N-polar Growth Research Articles

Chromatic aberration in homogeneous diameter expansion growth of AlN crystals by the PVT method

AbstractHomogeneous diameter expansion growth is the preferred method for preparing large-sized AlN single crystals using the PVT method. However, chromatic aberration between the expansion and seed regions, as well as localized chromatic aberrations within the expansion region, are commonly observed, but not deeply investigated yet. This paper investigates the homoepitaxial lateral growth of AlN crystals using Al-polar face seeds and analyzes the crystal structure, composition, and optical properties of both the expansion and seed regions. XRD and Raman spectroscopy indicate no significant differences in crystallization quality between the expansion and seed regions. XPS and photoluminescence characterizations reveal that the light yellow color of the seed region is mainly due to $$V_{Al}$$ V Al defects. In contrast, the deeper yellow color in the expansion region is caused by $$V_{Al}$$ V Al –$$O_N$$ O N complex defects. Additionally, etching results exhibit a polarity reversal from the Al-polar to N-polar growth face in localized chromatic aberration region. The bonding angle of approximately $${90}^\circ$$ 90 ∘ at the $$B_{1}$$ B 1 bond (formation by the half-filled orbitals of the Al and N atoms) site, which occurs during the bonding of Al and N atoms, results in the polarity reversal during m-plane lateral growth. This work provides implications for the preparation of large-sized AlN single crystals.

III-Nitride Magnetron Sputter Epitaxy on Si: Controlling Morphology, Crystal Quality, and Polarity Using Al Seed Layers.

Group III-nitride semiconductors have been subject of intensive research, resulting in the maturing of the material system and adoption of III-nitrides in modern optoelectronics and power electronic devices. Defined film polarity is an important aspect of III-nitride epitaxy as the polarity affects the design of electronic devices. Magnetron sputtering is a novel approach for cost-effective epitaxy of III-nitrides nearing the technological maturity needed for device production; therefore, control of film polarity is an important technological milestone. In this study, we show the impact of Al seeding on the AlN/Si interface and resulting changes in crystal quality, film morphology, and polarity of GaN/AlN stacks grown by magnetron sputter epitaxy. X-ray diffraction measurements demonstrate the improvement of the crystal quality of the AlN and subsequently the GaN film by the Al seeding. Nanoscale structural and chemical investigations using scanning transmission electron microscopy reveal the inversion of the AlN film polarity. It is proposed that N-polar growth induced by Al seeding is related to the formation of a polycrystalline oxygen-rich AlN interlayer partially capped by an atomically thin Si-rich layer at the AlN/Si interface. Complementary aqueous KOH etch studies of GaN/AlN stacks demonstrate that purely metal-polar and N-polar layers can be grown on a macroscopic scale by controlling the amount of Al seeding.

N-Polar growth of nitride semiconductors with MOVPE and its applications

All white LEDs that largely contribute to the energy saving as well as high electron mobility transistors (HEMTs) powering the base stations of cellular phone networks are made from only Ga-polar materials epitaxially grown along the c-axis. N-polar material, which has the opposite crystal-direction, is considered having high potential for expanding the application field. In this paper, the present status of N-polar epitaxial growth in our lab. is reviewed. Especially, the growth mechanism of N-polar GaN on the widely used sapphire substrate is investigated in detail by using transmission electron microscopy. Finally, as device applications, solar cells, red LEDs, and inverted HEMT are described.

N-polar AlN nucleation layers grown by hot-wall MOCVD on SiC: Effects of substrate orientation on the polarity, surface morphology and crystal quality

Hot-wall metalorganic vapor phase epitaxy enables a superior quality of group-III nitride epitaxial layers and high electron mobility transistor structures, but has not yet been explored for N-polar growth. In this work, we aim at achieving N-polar AlN nucleation layers (NLs) with optimized properties for subsequent growth of GaN device heterostructures. The effects of substrate orientation on the polarity, surface morphology and crystalline quality of AlN NLs on on-axis C-face SiC (0001̄), C-face SiC (0001̄) off-cut towards the [112̄0] by 4°, and Si-face SiC (0001) are investigated. The results are discussed in view of growth mode evolution with growth temperature and substrate orientation. It is demonstrated that N-polar AlN NLs with step-flow growth mode and 0002 rocking curve widths below 20 arcsec can be achieved on off-axis C-face SiC substrates.

Effect of substrate misorientation on the concentration of impurities and surface morphology of an epitaxial GaN layer on N-polar GaN substrate by MOVPE

This study examines the effect of (0 0 0 −1) GaN substrate misorientation on the residual impurities and surface morphology of N-polar GaN grown by metalorganic vapor-phase epitaxy. Carbon, silicon, and oxygen concentrations decreased with increasing GaN substrate misorientation angle, with the lowest impurity concentration achieved for a misorientation angle of 2° toward the m-axis, with 6 × 1015 cm−3 carbon, 6 × 1015 cm−3 silicon, and 4 × 1017 cm−3 oxygen atoms. The oxygen concentration was measured at a depth of 0.5 μm below the wafer surface, and the oxygen concentration decreased with increasing thickness. The incorporation of carbon and oxygen revealed a strong dependence on the misorientation angle. The step distance height of the steps parallel to the [1 1 −2 0] direction (or perpendicular to the [1 −1 0 0] m-direction) was confirmed to be a double-height layer step. This phenomenon indicated that m-direction steps are stable for N-polar growth in GaN. In cases of large misorientation toward the m-axis in of the GaN substrate it was difficult to control the misorientation perpendicular to the nominal direction leading to a-axis direction by wafer bowing at wafer manufacturing. Therefore, step-bunching was generated for each symmetric m-axis due to an increase in the compound's off-angle, thus causing the surface roughness to become large.

