Ryan G. Banal

Initial nucleation of AlN grown directly on sapphire substrates by metal-organic vapor phase epitaxy

AlN layers were grown directly on sapphire (0001) substrates using three different growth sequences based on metal-organic vapor phase epitaxy with an emphasis on initial nucleation processes. These three methods were simultaneous, alternating supply of aluminum and nitrogen sources, and a combination of the two. In all the methods, nucleation was initiated by three-dimensional (3D) islands with a typical diameter of ∼20nm. Enhanced migration by the alternating source supply caused highly 3D AlN ridge structures at the sapphire molecular steps. These ridge structures prevented a flattened AlN surface and, in addition, moderated lattice relaxation, suggesting the importance of controlling the initial nucleation in determining the film’s properties. In fact, the hybridized method, derived from the simultaneous and alternating supply methods, was able to control the initial nucleation, and provided the best film quality; the 600-nm-thick AlN grown by this method had an atomically flat surface free of pits an...

Extremely high internal quantum efficiencies from AlGaN/AlN quantum wells emitting in the deep ultraviolet spectral region

Internal quantum efficiencies (IQEs) as high as 69% were realized at room temperature from AlGaN/AlN quantum wells (QWs) emitting at 247 nm grown by metalorganic vapor phase epitaxy. The extremely high IQEs were achieved by examining the source–supply sequence. QWs fabricated by a continuous source–supply method have longer emission wavelengths (λ) and higher IQEs compared to QWs fabricated by modified migration enhanced epitaxy (MMEE). MMEE is an alternating source–supply method where the NH3 interruption promotes Ga evaporation. Thus, to obtain the same λ, MMEE requires a lower growth temperature than the continuous method, compromising the quality of the AlN and AlGaN layers as well as the IQE of QWs.

Emission mechanisms in Al-rich AlGaN/AlN quantum wells assessed by excitation power dependent photoluminescence spectroscopy

The optical properties of Al-rich AlGaN/AlN quantum wells are assessed by excitation-power-dependent time-integrated (TI) and time-resolved (TR) photoluminescence (PL) measurements. Two excitation sources, an optical parametric oscillator and the 4th harmonics of a Ti:sapphire laser, realize a wide range of excited carrier densities between 1012 and 1021 cm−3. The emission mechanisms change from an exciton to an electron-hole plasma as the excitation power increases. Accordingly, the PL decay time is drastically reduced, and the integrated PL intensities increase in the following order: linearly, super-linearly, linearly again, and sub-linearly. The observed results are well accounted for by rate equations that consider the saturation effect of non-radiative recombination processes. Using both TIPL and TRPL measurements allows the density of non-radiative recombination centers, the internal quantum efficiency, and the radiative recombination coefficient to be reliably extracted.

Homoepitaxy and Photoluminescence Properties of (0001) AlN

AlN layers are homoepitaxially grown on (0001) AlN substrates. The surfaces are atomically smooth, and the X-ray diffraction rocking curves for the symmetric and asymmetric planes indicate narrow line widths in the range of 10–30 arcsec. The oxygen, silicon, and carbon concentrations are below the detection limits of secondary ion mass spectroscopy. Due to these superior structural properties and low impurity concentrations, sharp free and donor-bound excitons dominate the photoluminescence spectra at low temperatures, while free excitons dominate at elevated temperatures.

Deep-ultraviolet light emission properties of nonpolar M-plane AlGaN quantum wells

Deep-ultraviolet (deep-UV) light emissions from nonpolar (10-10) M-plane AlxGa1−xN/AlyGa1−yN multiple quantum wells (MQWs) were studied by photoluminescence spectroscopy. The nonpolar M-plane AlGaN MQWs showed shorter emission wavelength than the polar (0001) C-plane ones, mainly because of the absence of the quantum-confined Stark effect. The deep-UV light emissions from the M-plane AlGaN MQWs showed stronger polarization with electric field E parallel to the c-axis (E||c) than the C-plane ones. The different polarization properties between the M- and C-plane AlGaN MQWs can be explained in terms of in-plane lattice strain and anisotropy of the effective hole mass.

