Showing papers by "Oliver Ambacher published in 1996"

Thermal stability and desorption of Group III nitrides prepared by metal organic chemical vapor deposition

Abstract: We present results on the thermal stability as well as the thermally induced hydrogen, hydrocarbon, and nitrogen–hydrogen effusion from thin films of Group III nitrides prepared by low‐pressure chemical vapor deposition from organometallic precursors. We have deposited amorphous, polycrystalline, and epitaxial InN, GaN, and AIN films on (0001) Al2O3 substrates using the chemical reaction of azido[bis(3‐dimethylamino)propyl]indium, triethylgallium, and tritertiarybutylaluminium with ammonia. The substrate temperature was varied between 400 °C and 1100 °C. The elemental composition, in particular its dependence on the growth temperature, was investigated by elastic recoil detection analysis (ERDA). The influence of growth rate and crystallite size on the concentration of surface adsorbed hydrocarbons and carbon oxides is determined by a combination of ERDA and thermal desorption measurements. In addition, the stability of and the nitrogen flux from the InN, GaN, and AIN surfaces was determined by x‐ray diffraction and thermal decomposition experiments.

Influence of substrate‐induced biaxial compressive stress on the optical properties of thin GaN films

Abstract: The influence of biaxial stress on the optical properties of thin GaN films is studied by x‐ray diffraction and Raman and photoluminescence spectroscopy. The stress is caused by differences in the thermal expansion coefficient and lattice mismatch between the film and c‐plane sapphire substrates. In particular, the influence of various thicknesses of AlN buffer layers on the strain in GaN films is studied. GaN/AlN films were deposited by low pressure metal organic chemical vapor deposition using triethylgallium and tritertbutylaluminum and ammonia. We observe a pronounced reduction of strain in the GaN films with increasing buffer thickness: An AlN buffer layer thicker than 200 nm eliminates the stress completely. Estimates of the linear coefficient for the near band gap luminescence shift due to biaxial compressive strain yield a value of 24 meV/GPa.

Optical patterning of GaN films

Abstract: Patterned etching of GaN films was achieved with laser‐induced thermal decomposition. High‐energy laser pulses are used to locally heat the film above 900 °C, causing rapid nitrogen effusion. Excess gallium is then removed by conventional etching. At exposures of 0.4 J/cm2 with 355 nm light, etch rates of 50–70 nm per pulse were obtained. Illumination with an interference grating was used to produce trenches as narrow as 100 nm.

Defect-related optical transitions in GaN.

Abstract: Sub-band-gap absorption of GaN grown by metal-organic chemical vapor deposition on sapphire was investigated by photothermal deflection spectroscopy (PDS), transmission measurements, and the constant photocurrent method (CPM). We determine acceptor binding energies in undoped GaN at 220 and about 720 meV. A comparison between absorption and CPM spectra yields the dependence of the quantum efficiency-mobility-lifetime-product ($\ensuremath{\eta}\ensuremath{\mu}\ensuremath{\tau}$) versus energy and gives relevant information about the excitation mechanisms. CPM spectra show a significantly smaller absorption (up to a factor of $\frac{1}{10}$) in the range between 3.0 and 3.3 eV as compared to PDS. This indicates that the majority of carriers excited with these photon energies have a relatively small $\ensuremath{\eta}\ensuremath{\mu}\ensuremath{\tau}$ product and thus do not contribute to the externally detected photocurrent. We propose that in this energy range the spectrum is dominated by interband absorption in isolated cubic-phase crystallites in the hexagonal matrix and by excitation of electrons from occupied acceptors into the conduction band of the main hexagonal crystal modification ($h$-GaN). Temperature-dependent photoluminescence measurements, excited with energies below and above the direct band gap of hexagonal GaN, confirm this interpretation and can be correlated with the subgap absorption detected by PDS. Transient photocurrent measurements show a persistent photoconductivity, which can also be explained by the existence of isolated cubic-phase inclusions.