N-polar GaN/AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistor formed on sapphire substrate with minimal step bunching

The metal–insulator–semiconductor (MIS) gate N-polar GaN/AlGaN/GaN high-electron-mobility transistor (HEMT) on a (0001) sapphire substrate, which can be expected to operate with lower on-resistance and more easily work on the pinch-off operation than an N-polar AlGaN/GaN HEMT, was fabricated. For suppressing the step bunching and hillocks peculiar in the N-polar growth, a sapphire substrate with an off-cut angle as small as 0.8° was introduced and an N-polar GaN/AlGaN/GaN HEMT without the step bunching was firstly obtained by optimizing the growth conditions. The previously reported anisotropy of transconductance related to the step was eliminated. The pinch-off operation was also realized. These results indicate that this device is promising.

Origin and effective reduction of inversion domains in aluminum nitride grown by a sublimation method

Owing to the large differences in the chemical properties between Al and N polarities in aluminum nitride (AlN), the choice of the polar direction for crystal growth strongly affects not only the quality but also the shape (facet formation) of the grown crystal. In particular, N-polar (000−1) has been considered to be a more preferable direction than Al-polar (0001) for sublimation growth because compared to Al-polar (0001), N-polar (000−1) exhibits better stability at high growth rate (high supersaturation) conditions and enables easier lateral enlargement of the crystal. However, some critical growth conditions induce polarity inversion and hinder stable N-polar growth. Furthermore, the origin of the polarity inversion in AlN growth by the sublimation method is still unclear. To ensure stable N-polar growth without polarity inversion, the formation mechanism of the inversion domain during AlN sublimation growth must be elucidated. Therefore, herein, we demonstrate homoepitaxial growth on an N-polar seed and carefully investigate the obtained crystal that shows polarity inversion. Annular bright-field scanning transmission electron microscopy reveals that polarity is completely converted to the Al polarity via the formation of a 30nm thick mixed polar layer (MPL) just above the seed. Moreover, three-dimensional atom probe tomography shows the segregation of the oxygen impurities in the MPL with a high concentration of about 3atom%. Finally, by avoiding the incorporation of oxygen impurity into the crystal at the initial stage of the growth, we demonstrate an effective reduction (seven orders of magnitude) of the inversion domain boundary formation.

(Invited) Fabrication of N-Polar (Al,Ga,In)N Heterostructures for Transistor Applications

This presentation will address the growth of N-polar (000-1) (Al,Ga,In)N/GaN heterostructures by metal-organic chemical vapor deposition (MOCVD) with emphasis on transistor applications. While the N-polar growth direction is in particular advantageous for highly scaled devices operating at frequencies of 30GHz and beyond, results for high power transistors working at lower frequencies will be discussed as well. Differences in the MOCVD growth of N-polar in comparison to metal-polar heterostructures will be described. Prerequisites for the fabrication of high quality (Al,Ga,In)N were the use of misoriented substrates in combination with a careful process optimization.

Role of SiC substrate polarity on the growth and properties of bulk AlN single crystals

Two symmetrically nonequivalent silicon carbide (SiC) substrate orientations, (0001) Si-terminated and \((000\overline{1} )\) C-terminated, were used in the physical vapour transport growth of bulk aluminium nitride (AlN) single crystals. The crystals grown on Si-faces always exhibit an Al-polar growth surface. AlN growth on \((000\overline{1} )\) C-terminated surfaces of the SiC substrates was performed to obtain N-polar growth surfaces. An abrupt interface was observed between the AlN crystal and the C-face substrate which is in contrast to the growth on Si-faces where hexagonally shaped SiC hillocks are formed. The growth on C-faces is usually dominated by multi-site nucleation. Applying similar supersaturation conditions that led to step-flow growth on Si-faces to the C-faces resulted in a spiral growth mode, even on highly off-oriented substrates. The obtained broad X-ray diffraction rocking curves of such samples (full-width at half-maximum ≈380 arcsec) indicate the presence of more misfit dislocations and significant misfit stress. In addition, polarity inversion is observed in C-face grown crystals. Though the structural properties of the crystals grown on C-face are inferior to that of the crystals grown on Si-face, the incorporation of unintentional Si impurity was found to be lower (<2 wt%).

