AlN GaN Research Articles

Crack-free GaN∕AlN distributed Bragg reflectors incorporated with GaN∕AlN superlattices grown by metalorganic chemical vapor deposition

A crack-free GaN∕AlN distributed Bragg reflector (DBR) incorporated with GaN∕AlN superlattice (SL) layers was grown on a c-plane sapphire substrate by metalorganic chemical vapor deposition. Three sets of half-wave layers consisting of 5.5 periods of GaN∕AlN SL layers and GaN layer were inserted in every five pairs of the 20 pair GaN∕AlN DBR structure to suppress the crack generation. The grown GaN∕AlN DBRs with SL insertion layers showed no observable cracks in the structure and achieved high peak reflectivity of 97% at 399nm with a stop band width of 14nm. Based on the x-ray analysis, the reduction in the in-plane tensile stress in the DBR structure with insertion of SL layers could be responsible for the suppression of crack formation and achievement of high reflectivity.

Efficacy of single and double SiNx interlayers on defect reduction in GaN overlayers grown by organometallic vapor-phase epitaxy

We report on the growth of and evolution of defects in GaN epilayers having single- and double-layer SiNx nanoporous insertion layers. The SiNx was formed in situ in the growth chamber of an organometallic vapor-phase epitaxy system by simultaneous flow of diluted silane and ammonia. The GaN epilayers and SiNx interlayers were grown on 6H-SiC substrates using three different nucleation layers, namely, low-temperature GaN, high-temperature GaN, and high-temperature AlN nucleation layers. X-ray-diffraction rocking curves and cross-sectional and plan-view transmission electron microscope analyses indicated that a nanoporous SiNx layer can reduce the dislocations density in the GaN overgrown layer to ∼3×108cm−2 range; below this level the defect blocking effect of SiNx would saturate. Therefore the insertion of a second SiNx layer becomes much less effective in reducing dislocations, although it continues to reduce the point defects, as suggested by time-resolved photoluminescence measurements. The insertion of SiNx interlayers was found to improve significantly the mechanical strength of the GaN epilayers resulting in a much lower crack line density.

A multifractal analysis of optical phonon excitations in quasicrystals

It is shown that the allowed frequencies of optical phonon modes propagating in quasiperiodic AlN–GaN structures display a multifractal behavior. This feature is confirmed through a scaling-exponent analysis of a parametric spectrum of singularities f ( α ) . This is done by using an alternative counting method, avoiding the use of a Legendre transform, which could lead to distortions. The Cantor-like spectrum is also investigated, and scaling properties are shown for Fibonacci, Thue–Morse and double-period quasiperiodic superlattices.

Stability and electronic structure of single-walled InN nanotubes

Based on density functional theory calculations, we predict the stability and electronic structures of single-walled indium nitride (InN) nanotubes. Compared with other group III-nitride nanotubes with a similar diameter, strain energies of InN nanotubes relative to their graphitic sheet are the lowest, suggesting the possibility of the formation of InN nanotubes. Considering the stability of a graphitic InN sheet, InN nanotubes are in metastable states with the stability between GaN nanotubes and AlN nanotubes. Contrary to the case of carbon nanotubes and BN nanotubes, the bond-length of both horizontal and vertical In–N bonds in InN nanotubes decreases as the tube diameter increases. InN nanotubes are all semiconductors with an almost constant band gap of about 1 eV. The existence of a direct gap in zigzag InN nanotubes and the small band gap indicate that they may have potential applications in light emitting devices and solar cells.

Growth of AlGaN/GaN based electronic device structures with semi‐insulating GaN buffer and AlN interlayer

Heterostructures of GaN/AlGaN for power applications are grown by metal organic vapor phase epitaxy in a multiwafer reactor on sapphire and SiC substrates. The insulating properties of the GaN buffer and the influence of an AlN interlayer on the two-dimensional electron gas transport properties of high electron mobility transistor structures have been studied. By optimizing the growth conditions the resistivity could be increased to 1011 Ωcm. For GaN/AlGaN heterostructures on sapphire with a 1 nm AlN interlayer a room temperature mobility of 1780 cm2/Vs and an electron sheet carrier density of 1.2 ×1013 cm–2 were achieved. Power devices were fabricated on s.i. SiC showing a power density of 4.9 W/mm at 10 GHz for a multifinger transistor with a gate width of 1 mm. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal conductivity of AlN and AlN–GaN thin films deposited on Si and GaAs substrates

Thermal properties of thin AlN, AlN–GaN films are analyzed. The investigated films are deposited on different substrates (Si and GaAs). The photothermal measuring method based on thermal wave propagation in layered structure in 3D geometry is used. A new method for experimental data analysis is proposed. The analysis is carried out after the transformation of measured dependences to the inverse space. Obtained results show that the thermal conductivity is higher for films deposited on GaAs substrates and for AlN–GaN films.

