire substrate (PSS) covered by a sputtered thin AlN buffer layer by using hydride vapor phase epitaxy (HVPE) growth technology. It was found that the morphology and crystal quality of the top GaN layer were significantly different with the variation of V/III ratio of source gases in initial nucleation growth step. Transparent single crystalline GaN layer could be obtained under relatively low V/III ratio of source gases, while it transferred to yellow and polycrystalline structure with increasing of V/III ratio of source gases. This is mainly attributed to the significant transition of GaN nucleation process with different crystallographic plane of AlN buffer layer, and corresponding variation of GaN growth mode at different V/III ratio of source gases, supported by surface, lateral SEM characterization, high-resolution X-ray diffraction (XRD) measurement, and detailed growth mechanism analysis. Moreover, the crystal quality of GaN grown under optimized condition can be further improved by increasing the thickness of GaN. It would offer a simple way to obtain GaN templates with high crystal quality at relatively low cost.

Properties of AlN layers grown on c-sapphire substrate using ammonia assisted MBE

AlN epilayer properties (120 nm thick) grown by ammonia assisted molecular beam epitaxy on c-sapphire substrates with different low temperature AlN buffer layers (LT-BL) have been studied. The role of the LT-BL on the AlN structural and optical properties was investigated as a function of the LT-BL thickness and growth temperature. Optimum growth conditions were identified with LT-BL thickness of 3 nm and growth temperature between 480 °C and 520 °C. It was shown that by optimizing these conditions, a reduction of both mixed and edge threading dislocation densities up to 75% is achieved. The impact of the growth temperature of the AlN epilayer was also studied showing an additional improvement of the AlN crystal and morphological properties while growing at higher temperature. A correlation between the epilayer strain and the PL emission was also investigated. Finally, an Al0.7Ga0.3N:Si doped layer was grown on the top of the optimized AlN template showing a smooth surface with monoatomic steps and a roughness ∼0.2 nm, confirming the potential of such templates for the fabrication of AlGaN based heterostructures.

Enhanced process stability for the low temperature sputter deposition of aluminium nitride thin films

MEMS (micro electro-mechanical systems) operated in resonance and excited piezoelectrically are nowadays used for a broad range of different application scenarios. To enhance the process stability and hence, the reproducibility of key film parameters of sputter-deposited aluminium nitride such as the film stress, the piezoelectric coefficient d33 and low leakage current levels, a novel aluminium clamped substrate holder is reported. Compared to the standard molybdenum based solution, where the thermal contact between the wafer and substrate holder varies during deposition, as the wafer can move freely, the substrate temperature variations are substantially reduced due to clamped configuration. Independent of AlN film thickness ranging between 0.5 μm and 2.0 μm the scatter in piezoelectric constant d33 and leakage current characteristics represented by the barrier height and the activation energy is reduced up to a factor of 3. These results demonstrate the importance to control carefully the temperature conditions during low-temperature AlN deposition to ensure a high reproducibility in film properties.

The preparation of different pairs near-ultraviolet AlGaN/GaN DBRs with AlN interlayer

In this paper, near-ultraviolet AlGaN/GaN distributed Bragg reflectors (DBRs) were prepared on GaN/sapphire templates by metal organic chemical vapor deposition. A single low-temperature AlN interlayer was adopted to prevent the formation of cracks in DBRs. Moreover, different pairs of DBRs with AlN interlayer were grown to investigate the stress distribution through the epilayer, the surface morphology evolution and the variation of reflectivity spectra. The in-situ stress monitoring shows a compressive-to-tensile stress transition with the increase of DBRs pairs. The optical microscope and atomic force microscope images show that the AlN interlayer induces the appearance of trenches on the surface of DBRs and the trenches gradually coalesce when DBRs grow. Meanwhile, the experimental reflectivity spectra get closer to the simulated results. Finally, we obtain a smooth-surface 25-pair Al.32Ga.68N/GaN DBRs with a reflectance of 94% at 390 nm and a 16 nm stopband bandwidth. The preparation of high-quality DBRs lays the foundation for the future development of high efficiency resonant cavity UV LEDs.

On the Role of AlN Insertion Layer in Stress Control of GaN on 150-mm Si (111) Substrate

In this study, low-temperature (LT) and high-temperature (HT) AlN insertion layers (ILs) grown at 680 and 970 °C were integrated with 3.7-μm GaN-based heterostructure grown on 150-mm Si (111) substrates by metalorganic chemical vapor deposition. Under a V/III flow ratio of 1960, the GaN epilayer with a continuous interface resulting from the LT AlN IL was subject to a compressive stress of −0.109 GPa. However, the GaN epilayer with discontinuous interfaces resulting from the HT AlN IL growth under the same flow ratio was subject to a tensile stress of 0.174 GPa. To realize continuous interfaces between the GaN epilayer and HT AlN IL, a higher V/III ratio of 5960 was utilized to suppress the decomposition of GaN. It results in changing the stress state of the GaN-based heterostructure from tensile to compressive. This strategic finding indicates that a stress-controllable GaN on Si can be achieved via the incorporation of HT AlN ILs. A minimum curvature at 5 km−1 is demonstrated for the 3.7-μm GaN-based heterostructure on a 150-mm Si (111) substrate, which has high potential for power switching device applications.

