Thick AlN Layers Research Articles

Comparison of ammonia plasma and AlN passivation by plasma-enhanced atomic layer deposition

Surface passivation of GaAs by ammonia plasma and AlN fabricated by plasma-enhanced atomic layer deposition are compared. It is shown that the deposition temperature can be reduced to 150 °C and effective passivation is still achieved. Samples passivated by AlN fabricated at 150 °C show four times higher photoluminescence intensity and longer time-resolved photoluminescence lifetime than ammonia plasma passivated samples. The passivation effect is shown to last for months. The dependence of charge carrier lifetime and integrated photoluminescence intensity on AlN layer thickness is studied using an exponential model to describe the tunneling probability from the near-surface quantum well to the GaAs surface.

Mobility-limiting mechanisms in polar semiconductor heterostructures

The mechanisms controlling the carrier mobility of two-dimensional electron gases (2DEGs) in ultrathin polar semiconductor heterostructures, such as III–V nitrides, have been analyzed. InxAl1−xN/AlN/GaN heterostructures with different AlN layer thicknesses have been investigated. These structures can be considered a very good benchmark for the analyses of III–V nitrides, due to the possibility of modulating the strain by varying the In composition. In order to determine an estimate of the mobility, charged dislocation and remote surface roughness scattering lifetimes have been calculated. Atomic force microscopy and scanning tunneling microscopy analyses have been used to measure the parameters required for the lifetime calculation, such as surface roughness, correlation length and dislocation density, and the total mobility has thus been calculated without the need of any a priori assumptions on the values of these parameters. The mobility of InxAl1−xN/AlN/GaN heterostructures has been measured at room temperature and liquid nitrogen temperature by the Hall effect. A comparison between the calculated and the Hall-effect-measured mobilities allowed us to establish, without using any ad hoc assumptions or fitting parameters, that the remote surface roughness is the most effective factor in controlling the transport properties of 2DEGs in nitride-based heterostructures at low temperature.

AlN homoepitaxial growth on sublimation-AlN substrate by low-pressure HVPE

Crack-free thick AlN layers with low impurity concentrations were grown on free-standing AlN substrates fabricated by a sublimation method. Cracks due to tensile stresses were generated in the overgrowth layer when using on-axis AlN (0001) substrates, as indicated by Raman scattering spectroscopy. In contrast, cracks were not generated when using 5° off-angle AlN (0001) substrates. High crystalline quality was indicated by X-ray rocking curve (XRC) analysis. The full width at half maximum (FWHM) values of the (0002) and (10–10) diffractions were 277 and 306arcsec, respectively. Secondary ion mass spectrometry (SIMS) measurements indicated that the Si and C impurity concentrations were reduced to half of those in the sublimation-grown AlN substrates.

Microstructure of Bulk AlN Grown by A New Solution Growth Method

We used transmission electron microscopy to analyse the microstructures in a thick AlN layer grown on a self-nucleated, columnar AlN seed crystal. The growth direction of the AlN layer grown by a new solution growth method was [1100]. The threading dislocation (TD) density near the epilayer-seed interface (on the seed crystal) was 109 cm-2. However, owing to dislocation annihilation, the TD density decreased to 108 cm-2 at a thickness of ∼5 µm. These results imply that the new solution growth method can grow high-crystalline-quality bulk AlN under moderate growth conditions (T ≈1200 °C, nitrogen pressure = 1 atm).

Crystallization of amorphous AlN and superhardness effect in VC/AlN nanomultilayers

VC/AlN nanomultilayers with various AlN layer thicknesses have been prepared by multi-target magnetron sputtering. Microstructure evolution and mechanical properties of the multilayers have been investigated. Under the “template effect” of cubic VC, as-deposited amorphous AlN has been crystallized to cubic structure with AlN layer thickness < 1 nm, correspondingly the multilayers exhibit coherent growth and obtain significantly enhanced hardness with the maximum of 40.1 GPa. A further increase of AlN layer thickness causes the formation of amorphous AlN, which blocks the coherent growth of the multilayers, resulting in a rapid decline of hardness.