Triazidogallium and derivatives : new precursors to thin films and nanoparticles of gan

Abstract: The synthesis and properties of [Ga(N3)3]∞ (1) and the related derivatives [(Do)nGa(N3)3] (2a-d: Do = THF, NEt3, NMe3, quinuclidine, n = 1; 2e: Do = pyridine; n = 3), Li[(CH3)Ga(N3)3] (3), [(N3)2Ga{(CH2)3NMe2}] (4), [Cp(CO)2-Fe-Ga(N3)2(py)] (5), and [(CO)4Co-Ga(N3)2(NMe3)] (6) are reported. Compounds 2e and 4 were characterized by single-crystal X-ray diffraction. The deposition of polycrystalline GaN thin films from 2a-e by solution methods (spin-on pyrolysis) and the solid-state pyrolysis of 1 to give GaN nanoparticles are described.

Triazidogallium and derivatives : new precursors to thin films and nanoparticles of gan

Abstract: The synthesis and properties of [Ga(N3)3]∞ (1) and the related derivatives [(Do)nGa(N3)3] (2a-d: Do = THF, NEt3, NMe3, quinuclidine, n = 1; 2e: Do = pyridine; n = 3), Li[(CH3)Ga(N3)3] (3), [(N3)2Ga{(CH2)3NMe2}] (4), [Cp(CO)2-Fe-Ga(N3)2(py)] (5), and [(CO)4Co-Ga(N3)2(NMe3)] (6) are reported. Compounds 2e and 4 were characterized by single-crystal X-ray diffraction. The deposition of polycrystalline GaN thin films from 2a-e by solution methods (spin-on pyrolysis) and the solid-state pyrolysis of 1 to give GaN nanoparticles are described.

Low-Temperature OMCVD of InN Thin Films from the Novel Air-Stable Single-Molecule Precursor Azido{bis[(3-dimethylamino)propyl]}indium, (N3)In[(CH2)3NMe2]2

Abstract: Hexagonal InN and InGaN are interesting wide-bandgap semiconductors for optoelectronic applications. The first example of an air-stable, fairly volatile, and low melting precursor, (N3)In[(CH2)3NMe2]2 (1), to deposit crystalline InN at low temperatures (350−450 °C) is presented.

X‐ray diffraction study of gallium nitride grown by MOCVD

Abstract: An X-ray diffraction study of gallium nitride grown on the c-plane (0001) and r-plane (0112) of sapphire substrates by metal organic chemical vapour deposition using triethylgallium and ammonia was performed. By measuring asymmetric reflections with a ω-2θ two-circle diffractometer, lattice constants of α-GaN could be resolved for both substrate orientations and the epitaxial relationships could be confirmed. Additionally, the existence of β-GaN on the r-plane sapphire substrate could be deterimined. Pole figure measurements show different qualities of in-plane orientation of the α-GaN layers for the two substrate orientations.

Metalorganic vapor phase epitaxial growth of GaInN/GaN hetero structures and quantum wells

Abstract: GaInN/GaN heterostructures and quantum wells have been grown by low pressure metalorganic vapor phase epitaxy on sapphire using an AIN nucleation layer. We found a significant In incorporation only for growth temperatures of 700°C, although still very high In/Ga ratios in the gas phase had to be adjusted. The In content could be increased by reducing the H2/N2 flow ratio in the main carrier gas. GaInN layers typically show two lines in low temperature photoluminescence which are identified as excitonic-like (high energy peak) and impurity-related-like (low energy) by time-resolved spectroscopy. Quantum wells with a thickness between 8 and 0.5 nm showed only one emission line. The peak of the thinnest wells shows excitonic-like behaviour, whereas we found a smooth transition to an impurity-related-like type with increasing thickness. By scanning transmission electron microscopy studies we found indications for composition fluctuations in these thicker quantum wells which may cause localization effects for the excitons and thus be responsible for the observed optical spectra.