Anisotropic strain relaxation and high quality AlGaN/GaN heterostructures on Si (110) substrates

We have investigated the growth and relaxation mechanisms of anisotropic lattice misfit strain in AlN and GaN layers on Si (110) substrates. A qualitative model is proposed to explain the relaxation process. It is revealed that the anisotropic misfit strain is quickly relaxed in the low temperature AlN layer by the formation of interface misfit dislocations, small misoriented grains, and lattice distortion. As a result, isotropic properties and atomically smooth surface are observed in the high temperature AlN layer. Based on this isotropic AlN layer, a high quality GaN layer and AlGaN/GaN heterostructures with a high electron mobility of 2160 cm2/(V · s) have been obtained. This work will have important impacts on the understanding of the epitaxy of isotropic semiconductor films on anisotropic substrates.

Growth mechanisms of GaN epitaxial films grown on ex situ low-temperature AlN templates on Si substrates by the combination methods of PLD and MOCVD

Three kinds of ex situ low-temperature AlN (LT-AlN) templates grown on Si substrates by pulsed laser deposition (PLD) are employed to grow GaN epitaxial films by metal organic chemical vapor deposition (MOCVD). The single-crystalline ex situ LT-AlN template with smooth surface shows the greatest advantage to achieve high-quality GaN epitaxial films. It can effectively promote the lateral growth of high-temperature AlN layer to obtain coalesced and smooth surface, and therefore reduce the GaN nucleation energy barrier to facilitate GaN nucleation process. The as-grown truncated GaN nuclei with large sizes and low density are favorable for the two-dimensional growth of GaN. The ∼1.5 μm-thick GaN epitaxial film grown on this ex situ LT-AlN template shows a flat surface and the best crystalline quality with the minimum full-width at half maximums (FWHMs) of GaN(0002) and GaN(10–12) X-ray rocking curves (XRCs) as 504 and 566 arcsec, respectively, and the minimum FWHM of near band emission peak as 20.62 nm. According to the XRC results, compared with the GaN film grown without ex situ LT-AlN templates, the screw-type threading dislocations (TDs) density, edge- and mixed-type TDs density in as-grown GaN film are greatly reduced by 20% and 49%, respectively, proving the superiority of ex situ LT-AlN template. This work is instructional for the growth of high-quality nitride films by combination methods, and significant for achieving high-performance GaN-based devices.

Low-temperature growth of AlN and GaN by metal organic vapor phase epitaxy for polarization engineered water splitting photocathode

Crystal properties of low-temperature grown AlN (LT-AlN) combined with low temperature GaN (LT-GaN) grown by metal organic vapor phase epitaxy (MOVPE) were investigated to obtain a high quality GaN/AlN/GaN structure with a few-nm-thick AlN layer. LT-AlN suppresses unintentional Ga incorporation and can be pseudomorphically grown on GaN with a relatively smooth surface morphology. The lattice of LT-AlN coherent to GaN, however, was found to relax after reactor conditions were changed to grow the subsequent GaN layer at higher temperature. The top GaN layer grown on the relaxed LT-AlN, thus, exhibited a rough surface morphology and a threading dislocation density (TDD) higher than 109cm−2 estimated from an X-ray diffraction measurement. An LT-GaN capping layer was found to be highly effective for avoiding such lattice relaxation of LT-AlN. The combination of LT-AlN and LT-GaN enables us to obtain a GaN/AlN/GaN junction with high Al content, a low TDD, and abrupt interfaces. As a result, introducing an LT-GaN layer improved the photoelectrochemical (PEC) property of a polarization engineered un-doped GaN/AlN/n-type GaN (u-GaN/AlN/n-GaN) photocathode for water splitting.

Buffer effects on the mosaic structure of the HR-GaN grown on 6H-SiC substrate by MOCVD

High-resistive GaN (>108 Ω cm) layers have been grown with different buffer structures on 6H-SiC substrate using metalorganic chemical vapor deposition reactor. Different combination of the GaN/AlN super lattice, low temperature AlN, high temperature AlN and AlxGa1−xN (x ≈ 0.67) layers were used in the buffer structures. The growth parameters of the buffer layers were optimized for obtaining a high-resistive GaN epilayer. The mosaic structure parameters, such as lateral and vertical coherence lengths, tilt and twist angle (and heterogeneous strain), and dislocation densities (edge and screw dislocations) of the high-resistive GaN epilayers have been investigated using x-ray diffraction measurements. In addition, the residual stress behaviors in the high-resistive GaN epilayers were determined using both x-ray diffraction and Raman measurements. It was found that the buffer structures between the HR-GaN and SiC substrate have been found to have significant effect on the surface morphology and the mosaic structures parameters. On the other hand, both XRD and Raman results confirmed that there is low residual stress in the high-resistive GaN epilayers grown on different buffer structures.