Use of Sub-nanometer Thick AlN to Arrest Diffusion of Ion-Implanted Mg into Regrown AlGaN/GaN Layers

Diffusion of Mg from Mg-ion-implanted GaN layer to metalorganic chemical vapor deposition (MOCVD) regrown AlGaN/GaN layers was detected and identified as a critical problem in devices which are dependent on layers implanted with Mg for its current blocking properties. Surface treatments done to etch away the Mg rich layer prior to the regrowth was not beneficial unlike in the case of the GaN doped with Mg. Remarkably, regrowth of a sub-nanometer thick (7 Å) AlN layer on top of the Mg-implanted GaN was found to be effective in arresting the Mg from diffusing out into the AlGaN/GaN layers grown on top at 1160 °C. This was verified from both secondary ion mass spectrometry (SIMS) analysis and electrical (capacitance–voltage) data. This result is significant because at such thickness the AlN would not impact the crystal quality of the overgrown material and serve as a viable method of achieving a current blocking structure by MOCVD growth technique.

Investigation on AlN epitaxial growth and related etching phenomenon at high temperature using high temperature chemical vapor deposition process

Thick AlN layers were grown by high temperature chemical vapor deposition (HTCVD) on 8° off-axis (0 0 0 1) 4H-SiC, on-axis (0 0 0 1) 6H-SiC and on-axis (0 0 0 1) AlN templates between 900 °C and 1600 °C. The experimental set-up consists of a vertical cold-wall reactor working at low pressure in which the reactions take place on a graphite susceptor heated by induction. The reactants used are ammonia (NH 3) and aluminum chlorides (AlCl x ) species in situ formed via Cl 2 reaction with high purity aluminum wire. As-grown AlN layers have been characterized by Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Optical Profilometry, Atomic Force Microscopy (AFM) and Raman spectroscopy. In this study, the influence of the deposition temperature and the N/Al ratio in the gas phase is studied in order to stabilize epitaxial growth. The epitaxy on AlN template is favored using a low N/Al ratio in the gas phase and a high temperature above 1400 °C. The crystalline quality of epitaxial AlN layers is found to increase with increasing deposition temperature from 1400 to 1500 °C. Growth rates up to 14 μm h −1 have been reached for epitaxial AlN layers. An important etching phenomenon is also observed at high temperature: apparition of pin holes certainly around threading dislocations at 1400–1500 °C and substrate etching at 1600 °C.

Atomistic and electrical simulations of a GaN–AlN–(4H)SiC heterostructure optically-triggered vertical power semiconductor device

In this paper, a comprehensive simulation study is conducted to investigate the switching characteristics, gain, and breakdown voltage of a GaN–AlN–(4H)SiC based optically-triggered (OT) heterostructure vertical power semiconductor device (PSD). It comprises a 1 nm AlN buffer layer between the GaN and SiC heterointerface to achieve a reasonable compromise between lattice mismatch and lower forward drop. The results are compared with an all-(4H)SiC OT PSD. The all-(4H)SiC homostructure PSD is based completely on SiC and has no buffer layer. Further, it has the same structure, dimensions, and doping densities as that of the GaN–AlN–(4H)SiC based heterostructure PSD. While there have been studies on GaN–AlN–SiC lateral heterostructures, their primary focus has been on lateral conduction in the GaN structure with a thick (typically >300 nm) AlN buffer layer residing on top of a SiC substrate. Such an approach will not be useful for our vertical PSD because of the thick AlN layer. As such, first, a scaled molecular dynamics simulation (MDS) is carried out in DMol 3 emulating the GaN–AlN–(4H)SiC heterointerface pn junction of the vertical PSD (with 1 nm AlN buffer) to assess the possibility of vertical conduction and stability of the heterointerface by calculating the density of states (DOS) at the Fermi level and the potential energy, respectively. Subsequently, detailed electrical simulations of the GaN–AlN–(4H)SiC and all-(4H)SiC vertical PSDs are carried out in Silvaco to assess their switching performances, gain, on-state drop, and blocking capabilities. The overall results indicate that, the GaN–AlN–(4H)SiC vertical PSD provides superior switching performance and optical absorption compared to the all-(4H)SiC vertical PSD, while the latter provides better gain. The blocking capabilities and forward drops are found to be comparable for both the PSDs from a practical standpoint.

Improvement of electroluminescent performance of n-ZnO/AlN/p-GaN light-emitting diodes by optimizing the AlN barrier layer

The effects of the growth temperature and thickness of AlN layer on the electroluminescence (EL) performance of n-ZnO/AlN/p-GaN devices have been systematically investigated. It is found that the higher growth temperature of AlN layer (TAlN) may facilitate the improvement of EL performance of the device, which is attributed to that the crystalline quality of AlN layer improves with increasing growth temperatures TAlN. Besides the crystallinity of AlN layer, the thickness of AlN barrier layer plays an important role on the performance of the device. The thinner AlN layer is not enough to cover the whole surface of GaN, while the thicker AlN layer is unfavorable to the tunneling of carriers and many of electrons will be captured and recombined nonradiatively via the deep donors within the thick AlN layer. We have demonstrated that the AlN layer at the growth temperature of 700 °C with an optimized thickness of around 10 nm could effectively confine the injected carriers and suppress the formation of interfacial layer, thus, the EL performance of n-ZnO/AlN/p-GaN device could be significantly improved.