Yellow Luminescence and Hydrocarbon Contamination in MOVPE‐Grown GaN

Abstract: Photoluminescence (PL) and cathodoluminescence (CL) measurements were performed on GaN films grown by metal organic vapor phase epitaxy (MOVPE) in order to study the origin of the yellow luminescence (YL) in this material. The development of the YL was followed as a function of deposition temperature between 650 and 1100 °C, in relation with the microstructure and the contamination in carbon, oxygen, and hydrogen determined by thermal effusion measurements. The YL was found to emanate from the grain boundaries, where high concentrations of hydrocarbons are present. This contamination could be removed upon thermal annealing up to 400 °C, whereas the YL remained constant. Thermal treatments up to 1000 °C increased the YL by a factor of 1.5. Since GaN decomposes above (850 ± 20) °C by evaporation of nitrogen molecules, the participation of nitrogen vacancies in the YL mechanism is suggested.

PEMBE-Growth of Gallium Nitride on (0001) Sapphire: A comparison to MOCVD grown GaN

Abstract: Thin films of GaN on c-plane sapphire were grown by plasma-enhanced molecular beam epitaxy (PEMBE). The influence of different growth conditions on the quality of the epitaxial layers was studied by x-ray diffraction (XRD), atomic force microscopy (AFM) and Hall measurements. For low deposition temperatures, the growth of a thin buffer layer of AlN results in a decrease of the XRD rocking curve full width at half maximum (FWHM) but also in poorer quality in electronic and optical properties. Samples of 3μm thickness with 570 arcsec FWHM in the XRD rocking curve, a near band gap PL-emission FWHM at 5 K of 7 meV, charge carrier densities of ne = 2 × 1017 cm−3, and Hall mobilities of 270 cm2/Vs at 300 K were grown without a buffer layer. A comparison of the morphology and XRD rocking curves with those of GaN films deposited by metalorganic chemical vapour deposition (MOCVD) shows that the two methods have different growth mechanisms.

Spin Resonance Investigations of GaN and AlGaN

Abstract: Electron Spin Resonance (ESR) and related methods have been used to study defects in GaN and in Al x Ga 1-x N ternary alloys. In particular, Light-induced ESR of a thick MOCVD grown layer of GaN shows that the ESR signature of the deep defect appears for excitation energies > 2.6 eV and saturates above 2.7 eV. This information provides a direct measure of the energetic level position of the diamagnetic to paramagnetic transition of this center. Furthermore, standard ESR investigations of MBE-grown layers of Al x Ga 1-x N alloys were performed with emphasis on the effective-mass donor resonance. Composition and temperature-dependent measurements of the resonance position are presented.

Defect transitions in GaN between 3.0 and 3.4 eV

Abstract: We have studied optical transitions (absorption and luminescence) in nominally undoped and Mg-doped GaN deposited by MOCVD and MBE. In the range between 3.0 and 3.4 eV, a variety of well known low-intensity luminescence lines are observed, whose origin is discussed. In particular, by comparing excitation with subgap versus above-gap laser lines as well as by combining optical subgap absorption with spectrally resolved photoconductivity, we identify localized optical transitions occuring in isolated cubic inclusions in the otherwise hexagonal GaN epitaxial layers. Implications of these strucural defects for photocurrent transients are also presented.

Improved Aluminum Nitride Thin Films Grown by MOCVD from Tritertiarybutylaluminum and Ammonia

Abstract: AIN thin films were grown on c-plane sapphire by metalorganic chemical vapor deposition from tritertiarybutylaluminum and ammonia at 1050°C. These films exhibit a full width at half maximum of the 002 X-ray rocking curve below 200 arcsec indicating high epitaxial quality. By measuring asymmetric reflections, a structural disorder of the lattice mainly due to edge dislocations can be observed. For further investigations, atomic force microscopy and photothermal deflection spectroscopy were performed. In order to study the effect of increasing A1N layer thickness on the optical and structural properties of GaN in an AIN/GaN heterostructure, A1N thin films with increasing thickness ranging from 0.02 to 0.36 were used as sublayers for the deposition of 0.75 /xm thick GaN layers. Photoluminescence, micro-Raman and X-ray diffraction measurements confirm the relaxation of biaxial compressive stress in the GaN layers due to different thermal expansion coefficients by increasing A1N layer thickness. The pressure dependence of the band gap shift was determined as 24 meV/GPa for biaxial compressive stress. Our results indicate that the growth of A1N with metal organic chemical vapor deposition from tritertiarybutylaluminum and ammonia is a promising method for obtaining high quality epitaxial films.