Fabrication of high-crystallinity a-plane AlN films grown on r-plane sapphire substrates by modulating buffer-layer growth temperature and thermal annealing conditions

High-crystallinity a-plane AlN(112̅0) films containing a low-temperature AlN (LT-AlN) buffer layer and a high-temperature AlN (HT-AlN) film were grown on r-plane sapphire(11̅02) substrates. We investigated the effect of the growth temperature and thermal annealing conditions for the LT-AlN buffer layers on the crystallinity and surface morphology. The surface roughness of the buffer layers became smooth with the decrease in growth temperature to 900°C, and the crystallinity of the buffer layers was improved by thermal annealing at temperatures over 1600°C. HT-AlN films were then grown on the annealed LT-AlN buffer layers at 1500°C. The optimum crystallinity of HT-AlN films without any facet formation at the surfaces was obtained with full width at half maximum values of the X-ray rocking curves for AlN(112̅0)//[11̅00]AlN at 770 and (0002) at 640″.

Low temperature aluminum nitride thin films for sensory applications

A low-temperature sputter deposition process for the synthesis of aluminum nitride (AlN) thin films that is attractive for applications with a limited temperature budget is presented. Influence of the reactive gas concentration, plasma treatment of the nucleation surface and film thickness on the microstructural, piezoelectric and dielectric properties of AlN is investigated. An improved crystal quality with respect to the increased film thickness was observed; where full width at half maximum (FWHM) of the AlN films decreased from 2.88 ± 0.16° down to 1.25 ± 0.07° and the effective longitudinal piezoelectric coefficient (d33,f) increased from 2.30 ± 0.32 pm/V up to 5.57 ± 0.34 pm/V for film thicknesses in the range of 30 nm to 2 μm. Dielectric loss angle (tan δ) decreased from 0.626% ± 0.005% to 0.025% ± 0.011% for the same thickness range. The average relative permittivity (εr) was calculated as 10.4 ± 0.05. An almost constant transversal piezoelectric coefficient (|e31,f|) of 1.39 ± 0.01 C/m2 was measured for samples in the range of 0.5 μm to 2 μm. Transmission electron microscopy (TEM) investigations performed on thin (100 nm) and thick (1.6 μm) films revealed an (002) oriented AlN nucleation and growth starting directly from the AlN-Pt interface independent of the film thickness and exhibit comparable quality with the state-of-the-art AlN thin films sputtered at much higher substrate temperatures.

Effects of AlN buffer layer thickness on the crystallinity and surface morphology of 10-µm-thick a-plane AlN films grown on r-plane sapphire substrates

10-µm-thick a-plane AlN films containing a low-temperature AlN (LT-AlN) buffer layer and a high-temperature AlN (HT-AlN) film were prepared on r-plane sapphire substrates. The crystallinity of all the samples with different LT-AlN buffer layer thicknesses was improved after thermal annealing and HT-AlN growth, mainly owing to the elimination of domain boundaries and the concurrent suppression of facet formation. The optimum crystallinity of HT-AlN films was obtained with full widths at half maximum of the X-ray rocking curves of 660 arcsec for AlNAlN and 840 arcsec for (0002) using a 200-nm-thick LT-AlN buffer layer.

Impact of growth conditions on stress and quality of aluminum nitride (AlN) thin buffer layers

In this study, we report about an analysis of the crystal quality and stress conditions of aluminum nitride (AlN) buffer layers by changing several process conditions during the growth at a metal‐organic chemical vapor deposition (MOCVD) reactor. A two‐steps process has been implemented for AlN growth on Si, consisting of a low‐temperature AlN (LT‐AlN) nucleation step followed by a high‐temperature AlN (HT‐AlN) growth step. An improvement of the crystal quality has been observed for lower V/III ratio used in the LT‐AlN step. Growth temperature of HT‐AlN was also examined showing a weak variation of AlN growth rates. Crystal quality and residual stress conditions as a function of the AlN layer thickness were studied. X‐ray diffraction (XRD) rocking curves and Raman spectroscopy suggest a better crystal quality when AlN thickness is increased, due to enlargement of islands and reduction of extended defects. After a certain thickness, namely 300 nm, slip lines occurs in epitaxial layers leading to reduction of the residual biaxial stress perpendicular to growth direction. Electrical measurements have been carried out with a critical electric field of 2.1 MV cm−1 achieved.