In this paper, the growth of epitaxial GaN layers on c-plane and a-plane bulk AIN substrates by metalorganic vapor phase epitaxy is reported. The AlN boules were grown by the sublimationrecondensation technique. Single crystal GaN films grown on the c-plane orientation replicate the substrate orientation. However the surface of the epilayer had a high density of cross-hatch defect lines, presumably caused by mechanical polishing damage. The low temperature PL spectra of these films were dominated by exciton emission at 3.470 eV with a FWHM of 14 meV at 7 K. On the other hand, GaN grown on the a-plane orientation AlN was polycrystalline and the surface was rough with ridge-like facets. The PL from this film showed a dominate peak at 3.406 eV which may originate from defect-bound excitons. The quality of the GaN layers grown on these AIN bulk substrates appeared to be limited by the surface preparation method, which has not been optimized.

The GaN on LGO system is the near perfect template (due to extremely high etch selectivity) for developing a viable thin film/compliant GaN substrate. Herein, we report on our efforts to grow GaN on LGO, including improvement of the microscopic surface morphology using pre-growthpretreatments. We also report on the first transferred thin film GaN substrate grown on LGO, transferred off of LGO, and mounted on GaAs. With this approach, (InAl)GaN alloys can be grown on thin GaN films, implementing a truly “compliant” substrate for the nitride alloy system. In addition, the flexibility of bonding to low cost Si, metal or standard ceramic IC packages is an attractive alternative to SiC and HVPE GaN substrates for optimizing cost verses thermal conductivity concerns. We have demonstratedhigh quality growth of GaN on LGO. X-Ray rocking curves of 145 arc-seconds are obtained with only a 0.28 μm thick film. We present data on the out of plane crystalline quality of GaN/LGO material. Likewise, we show 2 orders of magnitude improvement in residual doping concentration and factors of 4 improvement in electron mobility as compared to the only previously reported electrical data. We show substantial vendor to vendor and intra-vendor LGO material quality variations. We have also quantified the desorption of Ga and Li from the surface of LGO at typical growth temperatures using in situ desorption mass spectroscopy and XPS.

Using off-axis reactive rf sputtering, we have grown extremely smooth, nearly epitaxial, (001) oriented ZnO films on c-axis sapphire substrates. Atomic Force Microscopy was used to determine that these films are extremely smooth, having an rms roughness of only a few tenths of a nanometer. Based on high resolution x-ray diffraction (HXRD), the ZnO is highly oriented, with a rocking curve width of less than 400 arc seconds for the (006) diffraction peak, and only somewhat larger for the (112) reflection. HXRD Phi scans show that the ZnO (112) reflection is rotated in the a-b plane by 30 degrees from the sapphire (113) direction. These two measurements indicate excellent in-plane orientation. We are investigating the use of these buffer layers for subsequent GaN growth. Electrical resistivities of the films exceeded 100 kΩ-cm making ZnO a potential candidate as an insulating buffer layer.

We report on the growth of GaN on NdGaO3 (101) by plasma assisted molecular beam epitaxy (MBE). NdGaO3 (101) is an interesting alternative substrate compared to A12O3 due to its smaller lattice mismatch of +1.2 % to hexagonal (0001) GaN. Using photoluminescence (PL), X-ray diffraction (XRD), reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM) the optical and structural properties of the GaN thin films grown on NdGaO3 are investigated and compared to those grown on c-plane sapphire. The intended epitaxial relationship of [0001] GaN ║ [101] GaN is hard to realise as confirmed by several tilted or even polycrystalline films. However, the layer quality was found to improve considerably, when the NdGaO3 surface showed a (l × 1) instead of other reconstructions.