Magnetic behavior of CoPt–AlN granular structure laminated with AlN layers

The magnetic behavior of CoPt–AlN granular structure laminated with AlN layers has been studied. Ultrathin multilayer structure, [CoPt0.5 nm/AlN0.5nm]4, is used as the precursor of the magnetic layers, which are separated by 5-nm-thick AlN layers. Upon thermal annealing, the ultrathin multilayer transforms into CoPt–AlN granular structure, and the thick AlN layers remain to be spacers. When the film was annealed at 400 °C, the out-of-plane direction becomes the easy axis of magnetization, although the coercivity remains small. TEM observation has proved that CoPt shows disklike shape at such an annealing temperature. When increasing the annealing temperature to 600 °C and above, the films show “isotropic” magnetic behavior due to the formation of equiaxial CoPt particles in the magnetic layers.

Growth and Characterization of Thick Polycrystalline AlN Layers by HTCVD

Thick polycrystalline AlN layers were grown at low pressure using high temperature chemical vapor deposition (HTCVD). The experimental setup consists of a graphite susceptor heated by an induction coil surrounding a vertical cold wall reactor. The reactants used were ammonia and aluminum chloride species formed in situ via chlorine reaction with high purity aluminum wire. AlN films were deposited on a diameter graphite susceptor between 1200 and . AlN layers have been characterized by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and electron backscattered diffraction. The influence of temperature on growth rate, surface morphology, grain size, and crystalline structure is presented. Growth rates of up to have been reached. A nonpolar preferred orientation of AlN films is stabilized at a higher temperature. The potential of investigation in this new range of experimental conditions, i.e., high temperature and high growth rate, as well as deposition of nonpolar AlN crystals, is very promising for epitaxial growth and extends the field of applications.

Joining of Graphite and Aluminum Nitride Ceramics by using CBC Interlayer

Graphite material and AlN ceramic were joined successfully using an interlayer tape of Ceramic Bonded Carbon (CBC) by spark plasma sintering. The CBC tape was prepared from the slurry of graphite and AlN powders by tape casting method. The CBC interlayer sintered consists of graphite particles bonded with the three-dimensional network of AlN thin layer. The graphite and AlN joints were analyzed by SEM observation, XRD phase identification, and EDX elements distribution. There were no cracks and voids at interfaces of AlN ceramic layer, CBC interlayer and graphite materials. It was suggested that the joining was performed mainly by sintering of AlN through the AlN ceramics layer and the CBC interlayer, and physical bonding between the CBC layer and graphite. The thickness of AlN layer and the CBC interlayer was about 3 % and 4 %, respectively, against the entire thickness of graphite and AlN joint. Nevertheless, the joints showed much higher strength of 82 MPa than the graphite's strength of 41 MPa. Graphite-AlN joints would be expected to use as light, insulative and high heat-conductive substrates or components.

Inclined dislocation arrays in AlGaN/AlGaN quantum well structures emitting at 290 nm

We report on the structural and optical properties of deep ultraviolet emitting AlGaN/AlGaN multiple quantum wells (MQWs) grown on (0001) sapphire by metal-organic vapor phase epitaxy using two different buffer layer structures, one containing a thin (1 μm) AlN layer combined with a GaN interlayer and the other a thick (4 μm) AlN layer. Transmission electron microscopy analysis of both structures showed inclined arrays of dislocations running through the AlGaN layers at an angle of ∼30°, originating at bunched steps at the AlN surface and terminating at bunched steps at the surface of the MQW structure. In all layers, these inclined dislocation arrays are surrounded by AlGaN with a relatively higher Ga content, consistent with plan-view cathodoluminescence maps in which the bunched surface steps are associated with longer emission wavelengths. The structure with the 4 μm-thick AlN buffer layer had a dislocation density lower by a factor of 2 (at (1.7±0.1)×109 cm−2) compared to the structure with the 1 μm thick AlN buffer layer, despite the presence of the inclined dislocation arrays.

