AlN Superlattice Research Articles

Strain management and AlN crystal quality improvement with an alternating V/III ratio AlN superlattice

We report the metal-organic chemical vapor deposition growth of high-quality AlN on sapphire enabled by an alternating V/III ratio AlN superlattice. We demonstrated that the insertion of an alternating V/III ratio AlN superlattice facilitates the relaxation of tensile stress during growth and assists the annihilation of threading dislocations. Dislocation inclination was proposed to play a major role in the stress relaxation and dislocation reduction, which not only provides an effective misfit-dislocation component but also increases the dislocation reaction probability. By this method, crack-free 3.25 μm-thick AlN films were grown on flat sapphire substrate (FSS) and nano-patterned sapphire substrate (NPSS) at 1180 °C, and the full width at half maximum of (002)/(102) rocking curves was 204/408 and 152/323 arcsec for such AlN/FSS and AlN/NPSS templates.

Low-temperature/high-temperature AlN superlattice buffer layers for high-quality Al xGa 1−xN on Si (1 1 1)

We present MOVPE-grown, high-quality Al x Ga 1− x N layers with Al content up to x=0.65 on Si (1 1 1) substrates. Crack-free layers with smooth surface and low defect density are obtained with optimized AlN-based seeding and buffer layers. High-temperature AlN seeding layers and (low temperature (LT)/high temperature (HT)) AlN-based superlattices (SLs) as buffer layers are efficient in reducing the dislocation density and in-plane residual strain. The crystalline quality of Al x Ga 1− x N was characterized by high-resolution X-ray diffraction (XRD). With optimized AlN-based seeding and SL buffer layers, best ω-FWHMs of the (0 0 0 2) reflection of 540 and 1400 arcsec for the (1 0 1¯ 0) reflection were achieved for a ∼1-μm-thick Al 0.1Ga 0.9N layer and 1010 and 1560 arcsec for the (0 0 0 2) and (1 0 1¯ 0) reflection of a ∼500-nm-thick Al 0.65Ga 0.35N layer. AFM and FE-SEM measurements were used to study the surface morphology and TEM cross-section measurements to determine the dislocation behaviour. With a high crystalline quality and good optical properties, Al x Ga 1− x N layers can be applied to grow electronic and optoelectronic device structures on silicon substrates in further investigations.

Optically nonlinear effects in intersubband transitions of GaN∕AlN-based superlattice structures

We report optically nonlinear processes related to near-infrared intersubband transitions in short period GaN∕AlN superlattices. The strong piezo- and pyroelectric effects in this material lead to intrinsic asymmetries in the electronic potential of the superlattice, and thus to strong nonlinearities of the optical susceptibility. Because of the large intersubband transition energy of nearly 1eV and the short lifetime of excited electrons in the upper quantum state, these nonlinear effects can be exploited for the fabrication of room temperature operated high-frequency detectors in the telecommunication wavelength range. At the same time, saturation effects due to resonant two-photon absorption could be observed.

Near-infrared intersubband absorption in nonpolar cubic GaN∕AlN superlattices

Optical absorption spectra related to intersubband transitions in molecular beam epitaxially grown nonpolar cubic-GaN∕AlN superlattices were observed in the spectral range of 1.5–2.00μm. The background doping was measured using an electrochemical capacitance-voltage technique and found to be on the order of 1018cm−3. This doping level yields a Fermi energy level slightly above the ground state energy level enabling intersubband transitions to occur. The existence of the intersubband transition is verified in several samples with different well widths. The observed peak position energy of the intersubband transition is compared to those calculated using a transfer matrix method.

Ultrafast hole burning in intersubband absorption lines of GaN∕AlN superlattices

The authors present evidence for a distinct optical phonon progression in the nonlinear intersubband absorption spectra of electrons in a GaN∕AlN superlattice. Femtosecond two-color pump-probe experiments in the near infrared show spectral holes separated by the longitudinal optical (LO) phonon frequency and a homogeneous line broadening of approximately 50meV. The nonlinear bleaching signal decays with a time constant of 160fs due to intersubband scattering of delocalized electrons, followed by a weak picosecond component attributed to the relaxation of electrons from longer-lived localized states.