Effect of c-plane sapphire substrate miscut angle on indium content of MOVPE-grown N-polar InGaN

Nitrogen-polar (N-polar) InGaN films were grown on a GaN template/c-plane sapphire substrate by metal–organic vapor phase epitaxy (MOVPE). The effects of c-plane sapphire substrate miscut angle on the indium (In) content and crystal properties of N-polar InGaN films were investigated. The In content increased with increasing miscut angle in the vicinal region of less than 1.1°. This tendency is different from that of group-III-polar InGaN growth because of the difference in the atomic arrangement on the terraces and at step edges between these two inverted polar surfaces. In the case of N-polar growth, a spontaneous two-dimensional nucleation on terraces is difficult and the intentional introduction of steps is effective compared with group-III-polar growth. Furthermore, by observing the surface morphologies of GaN templates in view of both macroscopic and microscopic scales, a clear relationship between the macroscopic surface structure of GaN template and the In content of InGaN was revealed.

Understanding and controlling indium incorporation and surface segregation on InxGa1−xN surfaces: Anab initioapproach

The incorporation of In into the technologically relevant $(0001)$ (Ga-polar) and $(000\overline{1})$ (N-polar) surfaces of ${\mathrm{In}}_{0.25}{\mathrm{Ga}}_{0.75}\mathrm{N}$ is investigated using density functional theory. The cases of coherent pseudomorphic growth on GaN and on lattice-matched heterointerfaces are considered. For pseudomorphic growth on GaN, In incorporation into the ${0001}$ surface layers is limited to a tiny growth window corresponding to extreme In-rich growth conditions and at the In-rich/Ga-poor region of the metal chemical potentials. Lattice-matched growth, however, allows for a wider growth window. Surface phase diagrams are constructed as a function of growth conditions and reveal similarities between the two polar growth planes. However, a strong driving force is found for segregation of In atoms to the first III-N layer for Ga-polar growth, but not for N-polar growth. The former was found to be mainly due to chemical effects (stronger Ga-N as compared to In-N bonds), absent in the case of N-polar growth. Furthermore, finite-temperature calculations show that In incorporated into the first III-N layer is stable to $\ensuremath{\approx}150$ K higher temperatures in the N-polar surface than in the Ga-polar surface, indicating that for a given level of In incorporation, higher temperatures can be used for N-polar growth as compared to Ga-polar growth.

Efficiency droop in InGaN/GaN blue light-emitting diodes: Physical mechanisms and remedies

Physical mechanisms causing the efficiency droop in InGaN/GaN blue light-emitting diodes and remedies proposed for droop mitigation are classified and reviewed. Droop mechanisms taken into consideration are Auger recombination, reduced active volume effects, carrier delocalization, and carrier leakage. The latter can in turn be promoted by polarization charges, inefficient hole injection, asymmetry between electron and hole densities and transport properties, lateral current crowding, quantum-well overfly by ballistic electrons, defect-related tunneling, and saturation of radiative recombination. Reviewed droop remedies include increasing the thickness or number of the quantum wells, improving the lateral current uniformity, engineering the quantum barriers (including multi-layer and graded quantum barriers), using insertion or injection layers, engineering the electron-blocking layer (EBL) (including InAlN, graded, polarization-doped, and superlattice EBL), exploiting reversed polarization (by either inverted epitaxy or N-polar growth), and growing along semi- or non-polar orientations. Numerical device simulations of a reference device are used through the paper as a proof of concept for selected mechanisms and remedies.

Photoluminescence and positron annihilation studies on Mg-doped nitrogen-polarity semipolar (101¯1¯) GaN heteroepitaxial layers grown by metalorganic vapor phase epitaxy

The influences of enhanced stacking fault (SF) formation, which is peculiar to nitrogen-(N-) polarity growth and lattice-mismatched semipolar heteroepitaxy, on the electrical properties of (101¯1¯) Mg-doped GaN (GaN:Mg) epilayers were investigated. Although the residual donor concentration was higher than (0001) GaN because of N-polar growth, comparatively low Mg doping (3×1019 cm−3) gave a hole concentration approximately 1.5×1018 cm−3, which was an order of magnitude higher than (0001) GaN:Mg. As the acceptor ionization energy estimated from low temperature photoluminescence was quite similar for (101¯1¯) and (0001) GaN:Mg, the high Mg activation seems to result with the aid of high density SFs. Because the Doppler broadening S parameter for the positron annihilation measurement, which reflects the concentration or size of negatively charged cation vacancies, of (101¯1¯) GaN:Mg was smaller than (0001) case, (101¯1¯) orientation is well suited to Mg-doping.

Electrical and luminescent properties and deep traps spectra of N-polar GaN films

Electrical and luminescent properties of N-polar undoped GaN films grown using low temperature GaN buffers on on-axis and miscut sapphire and on-axis AlN buffers are compared to the properties of Ga-polar films grown on low temperature GaN buffers. It is shown that the concentration of residual donors increases by about an order of magnitude for on-axis N-polar growth and by two orders of magnitude for off-axis growth compared to Ga-polar films. On-axis films for both Ga-polar and N-polar polarities show the presence of n + interfacial layers greatly influencing the apparent electron concentration and mobility deduced from capacitance–voltage C– V measurements. These interfacial layers are much less prominent in the miscut N-polar films. Growth on N-polar greatly increases the concentration of electron traps with activation energy of 0.9 eV possibly related to Ga-interstitials.