Ion channeling and transmission electron microscopy were used to examine the microstructure of GaN implanted with deuterium (D) at high (>1 at. %) and low (< 0.1 at. %) D concentrations. At high concentrations, bubbles and basal-plane stacking faults were observed. Ion channeling showed the D was disordered relative to the GaN lattice, consistent with precipitation of D2 into bubbles. At low D concentrations, bubbles and stacking faults are absent and ion channeling shows that a large fraction of the D occupies sites near the center of the c-axis channel.

We demonstrate strong ultraviolet (UV) (280-330nm) photoluminescence (PL) emission from multi-quantum-well (MQW) structures consisting of AlGaN active layers fabricated by metal-organic chemical-vapor-deposition (MOCVD). Si-doping is shown to be very effective in order to enhance the PL emission of AlGaN QWs. We found that the optimum values of well thickness and Si-doping concentration of AlxGa1−xN/AlyGa1−yN (x=0.24−0.53, y=0.11) MQW structure for efficient emission were approximately 3nm and 2×1019cm−3, respectively. In addition, the PL intensities of AlGaN, GaN and InGaN quantum well structures are compared. We have found that the PL emission at 77K from a Al0.53Ga0.47N/Al0.11Ga0.89N MQW is as strong as that of InGaN QWs.

High quality, epitaxial growth of AlN and AlxGa1−xN by OMVPE has been demonstrated on single-crystal AlN substrates. Here we report characterization of epitaxial layers on an a-face AlN substrate using Rutherford Backscattering/ion channeling, atomic force microscopy (AFM), x-ray rocking curves, and preliminary electrical characterization. Ion channeling along the [100] axis gives a channeling minimum yield of 1.5% indicating a very high quality epitaxial layer.

The surface morphology of GaN is observed by atomic force microscopy for growth on GaN and AlN buffer layers and as a function of III/V flux ratio. Films are grown on sapphire substrates by molecular beam epitaxy using a radio frequency nitrogen plasma source. Growth using GaN buffer layers leads to N-polar films, with surfaces strongly dependent on the flux conditions used. Flat surfaces can be obtained by growing as Ga-rich as possible, although Ga droplets tend to form. Ga-polar films can be grown on AlN buffer layers, with the surface morphology determined by the conditions of buffer layer deposition as well as the III/V ratio for growth of the GaN layer. Near-stoichiometric buffer layer growth conditions appear to support the flattest surfaces in this case. Three defect types are typically observed in GaN films on AlN buffers, including large and small pits and “loop” defects. It is possible to produce surfaces free from large pit defects by growing thicker films under more Ga-rich conditions. In such cases the surface roughness can be reduced to less than 1 nm RMS.

We have developed a method to modulate the strain state (normally > 4 kbar, tensile) of moderately thick (∼2 μm) GaN based structures grown on 6H-SiC to a range 0 to -2 kbar of compressive stresses by introducing a strain-mediating layer (SML) above the standard high-temperature AlN buffer layer. The strain characteristics of subsequently deposited nitride layers can be modulated by changing the growth parameters of the SML layer. This is achieved by in-situ techiniques during crystal growth without degrading the optical and structural properties of the deposited layers.

Boron was incorporated into GaN in order to determine its limits of solubility, its ability of reducing the lattice constant mismatch with 6H-SiC, as well as its effects on the structural and optical properties of GaN epilayers. BxGa1−xN films were deposited on 6H-SiC (0001) substrates at 950 °C by low pressure MOVPE using diborane, trimethylgallium, and ammonia as precursors. A single phase alloy with x=0.015 was successfully produced at a gas reactant B/Ga ratio of 0.005. Phase separation into pure GaN and BxGa1−xN alloy with x=0.30 was deposited for a B/Ga reactant ratio of 0.01. This is the highest B fraction of the wurtzite structure alloy ever reported. For B/Ga ratio ≥ 0.02, no BxGa1−xN was formed, and the solid solution contained two phases: wurtzite GaN and BN based on the results of Auger and x-ray diffraction. The band edge emission of BxGa1−xN varied from 3.451 eV for x=0 with FWHM of 39.2 meV to 3.465 eV for x=0.015 with FWHM of 35.1 meV. The narrower FWHM indicated that the quality of GaN epilayer was improved with small amount of boron incorporation.

