M Mayer

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.

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.

Photocurrent and photoluminescence measurements in the near-band-edge region of 6H GaN

The optical transitions near the band edge in molecular-beam epitaxy grown 6H GaN films on sapphire have been studied by photocurrent (PC) and photoluminescence (PL) measurements. The low-temperature PC spectra show well-resolved free-exciton transitions, which allow us to determine the A, B, and C free-exciton energies and the corresponding direct band gaps. PL measurements were performed to check the assignment of the excitonic transitions and to derive strain-dependent excitonic parameters from the energetic position of the free A exciton transition, which serve as an input to the fit function for the PC spectra. Variation of the sample temperature between 5 and 300 K allows us to extract the temperature dependence of the band gap.

Deep Level Related Yellow Luminescence in P-Type GaN Grown by MBE on (0001) Sapphire

Yellow luminescence (YL) has been studied in GaN:Mg doped with Mg concentrations ranging from 10 to 10 cm by spectral CL (T=5K) and TEM and explained by suggesting that a different mechanism could be responsible for the YL in p-type GaN with respect to that acting in n-type GaN. Transitions at 2.2, 2.8, 3.27, 3.21, and 3.44 eV were found. In addition to the wurtzite phase, TEM showed a different amount of the cubic phase in the samples. Nano tubes with a density of 3x10 cm were also observed by approaching the layer/substrate interface. Besides this, coherent inclusions were found with a diameter in the nm range and a volume fraction of about 1%. The 2.8 eV transition was correlated to a deep level at 600 meV below the conduction band (CB) due to MgGa-VN complexes. The 3.27 eV emission was ascribed to a shallow acceptor at about 170-190 meV above the valence band (VB) due to MgGa. The 2.2 eV yellow band, not present in low doped samples, increased by increasing the Mg concentration. It was ascribed to a transition between a deep donor level at 0.8-1.1 eV below the CB edge due to NGa and the shallow acceptor due to MgGa. This assumption was checked by studying the role of C in Mg compensation. CL spectra from a sample with high C content showed transitions between a C-related 200 meV shallow donor and a deep donor level at about 0.91.1 eV below the CB due to a NGa-VN complex. In our hypothesis this should induce a decrease of the integrated intensity in both the 2.2 and 2.8 eV bands, as actually shown by CL investigations.

Ion Scattering Studies of Defects In Gan Thin Films on C-Oriented Sapphire

Epitaxial GaN films grown by metal organic vapour phase epitaxy (MOVPE) or gas source molecular beam epitaxy (GSMBE) have opened up new applications in short wavelength photonic devices as well as high-power and high-temperature devices. The large lattice mismatch of 14% between GaN and sapphire, which is frequently used as the substrate, and the different thermal expansion coefficients generally lead to high densities of structural defects. We investigate the defect structure of MOVPE- and GSMBE-grown GaN layers on sapphire by Rutherford backscattering and ion channeling measurements. Channeling along the c-axis revealed χ min -values as low as 1.2% even in samples with dislocation densities in the order of 10 9 cm −2 . Channeling measurements along different crystal planes were performed in order to improve the sensitivity to dechanneling by crystalline defects. Angular yield scans around the c-axis indicate clearly the hexagonal symmetry of the GaN lattice. Dechanneling results were combined with transmission electron microscopy investigations (TEM). The results suggest that the dechanneling-cross-section of edge dislocations is about 4 times larger than the dechannelingcross-section of screw dislocations. screw dislocations.