Lateral epitaxial overgrowth (LEO) of GaN layers has been achieved on 3 μm wide and 7 μm spaced stripe windows contained in SiO2 masks on GaN/AIN/6H-SiC(0001) substrates via organometallic vapor phase epitaxy (OMVPE). The extent and microstructural characteristics of lateral overgrowth were a complex function of stripe orientation, growth temperature and triethylgallium (TEG) flow rate. A high density of threading dislocations, originating from the interface of the underlying GaN with the AIN buffer layer, were contained in the GaN grown in the window regions. The overgrowth regions, by contrast, contained a very low density of dislocations. The second lateral epitaxial overgrowth layers were obtained on the first laterally grown layers by the repetition of SiO2 deposition, lithography and lateral epitaxy.

GaN layers were grown by MOCVD on Silicon on Insulator (SOI) substrates in an effort to improve the material quality compared to more traditionally employed sapphire substrates. Their photoluminescence properties are reported and found to exhibit an intense and relatively large PL band around 3.47eV at low temperature (7K). This is about 10meV lower than the PL energy of samples grown on sapphire substrates and suggests the presence of lower strain in the layers which is expected for compliant growth on SOI substrates. The shape of the main PL peak appears to indicate that Silicon diffusion takes place from the substrate during growth. The behavior of the PL spectra is studied as a function of temperature. The GaN films show good overall electrical properties with Hall mobilities at room temperature in the range of 150 to 300cm2/Vs and background carrier concentration from 2.9 to 3.9×1019cm−3.The promising optical and electronic features of these layers could be of great interest for the development of high quality optical and electronic devices.

GaN films have been grown atop SiC intermediate layers on Si(001) and Si(111) substrates using supersonic jet epitaxy (SJE). GaN growth temperatures ranged between 600 °C and 775 °C. Methylsilane (H3SiCH3) was used as the single source precursor for SiC growth and triethylgallium (TEG) and ammonia (NH 3) were the sources for GaN epitaxy. The GaN growth rate was found to depend strongly on substrate orientation, growth temperature, and flux. Structural characterization of the films was done by transmission electron diffraction (TED) and x-ray diffraction (XRD). Growth of GaN on SiC(002) produces a cubic or mixed phase of cubic and wurtzite depending on growth conditions. Growth on SiC(111) produces predominantly wurtzite GaN(0002). Minimum rocking curve widths for GaN(0002) on SiC/Si(111) and GaN(002) on SiC/Si(001) are 0.6° and 1.5°, respectively. Cross Sectional Transmission Electron Microscopy (XTEM) was also performed.

AIN films were grown on Si< 100 >, using unskimmed pulsed supersonic jets of ammonia and trimethylaluminum (TMA). By seeding the ammonia gas in hydrogen or helium, several different energies of the N precursor were used to examine the effect of N kinetic energy on the growth rate of AIN. The energy of the Al precursor, TMA, was 130 meV in all cases. The highest growth rate (0.115 μm/hr) was achieved with the high energy ammonia jet. The role of number density on film growth is discussed.

A UHV-compatible nitrogen arcjet suitable for the growth of III-nitrides by molecular beam epitaxy is described and characterized. The arcjet operates at powers between 10W and 300W (the highest power used for these studies); typical nitrogen flows range between 5sccm and 100sccm. Optical emission spectra show the presence of activated atomic (N*) and molecular (N2*) nitrogen. A collisional radiative equilibrium model has been employed to provide insight into the excitation state of the active nitrogen. These results indicate that the arcjet is capable of supplying atomic nitrogen fluxes consistent with growth rates on the order of several monolayers per second. Langmuir probe measurements conducted near the position of the sample holder in the MBE chamber show the charged particle flux density is very low. The arcjet operates over a large powerpressure parameter space, and properties of the arc can be systematically “tuned” to provide a source suitable for selected-energy-epitaxy.

Homoepitaxial growth of GaN on MOCVD-grown GaN/AlN/6H-SiC substrates was investigated using NH3-seeded supersonic molecular beams and an effusive Ga source. Ga-limited growth is observed at 730 and 770°C for incident Ga fluxes ≤ 1.2×1015 cm−2 s−1 using a 0.25 eV NH3 beam. A Ga incorporation efficiency of 20–25% is observed under these conditions. Increasing NH3 kinetic energy in the 0.25 to 0.61 eV range results in a modest increase in the GaN growth rate which we ascribe to an enhancement in NH3 reactivity. A concomitant increase in surface roughness is observed with increasing GaN growth rate.