We report the morphological and compositional characteristics of Er-doped GaN grown by MBE on Si(111) substrates and their effect on optical properties. The GaN was grown by molecular beam epitaxy using solid sources (for Ga and Er) and a plasma gas source for N2. The films emit by photoexcitation in the visible and near infrared wavelengths from the Er atomic levels. The morphology of the GaN:Er films was examined by AFM. Composition was determined by SIMS depth profiling that revealed a large Er concentration at 4.5×1021 atoms/cm3 accompanied by a high oxygen impurity concentration.

Selective area growth (SAG) and epitaxial lateral overgrowth (ELO) of GaN using tungsten (W) mask by metalorganic vapor phase epitaxy (MOVPE) and hydride vapor phase epitaxy (HVPE) have been studied. The selectivity of the GaN growth on the W mask as well as the SiO2 mask is excellent for both MOVPE and HVPE. The ELO-GaN layers are successfully obtained by HVPE on the stripe patterns along the <1 00> crystal axis with the W mask as well as the SiO2 mask. There are no voids between the SiO2 mask and the overgrown GaN layer, while there are triangular voids between the W mask and the overgrown layer. The surface of the ELO-GaN layer is quite uniform for both mask materials. In the case of MOVPE, the structures of ELO layers on the W mask are the same as those on the SiO2 mask for the <11 0> and <1 00> stripe patterns. No voids are observed between the W or SiO2 mask and the overgrown GaN layer by using MOVPE.

The epitaxial lateral overgrowth (ELO) process for GaN has been studied using SiC and sapphire substrates. Both MBE and MOVPE growth processes were employed in the study. The use of SiO2 versus SiNx insulator stripes was investigated using window/stripe widths ranging from 20 μm/4 μm to 3 μm/15 μm. GaN film depositions were completed at temperatures ranging from 800 °C to 1120 °C. Characterization experiments included RHEED, TEM, SEM and cathodolumenescence studies. The MBE growth experiments produced polycrystalline GaN over the insulator stripes even at deposition temperatures as high as 990 °C. In contrast, MOVPE growth produced single-crystal GaN stripes with no observable threading dislocations.

We demonstrate a two-step process wherein the lateral epitaxial growth (LEO) of GaN from <100>-oriented stripes is initiated at a low V/III ratio to produce smooth, vertical <110> sidewalls, and where the V/III ratio is subsequently raised in order to increase the lateral growth rate. We find that the formation of the <101> facets is inhibited using this two-step process, and that it is possible to maintain the <110> sidewalls while achieving a large lateral growth rate. The ratio of lateral to vertical growth rate has been increased by up to factor of 2.6 using this approach relative to identical growth conditions without the initiation at low V/III ratio. The effect of lateral growth rate on the structural properties of the stripes is discussed.

Transmission electron microscopy was applied to study defects in laterally overgrown GaN layers, with initial growth on Al2O3 substrates followed by further growth over SiO2 masks. Dislocations found in the overgrown areas show changes in line direction. Most dislocations propagate along c-planes. In the overgrown material planar defects (faulted loops) are present on c-planes and their presence is most probably related to segregation of excess point defects and impurities present in this material. They appear to be initiated by the fast lateral growth. Some dislocations with screw orientation become helical resulting from climb motion.

Formation of voids and also a high dislocation density was observed at the boundaries where two overgrowing fronts meet. Tilt and twist components were observed for these boundaries that were different for different overgrown strips grown in the same crystallographic direction suggesting that the GaN subgrain orientations on the two sides of a SiO2 mask are responsible for the final tilt and twist value.

Epitaxial lateral overgrowth (ELO) of GaN on SiO2-masked (0001) GaN substrates has been investigated by using chloride-based growth chemistries via hydride vapor phase epitaxy (HVPE) and metal organic vapor phase epitaxy (MOVPE). Diethyl gallium chloride, (C2H5)2GaCl, was used in as the MOVPE Ga precursor. The lateral and vertical growth rates as well as the overgrowth morphology of ELO GaN structures are dependent on growth temperature, V/III ratio and the in-plane orientation of the mask opening. A high growth temperature and low V/III ratio increase the lateral growth rate and produce ELO structures with a planar surface to the GaN prisms. High-quality coalesced and planar ELO GaN has been fabricated by both growth chemistries. The use of the diethyl gallium chloride source allows for the benefits of HVPE growth to be realized within the MOVPE growth environment.