Coupling of spontaneous emission from GaN–AlN quantum dots into silver surface plasmons

We have demonstrated the decay of spontaneous emission (SE) from AlN-GaN quantum dots (QDs) into silver surface plasmon (SP) modes in the ultraviolet at approximately 375-380 nm. Using time-resolved photoluminescence (PL), we show that the electron-hole recombination rate in AlN-GaN QDs is enhanced when SE is resonantly coupled to a metal SP mode, corresponding to the dip in the continuous-wave PL spectrum. Exciton recombination by means of silver SP modes is as much as 3-7 times faster than in normal QD SE and depends strongly on emission wavelength and thickness of the silver.

Optical and structural properties of rare earth doped GaN quantum dots

We have studied the structural and optical properties of GaN quantum dots (QDs) doped with Tm and Eu. It has been found that the morphology of the dots was affected by the presence of the rare earth atoms. Differences in incorporation have also been pointed out, Eu being located inside the QDs while Tm is more likely to be found at the interface between GaN QDs and AlN matrix. Intense and sharp Eu- and Tm-related transition lines are observed in the photoluminescence and cathodoluminescence spectra of doped QDs, with no thermal quenching between 20 and 300 K. They exhibit a saturation effect with increasing excitation power, independently of the temperature. It is tentatively proposed that some transition lines in GaN:Tm QDs could be induced by the presence of the internal electric field.

Strain relaxation in AlN/GaN bilayer films grown on γ-LiAlO2(100) for nanoelectromechanical systems

We fabricate submicrometer-wide cantilevers and beams from M-plane AlN/GaN bilayer films grown on γ-LiAlO2(100) substrates. The chemically reactive substrate is ideal for fabrication of nanoelectromechanical systems using the light, stiff, and piezoelectrically active AlN. The absence of polarization fields in M-plane quantum wells allows us to incorporate optical functionalities in the (Al,Ga)N-based nanoelectromechanical systems. Self-rolling of the cantilevers indicates that the bilayer films are strained at the AlN–GaN interface along the a axis, whereas the strain is roughly completely relaxed along the c axis. We examine the partial relaxation of the strain along the a axis when the layer thickness is varied.

Self-Organized GaN/AlN Superlattice Nanocolumn Crystals Grown by RF-MBE

ABSTRACTGaN nanocolumns including a GaN/AlN superlattice (SL) region were grown by rf-plasma assisted molecular beam epitaxy. The photoluminescence (PL) peak intensity of the GaN/AlN SL nanocolumns was 300∼500 times stronger than that of conventional GaN continuous films with a dislocation density of 3∼5×109 cm−2 and thickness of 3.75 μm grown by metalorganic chemical vapor deposition (MOCVD). The peak wavelengths of the GaN (10.2 ML)/AlN (15.2 ML) SL and GaN (7.7 ML)/AlN (12.4 ML) SL were 420 and 380 nm, respectively. The theoretically calculated transition wavelength agreed well with experimental values, suggesting that GaN/AlN SL nanocolumns involve a large built-in electrostatic field of about 5.8 MV/cm. The effect of the surface morphology of nanocolumns on the PL intensity was studied using GaN/AlN SL nanocolumns with different surface morphologies but with the same nanocolumn structure. Integrated PL intensity was increased by a factor of 2.2 upon changing the surface morphology from continuous to columnar.