Thin films of nitride semiconductors are usually grown by means requiring high substrate temperatures. Deposition techniques providing higher kinetic energies of incident species offer an alternative route which might allow growth of good quality films at lower temperatures. Pulsed Laser Deposition can provide higher kinetic energies than most thin film growth methods. However, III-nitride thin films grown by PLD are often nitrogen deficient. We have been able to obtain good stoichiometry for aluminum nitride films even at room temperature by providing atomic nitrogen at low (thermal) energies during growth. Very good orientation can be obtained on (001) sapphire substrates at moderate temperatures (∼ 500 C). AIN films were grown from either AIN or Al targets. We also report on preliminary work by the same method with GaN film growth from a liquid Ga target.

We report high quality epitaxial growth of GaN film by pulsed laser deposition technique. In this method, a KrF pulsed excimer laser was used for ablation of a polycrystalline, stoichiometric GaN target. The ablated material was deposited on a substrate kept at a distance of ∼ 7 cm from the target surface and in an NH3 background pressure of 10−5 Torr and temperature of 750°C. The films (∼0.5 μm thick) grown on AIN buffered sapphire showed a x-ray diffraction rocking curve FWHM of 4–6 arc minutes. The ion channeling minimum yield in the surface region was ∼3% indicating a high degree of crystallinity. The optical band gap was found to be 3.4 eV. The epitaxial films were shiny, and the surface RMS roughness was ∼ 5–15 nm. The electrical resistivity of these films was in the range of 10−2–102 Ω-cm with a mobility in excess of 60 cm2V-1s−1 and carrier concentration of 1017–1019cm−3.

Wurtzite InGaN/GaN and AlGaN/GaN heterostructures grown on sapphire by metal organic vapor phase epitaxy were studied using scanning transmission electron microscopy (STEM), cathodoluminescence (CL) combined with secondary electron (SE) imaging, high resolution x-ray diffractometry (HRXRD), and atomic force microscopy (AFM).

SE imaging and AFM were used to study the surface morphology. The results indicate the presence of the following structural defects on the surface of InGaN/GaN heterostructures: hexagonal mesa-like structures, hexagonal pyramids and micropipes, while the surface of the AlGaN/GaN heterostructures are mirror-like smooth. The local optical properties of defects and defect free regions were studied using spatially resolved CL at low temperature. In addition, the dependence of the optical properties of both sorts of heterostructures on the quantum well width or chemical composition of ternary materials was investigated. The structural properties of the heterostructures were studied by STEM and HRXRD. Convergent beam electron diffraction (CBED) and corresponding simulations, convergent beam imaging (CBIM), and high resolution x-ray diffraction (HRXRD) were used to study the strained layers. Dislocations and interface properties were characterized using bright-field imaging, while the chemical compositions fluctuations were analyzed by Z-contrast imaging and energy dispersive x-ray microanalysis (EDX).

We report studies of the surface structure of MBE-grown GaN layers on sapphire (0001) and 6H-(0001) SiC substrates. A different set of reconstructions is observed for nitrogen-face and gallium-face layers. The gallium-face has so far only been grown on MOCVD GaN/ sapphire substrates, while the nitrogen-face has been obtained on SiC and bare sapphire substrates.

A GaN/AlxGalxN multi-quantum well test structure with Al concentrations 0 ≤ xAl ≤ 1 was utilized to investigate the growth of AlxGal–xN barrier layers deposited by metal organic chemical vapor deposition (MOCVD). A transition from a two dimensional (2D) to a three dimensional (3D) growth mode was observed in AlxGa1–xN barriers with XAl ≥ 0.75. It is argued that the transition occurs because of growth at temperatures that are low compared with the materials melting points Tmelt. The resulting rough AlxGa1–xN surfaces can be planarized by overgrowth with GaN. Quantitative high resolution electron microscopy (HREM) was applied to measure composition and strain profiles across the GaN/AlxGa1−xN stacks at an atomic level. The measurements reveal a substantial variation of lattice constants at the AlxGa1−xN/GaN interfaces that is attributed to an Al accumulation.