Epitaxial growth of GdN on silicon substrate using an AlN buffer layer

We report on the epitaxial growth of the intrinsic ferromagnetic semiconductor GdN on Si (1 1 1) substrates buffered by a thick AlN layer, forming a heteroepitaxial system with promise for spintronics. Growth is achieved by depositing Gd in the presence of unactivated N 2 gas, demonstrating a reactivity at the surface that is sufficient to grow near stoichiometric GdN only when the N 2:Gd flux ratio is at least 100. Reflection high-energy electron diffraction and X-ray diffraction show fully (1 1 1)-oriented epitaxial GdN films. The epitaxial quality of the films is assessed by Rutherford backscattering spectroscopy carried out in random and channelling conditions. Magnetic measurements exhibit a Curie temperature at 65 K and saturation magnetisation of 7 μ B/Gd in agreement with previous bulk and thin-film data. Hall effect and resistance data establish that the films are heavily doped semiconductors, suggesting that up to 1% of the N sites are vacant.

High performance InGaN LEDs on Si (1 1 1) substrates grown by MOCVD

We report high performance InGaN multiple-quantum well (MQW) light-emitting diodes (LEDs) grown on Si (1 1 1) substrates using metalorganic chemical vapour deposition (MOCVD). A high-temperature thin AlN layer and AlN/GaN multilayers have been used for the growth of a high-quality GaN-based LED structure on Si substrates. Reduction of the high-temperature AlN layer thickness promotes the formation of a tunnel junction at the AlN/Si interface which reduces the LED operating voltage. Optical output power of the LED on Si saturates at a higher injected current density due to higher thermal conductivity of Si than that of a sapphire substrate. At a high injection current, output power of the LED on Si is higher than that of the LED on sapphire. Cross-sectional transmission electron microscopy (TEM) indicates that the active layer of these LEDs consists of a dislocation-free pyramid-shaped (quantum-dot-like) structure. Additionally, the crack-free thin-film LED epilayer region was transferred onto a copper carrier using metal-to-metal bonding and the selective lift-off technique. A LED with high output power, low operating voltage and low series resistance was realized by this technique. Furthermore, optimization of LED on Si by insertion of an Al0.06Ga0.94N/GaN strained-layer superlattice underlayer into the structure exhibits improved internal quantum efficiency (ηiqe) in the MQW, higher optical emission intensity with higher saturation current, lower operation voltage of 3.2 V at 20 mA and a series resistance of 16 Ω, as well as narrower electroluminescence spectra.

Investigation of void formation beneath thin AlN layers by decomposition of sapphire substrates for self-separation of thick AlN layers grown by HVPE

Void formation at the interface between thick AlN layers and (0 0 0 1) sapphire substrates was investigated to form a predefined separation point of the thick AlN layers for the preparation of freestanding AlN substrates by hydride vapor phase epitaxy (HVPE). By heating 50–200 nm thick intermediate AlN layers above 1400 °C in a gas flow containing H 2 and NH 3, voids were formed beneath the AlN layers by the decomposition reaction of sapphire with hydrogen diffusing to the interface. The volume of the sapphire decomposed at the interface increased as the temperature and time of the heat treatment was increased and as the thickness of the AlN layer decreased. Thick AlN layers subsequently grown at 1450 °C after the formation of voids beneath the intermediate AlN layer with a thickness of 100 nm or above self-separated from the sapphire substrates during post-growth cooling with the aid of voids. The 79 μm thick freestanding AlN substrate obtained using a 200 nm thick intermediate AlN layer had a flat surface with no pits, high optical transparency at wavelengths above 208.1 nm, and a dislocation density of 1.5×10 8 cm −2.