Crack-Free Thick AlN Films Obtained by NH3 Nitridation of Sapphire Substrates

We demonstrate that NH3 nitridation of sapphire substrates effectively suppresses cracks in AlN epilayers. The sapphire nitridation promoted three-dimensional (3D) growth at the initial stage, in contrast to the 2D growth mode on non-nitrided sapphire. The coalescence of 3D columnar grains in the process created voids, which act as strain absorber and thus crack-free thick epilayers were obtained. The control of nitridation period is also found important. The optimum nitridation period realized an atomically-smooth epilayer with superior structural quality. On the other hand, the least nitrided sapphire exhibited high twist mosaic of the grains which complicate the epilayer smoothing process, and the longer nitridation promoted slow recovery of a smooth epilayer.

Co-existence of a few and sub micron inhomogeneities in Al-rich AlGaN/AlN quantum wells

Inhomogeneity in Al-rich AlGaN/AlN quantum wells is directly observed using our custom-built confocal microscopy photoluminescence (μ-PL) apparatus with a reflective system. The μ-PL system can reach the AlN bandgap in the deep ultra-violet spectral range with a spatial resolution of 1.8 μm. In addition, cathodoluminescence (CL) measurements with a higher spatial resolution of about 100 nm are performed. A comparison of the μ-PL and CL measurements reveals that inhomogeneities, which have different spatial distributions of a few- and sub-micron scales that are superimposed, play key roles in determining the optical properties.

Structural properties and transfer characteristics of sputter deposition AlN and atomic layer deposition Al2O3 bilayer gate materials for H-terminated diamond field effect transistors

Significant improvements in electrical properties are achieved from AlN/Al2O3 stack gate H-terminated diamond metal-insulator-semiconductor field-effect transistors (MISFETs) upon improving the structural quality of an AlN insulating layer. The 5-nm-thick Al2O3 layer and 175-nm-thick AlN film are successively deposited by atomic layer deposition and sputter deposition techniques, respectively, on a (100) H-diamond epitaxial layer substrate. The AlN layer exhibits a poly-crystalline structure with the hexagonal wurtzite phase. The crystallite growth proceeds along the c-axis direction and perpendicular to the substrate surface, resulting in a columnar grain structure with an average grain size of around ∼40 nm. The MIS diode fabricated provides a leak current density as low as ∼10−5 A/cm2 at gate voltage bias in the range of −8 V and +4 V. The MISFET fabricated shows normally off enhancement mode transfer characteristic. The drain-source current maximum, threshold voltage, and maximum extrinsic conductance...

Heteroepitaxy between wurtzite and corundum materials

Heteroepitaxy of wurtzite semiconductors on corundum substrates is widely used in modern optoelectronic devices, because both crystals belong to the same hexagonal close-packed system. However, the constituent atoms in the wurtzite structure align in an ideal hexagon within the (0001) plane, whereas those in the corundum structure are displaced due to empty octahedral sites. Herein, we demonstrate that this atomic arrangement mismatch at the interface generates low-angle grain boundaries in epilayers, and step bunching of corundum substrates with an even number of molecular layers can eliminate the boundaries. Furthermore, we propose that the weakened epitaxial relationship between epilayers and substrates also eliminates low-angle grain boundaries, which may be useful for practical applications.

Effect of off-cut angle of hydrogen-terminated diamond(111) substrate on the quality of AlN towards high-density AlN/diamond(111) interface hole channel

Single-crystal AlN/diamond heterojunction with high-density interface hole channel is successfully obtained by metal-organic vapor phase epitaxy. The AlN layer is epitaxially grown on hydrogen-terminated (H-)diamond(111) substrate. The thermal treatment of diamond substrate just before AlN growth under hydrogen and ammonia mixture environment at 1250 °C leads to surface sheet hole density as high as ∼1.0 × 1014 cm−2 without structural reconstruction of diamond surface. In addition, the use of smaller off-cut angle (0.20 ± 0.25°) H-diamond(111) substrate combined with this treatment enables to obtain single-crystal epitaxial AlN layer, which simultaneously acts as passivation of the surface hole channel with such a high density. The AlN/H-diamond(111) heterojunction reveals type-II staggered energy band configuration with valence band offset of ∼2.0 eV, which is suitable for the fabrication of p-channel field-effect transistor using AlN-gate-insulator/diamond heterojunction. These results are promising for...