A. Pelzmann

Dry etching of GaN substrates for high-quality homoepitaxy

Chemically assisted ion-beam etching (CAIBE) was used to remove subsurface damage from polished GaN bulk substrates prior to growth. Subsequently, GaN layers were deposited by metalorganic vapor phase epitaxy. Only the CAIBE-treated areas reveal a mirror-like surface without trenches, scratches, or holes. A dramatic increase of crystal quality is determined by low-temperature cathodoluminescence (CL). Compared to not CAIBE-treated material, the CL intensity is improved by a factor of 1000 and the linewidth is ten times narrower.

Luminescence Related to Stacking Faults in Heterepitaxially Grown Wurtzite GaN

We correlate structure analyzed by transmission electron microscopy with photo- and cathodoluminescence studies of GaN/Al 2 O 3 (0001) and GaN/SiC(0001) and show that an additional UV line at 364nm/3.4eV can be connected to the occurrence of stacking faults. We explain the occurrence of this line by a model that is based on the concept of excitons bound to stacking faults that form a quantum well of cubic material in the wurtzite lattice of the layer material. The model is in reasonable agreement with the experimental observations.

High Quality Homoepitaxial GaN Grown by Molecular Beam Epitaxy with NH3 on Surface Cracking.

Epitaxial GaN films have been grown on GaN single-crystal substrates, using on surface cracked ammonia as nitrogen precursor for molecular beam epitaxy. With this approach excellent optical and structural properties are achieved. Low-temperature photoluminescence shows well-resolved excitonic lines with record low linewidths as narrow as 0.5 meV. The transitions are attributed to excitons bound to neutral donors ((D°, X)1 at 3.4709 eV and (D°, X)2 at 3.4718 eV) and to a neutral acceptor ((A°, X) at 3.4663 eV). In addition, free exciton lines are observed at 3.4785 eV, 3.4832 eV, and 3.499 eV for excitons A, B, and C, respectively.

Fundamentals, Material Properties and Device Performances in GaN MBE using On-Surface Cracking of Ammonia

Ammonia is investigated as nitrogen precursor for molecular beam epitaxy of group III nitrides. With the particular on-surface cracking approach, NH 3 is dissociated directly on the growing surface. By this technique, molecular beam epitaxy becomes a serious competitor to metal organic vapor phase epitaxy. Thermodynamic calculations as well as experimental results reveal insights into the growth mechanisms and its differences to the conventional plasma approach. With this knowledge, homoepitaxially GaN can be grown with record linewidths of 0.5 meV in photoluminescence (4 K). GaN layers on c-plane sapphire also reveal reasonable material properties (photoluminescence linewidth 5 meV, n ≈ 10 17 cm −3 , μ ≈ 220 cm 2 /Vs). Beside GaN growth, p- and n-doping of GaN as well as the growth of ternary nitrides are discussed. Using the presented ammonia approach UV-LEDs emitting at 370 nm with linewidths as narrow as 12 nm have been achieved.

Optical activation and diffusivity of ion-implanted Zn acceptors in GaN under high-pressure, high-temperature annealing

Optical activation of Zn ions implanted to an epitaxial film of GaN was performed by means of sample annealing at N2 overpressure up to 1.6 GPa. By applying pressure we avoided GaN decomposition and could increase the annealing temperature up to 1550 °C versus a limit of 1000–1100 °C at ambient pressure. The Zn-acceptor-related photoluminescence (PL) intensity in implanted samples is maximized by annealing above 1350 °C after which the Zn PL intensity exceeds epitaxially doped GaN:Zn with comparable Zn concentration by factor 15. High-pressure annealing causes a significant diffusion of implanted Zn atoms in GaN films. It is also possible to diffuse Zn from the external source into the implanted/unimplanted layers. High dislocation densities strongly accelarate the Zn diffusion.