Transmission electron microscopy has been applied to study defects in epitaxial doped and undoped GaN layers grown by MOCVD on sapphire and SiC substrates. Samples with InGaN/GaN and AlGaN/InGaN heterostructures have also been investigated. The results of this study show that incorporation of “foreign” atoms increases formation of nano-tubes and pinholes. The highest density of these defects was formed close to the interface with sapphire where oxygen outdiffusion might be expected, or in the subsurface area in the samples where oxygen was added deliberately. Addition of In (or Al) at QW's also leads to pinhole formation. Increased In fraction or a larger number of QW's results in a greater density of pinholes and more surface roughness. Many of the “hollow” nanotube defects are terminated during growth.

The effect of p- and n-type doping (using Mg and Si, respectively) in the microstructure of GaN, grown epitaxially on (0001)Al2O3 and (111)Si, is studied with X-ray absorption measurements at the N-K-edge. A distortion in the local microstructure around the N atom is detected in the undoped and the Mg doped samples. The N atom is 4-fold coordinated with n Ga atoms in the expected distance and 4-n atoms at a distance longer by 0.28Å, where 2.9 < n < 3.3. Such a distortion, which is attributed to the inward relaxation and the strong interaction between the Ga atoms surrounding the nitrogen vacancies (VN), does not exist in the Si doped sample (carrier concentration=1.57×1018cm−3) where the formation of VN is suppressed due to the n-type doping. However, in GaN:Si the N atom is undercoordinated with 3.3 nearest neighbors instead of 4. This undercoordination indicates the presence of VGa and/or NGa antisite defects. Finally, from the nearest neigbohr distances the lattice parameters were calculated and it is found that although the a and c vary by about 1.5%, the ratio of the lattice constants, c/a, remains constant and equal to 1.63.

High resolution electron microscopy (HREM) was applied to study atomic structure of stacking faults, grain boundaries and interfaces in III-nitrides epitaxial layers grown by MOVPE on sapphire. Defects formed in GaN epitaxial layers grown by MOVPE were reviewed in comparison with those in MBE grown materials

The microstructure and the lateral epitaxy mechanism of formation of homoepitaxially and selectively grown GaN structures within windows in SiO2 masks have been investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Three types of samples and each of their microstructures as a function of the geometry of the mask pattern and the selective growth parameters have been studied: GaN pyramids, GaN stripes oriented along [1100] and [1120] directions, and GaN continuous layers. Observations via TEM showed in all three types of samples that the laterally overgrown GaN exhibit four-to-six orders of magnitude reduction in the dislocation density compared to the vertically grown GaN films. Owing to the lateral epitaxy, the threading dislocations bend when the growth front changes from vertical to lateral, thus changing their character from being mostly threading dislocations of mixed or edge character, to being basal plane dislocations with lines parallel to the interfacial planes. The underlying GaN provided the crystallographic template for the initial vertical selective growth through the openings (windows) in the SiO2 mask. The GaN structures in these areas had a threading dislocations density of 108-1010 cm−2. Lateral growth of the GaN films over the amorphous SiO2 mask, resulted in a reduction of the dislocation density to <106 cm−2. The primary materials source for both the vertical and lateral growth of the structures was the vapor phase. Essentially no GaN deposited on the SiO2 because of the very low sticking coefficients of Ga and N species on SiO2. Thermal stress relaxation is responsible for the final morphology and microstructure in these selectively grown structures.

Epitaxial GaN grown by MBE has been characterised using the combined techniques of scanning electron microscopy / cathodoluminescence, reflection high energy electron diffraction (RHEED), and conventional transmission electron microscopy. Variations in spatial and spectral distributions of luminescence can arise due to embedded cubic inclusions within the hexagonal GaN matrix. The strong effect of doping on the crystallinity of the GaN deposit is illustrated, as determined by RHEED in a TEM.