In situ stress measurements during the MOCVD growth of AlN buffer layers on (1 1 1) Si substrates

Stress evolution during MOCVD growth of AlN buffer layers and GaN epilayers on (1 1 1) Si substrates was investigated using in situ wafer curvature measurements in order to understand the impact of growth conditions on thin film stress. AlN layers up to 400 nm thick were deposited over a temperature range of 600–1100°C at growth rates of 0.1–1 nm/s. In all cases, the growth stresses were tensile and the stress value did not change during growth. A sharp drop in the value of the tensile growth stress is observed between 900°C and 800°C from >1 GPa to <0.4 GPa. This drop is due a transition in the structure of the film from an epitaxially oriented crystalline film above 900°C to a highly defective polycrystalline film at 600°C. At a constant temperature however, the tensile stress in the film remained relatively constant with a change in growth rate. The origin of these stress differences and their impact on residual stress in GaN epitaxial layers grown on Si are discussed.

Structural and electrical characterization of a ‐plane GaN grown on a ‐plane SiC

Planar nonpolar () a-plane GaN thin films were grown on () a-plane 6H-SiC substrates via metalorganic chemical vapor deposition by depositing a high temperature AlN buffer layer prior to the epitaxial GaN growth. The orientation of the GaN film and AlN buffer layer directly match that of the SiC substrate, as determined by on- and off-axis X-ray diffraction measurements. The morphological evolution of GaN grown on the AlN buffer layers was investigated using atomic force microscopy. Microstructural characterization of the coalesced a-plane GaN films provided by plan-view transmission electron microscopy revealed threading dislocation and stacking fault densities of ∼3 × 1010 cm−2 and ∼7 × 105 cm−1, respectively. Structural comparisons to a-plane GaN films grown on r-plane sapphire substrates are presented. Si-doped films were grown with a variety of Si/Ga ratios and electrically characterized using Hall effect measurements. A maximum Hall mobility of 109 cm2/Vs was attained at a carrier concentration of 1.8 × 1019 cm−3.

Positron-sensitive vacancy-type centres in the nitrides: 1D-ACAR data

The measurements of one-dimensional angular correlation of the annihilation radiation (1D-ACAR) have been carried out for BN, AlN, and GaN as well as for some related materials that have been used as the reference samples for the analysis of results. The electron–positron ion radii reconstructed by 1D-ACAR for the cation and anion sublattices of the nitrides as well as the estimated average electron density around the positron suggest that: (a) the positron annihilates in the vacancy complexes NGaVN in GaN and NAlVN in AlN, and (b) the cation nearest neighbours are, probably, shifted inward to the VN vacancy where the electron density is sufficiently lower in comparison with that estimated for the bulk.

Theoretical model for polarization superlattices: Energy levels and intersubband transitions

A theoretical model for stress-free polarization superlattices composed of wurtzite semiconductors is described, which exploits Airy function solutions of the Schrödinger equation for a superlattice in the absence of free carriers. The theory is applied to several stress-free structures consisting of AlN barriers and GaN wells. The part played by the crystal-field splitting of the valence band and its dependence on biaxial elastic strain in determining the conduction-band offset is fully taken into account. Electric fields were determined from the spontaneous and nonlinear piezoelectric polarization. Energy bands, transition energies between subbands 1 and 2, and associated eigenfunctions are calculated, and the sensitivity to the conduction-band discontinuity explored. Intersubband radiative and nonradiative transition rates are estimated taking into account the Bloch-function overlap integrals and the role of intervalley scattering. Upper limits to technologically useful subband separations set by conduction-band offsets and the energy of conduction-band valleys are discussed. An upper limit of around 1.5 eV for the energy of the second subband is indicated.

Growth of a-plane InN on r-plane sapphire with a GaN buffer by molecular-beam epitaxy

We report heteroepitaxial growth of InN on r-plane sapphire substrates with an AlN nucleation layer and GaN buffer using plasma-assisted molecular-beam epitaxy. The InN film was identified to be nonpolar (112̄0) a-plane which follows the a-plane GaN buffer. Optical absorption and photoluminescence measurements of this material show that InN has a fundamental band gap of about 0.7 eV, which is also seen for growth on c-plane sapphire. The room-temperature Hall mobility of undoped a-plane InN is around 250 cm2/V s with a carrier concentration around 6×1018 cm−3. We also studied the electrical properties of the a-plane InN as a function of film thickness. In contrast to c-plane InN grown on c-plane sapphire, we did not observe apparent improvement of electrical properties of a-plane InN by growing thicker films.