RF 마그네트론 스퍼터링법을 이용하여 사파이어 기판과 ZnO 박막 위에 증착한 AlN 박막의 특성분석

먼저 RF 마그네트론 스퍼터링법을 이용하여 사파이어 기판 위에 AlN 박막을 증착하였다. AlN 공급원으로는 분말소결된 AlN 타겟을 적용하였다. 플라즈마 파워를 50에서 110 W로 증가시켰을 때 AlN 층의 두께는 선형적으로 증가하였다. 그러나 동작압력을 3에서 10 mTorr로 증가시켰을 때는 동작기체인 아르곤 양이 증가함에 따라 AlN 타겟으로부터 스퍼터링되어 나온 AlN 입자들의 평균자유행정의 거리가 감소하기 때문에 AlN 층의 두께는 약간 감소하였다. 질소 기체를 아르곤과 섞어주었을 때는 질소의 낮은 스퍼터링 효율에 의해서 AlN의 두께는 크게 감소하였다. 다음으로는 ZnO 형판 위에 AlN를 증착하였다. 그러나 700도 이상의 열처리에 의해서 AlN와 ZnO의 계면이 약간 분리되어 계면의 열적 안정성이 낮다는 결과를 얻었다. 게다가 스퍼터링으로 증착한 AlN 박막의 나쁜 결정성으로 인하여 700도에서 MOCVD의 반응기 기체인 수소와 암모니아에 의해서 AlN 밑의 ZnO 층이 분해되는 현상도 관찰하였다. 그리고 900도 이상에서는 ZnO가 완전히 분해되어 AlN 박막이 완전히 분리되었다. AlN thin films were deposited on sapphire substrates and ZnO templates by rf-magnetron sputtering. Powder-sintered AlN target was adopted for source material. Thickness of AlN layer was linearly dependent on plasma power from 50 to 110 W, and it decreased slightly when working pressure increased from 3 to 10 mTorr due to short mean free path of source material sputtered from AlN target by Ar working gas. When <TEX>$N_2$</TEX> gas was mixed with Ar, the thickness of AlN layer decreased significantly because of low sputter yield of nitrogen. AlN layer was also deposited on ZnO template. However, it showed weak thermal stability that the interface between AlN and ZnO was deteriorated by rapid thermal annealing treatment above <TEX>$700^{\circ}C$</TEX>. In addition, ZnO layer was largely attacked by MOCVD ambient gas of hydrogen and ammonia around <TEX>$700^{\circ}C$</TEX> through inferior AlN layer deposited by sputtering. And AlN layers were fully peeled off above <TEX>$900^{\circ}C$</TEX>.

Hetero-structure coherent epitaxial growth in AlN/NbN nano-structured multilayers

Monolithic AlN，NbN films and AlN/NbN multilayers with different modulation periods were prepared by reactive magnetic sputtering. The films were characterized by X-ray diffraction, X-ray reflectivity and high-resolution transmission electron microscopy. The results showed that the crystal structure of monolithic AlN and NbN films is close-packed hexagonal (hcp) and face-centered cubic (fcc), respectively. The crystal structure of AlN and NbN is hcp and fcc, respectively, in AlN/NbN multilayers. The interfaces between AlN layers and NbN layers are coherent， i.e., c-NbN (111)∥h-AlN(0002). The lattice mismatch of AlN/NbN multilayers is 013%. The thermodynamic calculation revealed that no matter how thickness of AlN or NbN layer is, the AlN layer does not form nonequilibrium structure of fcc, but the equilibrium structure of hcp. The AlN layers grow in the way of hetero-epitaxial coherent growth with NbN layers.

Multi‐layer thickness determination using differential‐based enhanced Fourier transforms of X‐ray reflectivity data

AbstractLayer thickness determination of single and multi‐layer structures is achieved using a new method for generating Fourier transforms (FTs) of X‐ray reflectivity data. This enhanced Fourier analysis is compared to other techniques in the determination of AlN layer thickness deposited on sapphire. In addition to demonstrably improved results, the results also agree with thicknesses determined using simulations and TEM measurements. The effectiveness of the technique is further demonstrated using the more complicated metamorphic epitaxial multi‐layer AlSb/InAs structures deposited on GaAs. The approach reported here is based upon differentiating the specular intensity with respect to the vertical reciprocal space coordinate QZ. In general, differentiation is far more effective at removing the sloping background present in reflectivity scans than logarithmic compression alone, average subtraction alone, or other methods. When combined with any of the other enhancement techniques, however, differentiation yields distinguishable discrete Fourier transform (DFT) power spectrum peaks for even the weakest and most truncated of sloping oscillations that are present in many reflectivity scans from multi‐layer structures.

Controlled formation of voids at the AlN/sapphire interface by sapphire decomposition for self‐separation of the AlN layer

AbstractSelf‐separation of hydride vapor phase epitaxy (HVPE)‐grown AlN layer from sapphire substrate was achieved by the formation of voids at the AlN/sapphire interface. Voids could be formed through the decomposition of sapphire beneath a thin (100 nm) protective AlN layer grown at 1065 °C by heating at 1450 °C in a carrier gas (H2 + N2) containing NH3. Additionally, the void size could be controlled by varying the heating time. As the results of controlled formation of voids, after growth of a thick (85 μm) AlN layer on the thin protective AlN layer heated at 1450 °C for 30 min, self‐separation of the AlN layer from a sapphire substrate occurred by the compressive stress from difference in thermal expansion coefficients between AlN and sapphire during post‐growth cooling. The self‐separated thick AlN layer had low concentration of impurities, and showed clear photoluminescence peak at 208.4 nm. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)