Photoluminescence study on GaN homoepitaxial layers grown by molecular beam epitaxy

GaN epitaxial layers on GaN single crystals were grown using molecular beam epitaxy with an NH 3 source. The deposited layers were examined by high resolution x-ray diffraction and photoluminescence (PL) spectroscopy. We observed strong and extremely narrow (half-widths of 0.5 meV) lines related to the bound excitons. In the higher energy range we observed three strong lines. Two of them are commonly attributed to free exciton transitions A (3.4785 eV) and B (3.483 eV). Their energetic positions are characteristic of strain-free GaN material.

On Surface Cracking of Ammonia for MBE Growth of GaN

With the use of ammonia as nitrogen precursor for Group III-Nitrides in an on surface cracking (OSC) approach, MBE becomes a serious competitor to MOVPE. Homoepitaxial GaN exhibits record linewidths of 0.5 meV in PL (4K), whereas GaN grown on c-plane sapphire also reveals reasonable material properties (PL linewidth ≈ 5 meV, n ≈ 10 17 cm -3 , μ 220 cm 2 /Vs). Beside results on hetero- and homoepitaxial growth of GaN, insights into the growth mechanisms are presented. The growth of ternary nitrides is discussed, p- and n-doping as well as first LED results are reported.

NH3 as Nitrogen Source in MBE growth of GaN

The authors report on the growth of GaN in GSMBE using NH{sub 3} as nitrogen source. Special focus will be on the NH{sub 3} cracking, where they applied an On Surface Cracking technique (OSC). Using OSC they achieve photoluminescence linewidths as narrow as 5.5 meV (5K) and mobilities of 220 cm{sup 2}/Vs at room temperature.

Microstructure and growth morphology as related to electro-optical properties of heteroepitaxial wurtzite GaN on sapphire (0001) substrates

We investigate the microstructural development during growth and the related electro-optical properties of gallium nitride (GaN) films deposited on (0001) sapphire substrates by gas source molecular beam epitaxy. We use transmission electron microscopy, atomic force microscopy, scanning tunnelling microscopy, spectrally resolved cathodoluminescence (CL) mapping and photoluminescence (PL). We report on specimens exhibiting UV luminescence (band-to-band transition at 358 nm/3.46 eV) in CL/PL and an additional strong yellow luminescence (Gaussian shaped CL peaks at around 528 nm/2.35 eV). The specimens show a surface topology with facetted hexagonal islands with a width of 1–2 μm at a height of 50 nm. A correlation with spectrally resolved CL images shows: the yellow luminescence is homogeneously distributed over the whole of the specimens as are the pure screw dislocations with b→=〈0001〉, while the UV luminescence is confined to troughs between adjacent hexagonal islands where edge type dislocations with b→=1/3〈2¯110〉) or b→=1/3〈21¯1¯3〉) form small angle boundaries. These dislocations do favour or at least do not impair UV luminescence. Formation mechanisms for the different defect types and possibilities for their reduction are briefly considered.

Ion channeling studies of GaN layers on c-oriented sapphire

Abstract The successful growth of epitaxial GaN films by metal organic vapor phase epitaxy (MOVPE) or gas source molecular beam epitaxy (GSMBE) has opened up new applications in short wavelength photonic devices for displays and optical data storage systems. The large lattice mismatch of 14% between GaN and sapphire, usually used as the substrate, and the different thermal expansion coefficients generally lead to high densities of structural defects. In this paper we investigate the defect structure of MOVPE- and GSMBE-grown GaN layers on sapphire by Rutherford backscattering and ion channeling measurements. In addition to axial c-axis channeling, which reveals χmin values as low as 1.2%, channeling measurements along the (2 1 1 0) and (1 1 0 0) crystal planes were performed in order to improve the sensitivity to dechanneling by crystalline defects. Angular scans around the c-axis indicate clearly the hexagonal symmetry of the GaN lattice. Dechanneling results were compared to observations of defects by transmission electron microscopy (TEM). This comparison suggests that the cross-section of dechanneling by edge dislocations is about a factor of 4 larger than the dechanneling cross-section of screw dislocations.