High-Resolution X-Ray Diffractometry Investigation of Interface Layers in GaN/AlN Structures Grown on Sapphire Substrates

GaInN is an important wide band gap material with applications in short wavelength optoelectronic devices. The GaInN layer is often grown on a sapphire substrate, with low-temperature-deposited AlN and thick GaN used as buffer layers. The growth regime consists of many steps, each of which contributes to the overall properties of the device. The aim of our high-resolution X-ray diffraction experiments, conducted at the Photon Factory (Tsukuba, Japan), was to investigate the structural quality of the AlN buffer layer, which affects the final properties of the device. Reciprocal space mapping was used to study samples (having various layer thicknesses) from each stage of the growth process. Analysis of the experimental data provides parameters such as mosaic block dimensions and orientation, lattice strain distribution, and layer thickness.

The impact of substrate nitridation temperature and buffer design and synthesis on the polarity of GaN epitaxial films

The polarity of GaN epitaxial layers grown on GaN and AlN buffer layers was investigated and found to be dependent on nitridation temperature over the range of 200–700°C. When low temperature (LT), 500°C, GaN buffer layers are used, GaN epitaxial layers grown on 200°C nitrided sapphire have a higher density of N-polar inversion domains. However, a high density of dislocation pits was observed on GaN epitaxial layers based on AFM morphology when GaN epitaxial layers were grown on LT GaN buffer of 700°C nitrided sapphire substrate. With high temperature (HT), 850°C, AlN buffer layers, the density of N-polar inversion domains in GaN epitaxial layers depends on the thickness of AlN buffer layer. The structural quality of Ga-polar GaN epitaxial layer is dramatically improved when LT GaN and HT AlN buffer layers are combined with an optimized annealing time. The measured full-widths at half-maximum of (0 0 0 2) symmetric and (1 0 .4) asymmetric reflections are 68 and 246 arcsec, respectively, for 1.0 μm GaN epitaxial layers. The results presented here can be implemented to produce low dislocation density, single Ga-polar GaN epitaxial layers.

Lattice dynamics of a strained GaN–AlN quantum well structure

A wurtzite GaN (1.5 nm) –AlN (10 nm) superlattice (SL), grown on an AlN buffer layer (BL) and on a 6H-SiC substrate, was studied by X-ray diffraction and Raman spectroscopy. The measurements on the BL allowed one to determine for hexagonal AlN the deformation potentials of the E 2 phonon, which governs its strain-induced frequency shift. The components of the strain tensor of both GaN and AlN layers in the SL were derived.

Growth and characterization of GaN and AlN films on (1 1 1) and (0 0 1) Si substrates

GaN and AlN films were grown on (1 1 1) and (0 0 1) Si substrates by separate admittances of trimethylgallium (or trimethylaluminum) and ammonia (NH 3) at 1000°C. A high temperature (HT) or low temperature (LT) grown AlN thin layer was employed as the buffer layer between HT GaN (or HT AlN) film and Si substrate. Experimental results show that HT AlN and HT GaN films grown on the HT AlN-coated Si substrates exhibit better crystalline quality than those deposited on the LT AlN-coated Si substrates. Transmission electron microscopy (TEM) of the HT GaN/HT AlN buffer layer/(1 1 1)Si samples shows a particular orientation relationship between the (0 0 0 1) planes of GaN film and the (1 1 1) planes of Si substrate. High quality HT GaN films were achieved on (1 1 1) Si substrates using a 200 Å thick HT AlN buffer layer. Room temperature photoluminescence spectra of the high quality HT GaN films show strong near band edge luminescence at 3.41 eV with an emission linewidth of ∼110 meV and weak yellow luminescence.

Interfacial and defect structures in multilayered GaN/AlN films

A structural assessment of various interfaces formed between successive GaN/AlNlayers epitaxially grown on (0001) sapphire, as well as between n-type andp-type GaN is presented, using transmission and high-resolution electronmicroscopy. The structure of the interfaces between the thick GaN and thethin AlN interlayers (ILs) is investigated in terms of misfit difference andelastic fit of the two crystal lattices. Dense threading dislocations areobserved to originate from the substrate/buffer layer interface and theirinteraction with the successive AlN ILs is studied. Furthermore, basal inversiondomain boundaries localized in the n-GaN/p-GaN interface are evaluated.