Electrically adjustable intersubband absorption of a GaN∕AlN superlattice grown on a transistorlike structure

The authors report on electromodulated intersubband (ISB) absorption experiments on AlN∕GaN superlattices (SLs) grown on a transistorlike structure. A sample containing five SL periods shows two distinct absorption peaks related to ISB transitions in the SL and in the two dimensional electron gas located at the interface of the lowest SL barrier and the underlying GaN buffer. The ratio of those two absorption peaks can be adjusted by applying an external field, which influences the overall band structure and, more specifically, the free carrier density in the SL. This is a proof of concept of an on-off electro-optical modulator at 1.5μm.

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.

250 nm AlGaN light-emitting diodes

We report AlGaN deep ultraviolet light-emitting diodes (LEDs) at 250 and 255nm that have short emission wavelengths. For an unpackaged 200×200μm square geometry LED emitting at 255nm, we measured a peak power of 0.57mW at 1000mA of pulsed pump current. For a similar device emitting at 250nm the peak output power of 0.16mW was measured at 300mA of pulsed pump current. Progress is based on the development of high quality AlGaN cladding layers with an Al content up to 72%, which were grown over AlGaN∕AlN superlattice buffer layers on sapphire substrates. These n-Al0.72Ga0.28N layers were doped with Si up to about 1×1018cm−3 and electron mobilities up to 50cm2∕V∙s were estimated. High resolution x-ray diffraction studies gave a narrow (002) rocking curve with full width at half maximum of only 133arcsec.

Phonons in III–V nitrides: Confined phonons and interface phonons

Phonons in III–V nitrides are examined experimentally for dimensionally confined systems and for alloys of InGaN with a view towards understanding the phonon modes of these systems. Results are compared with the predictions of Loudon's model for uniaxial semiconductors. The modes of the InGaN system are compared with those of the AlGaN ternary alloy. The first Raman measurements of interface phonons in binary GaN–AlN superlattices are presented.

Angular dispersion of polar phonons in a hexagonal GaN–AlN superlattice

Raman spectra of a wurtzite GaN (6.3 nm)–AlN (5.1 nm) superlattice have been recorded under visible excitation. When the orientation of the phonon wavevector is varied, the angular dispersion of polar phonons from the superlattice is clearly evidenced. These experimental data are found to be in good agreement with the results of a previous calculation based on a dielectric continuum model, taking into account the strain of the two types of layers, which predicts dispersive interface and quasi-confined modes.

Inelastic Light Scattering by Phonons in Hexagonal GaN-AlN Nanostructures

Two selected examples have been chosen to illustrate the ability of non-resonant Raman scattering to probe phonons in hexagonal GaN–AlN artificial structures. The angular dispersion of polar phonons is investigated in a long period GaN–AlN superlattice and compared with the results of calculations based on a dielectric continuum model. On the other hand, the Raman signature of the self-assembled GaN quantum dots and of the AlN spacers of a multi-layered structure is used to determine the strain field in the stucture. The dots are shown to be fully strained on the AlN lattice parameter while the spacers exhibit on the average a slight tensile strain.

Phonons in a strained hexagonal GaN–AlN superlattice

A GaN (6.3 nm)-AlN (5.1 nm) superlattice, grown by molecular beam epitaxy on a sapphire substrate and an AlN buffer layer, has been studied by means of micro-Raman spectroscopy. Most of the observed features have been identified and assigned to optical phonons of the superlattice layers. The average biaxial strain in GaN layers has been deduced from the detailed analysis of the frequency shift observed on the phonon lines. Additional measurements on the bevelled sample clearly suggest the significant increase of this strain for decreasing distances from the interface with the buffer layer.