The evolution of the topography of GaN stripes as a function of stripe width (2 – 120 μm), fill factor and substrate smoothness has been explored. The spatially resolved optical properties of these structures have been characterized by cathodoluminescence imaging and line scans. Implications from the optical study have been discussed.

GaN films have been grown on 6H-SiC substrates employing a new form of selective lateral epitaxy, namely pendeo-epitaxy. This technique forces regrowth to start exclusively on sidewalls of GaN seed structures. Both discrete pendeo-epitaxial microstructures and coalesced single crystal layers of GaN have been achieved. SEM and TEM analysis are used to evaluate the morphology of the resulting GaN films. Process routes leading to GaN pendeo-epitaxial growth using silicon substrates have also been achieved and the preliminary results are discussed.

Various methods have been used to initiate growth by Molecular Beam Epitaxy (MBE) of GaN on sapphire, or other substrates, but there is always a problem with morphology and with a high defect density which results in the formation of a sub-grain boundary structure. We show that by using, homo-epitaxial growth on properly prepared bulk GaN substrates, combined with high temperature growth, we obtain a significant improvement in surface morphology. Growth at sufficiently high temperature leads to a rapid smoothing of the surface and to almost atomically flat surfaces over relatively large areas. Multi-Quantum Well structures grown on such GaN epitaxial films are dislocation free with abrupt interfaces.

Computational materials science has evolved in recent years into a reliable theory capable of predicting not only idealized materials and device performance properties, but also those that apply to practical engineering developments. The codes run on workstations and even now are fast enough to be useful design tools. A review will be presented of the current status of this rapidly advancing field. Examples of the accuracy of the codes are displayed by comparing the predicted atomic volumes, and cohesive and excess energies of several materials with experiment. As a further demonstration of the methods, the band structures of AlN, GaN, and InN in wurtzite and zinc blende structures will be presented. There are anomalies in the conduction and valence bands of these materials. Some consequences on light emitting and power devices made from these materials will be examined.

The design of optoelectronic devices fabricated from III-nitride materials is aided by knowledge of the refractive index and absorption coefficient of these materials. The optical properties of GaN, AlN and AlGaN grown by MOVPE on sapphire substrates were investigated by means of transmittance and reflectance measurements. Thin (less than 0.5 μm) single crystal films were employed to insure that transmission measurements could be obtained well above the optical band gap. The influence of alloy broadening on the absorption edge was investigated by using a series of AlGaN alloy samples with a range of Al compositions. The optical absorption coefficient above the band gap was obtained for AlGaN having up to 38% Al composition. The refractive index below the band gap was determined for the same series of samples. These properties provide information critical to the optimal design of solar blind detectors or other optoelectronic devices.

It is well known that hydrogen plays a key role in p-type doping of GaN. It is believed that H passivates substitutional Mg during growth by forming a Mgs-N-Hi complex; in subsequent annealing, H is removed, resulting in p-type doping. Several open questions have remained, however, such as experimental evidence for other complexes involving Mg and H and difficulties in accounting for the relatively high-temperature anneal needed to remove H. We present first principles calculations in terms of which we show that the doping process is in fact significantly more complex. In particular, interstitial Mg plays a major role in limiting p-type doping. Overall, several substitutional/interstitial complexes form and can bind H, with vibrational frequencies that account for hitherto unidentified observed lines. We predict that these defects, which limit doping efficiency, can be eliminated by annealing in an atmosphere of H and N prior to the final anneal that removes H.

A detailed photoluminescence (PL) study reveals a striking similarity in local vibrational properties of a defect center in GaN as compared to that for the substitutional OP donor in GaP. This observation could be interpreted as if the center is in fact related to the substitutional oxygen donor in GaN. The deep-level nature experimentally determined for the defect center calls for caution of a commonly referred model that the substitutional oxygen donor is responsible for the residual n-type conductivity in GaN.

Thin films of GaN and its alloy AlGaN are investigated with respect to their properties of the persistent photoconductivity (PPC). In this work, we show that the film-substrate interface plays an important role for the metastable electrical effect. Strongly absorbed bandgap light causes an increase of photoconductivity which is about one order of magnitude higher when the sample is illuminated from the substrate side near the interface than from the growth side. To access the interface properties at the substrate, we use temperature-dependent Hall effect measurements. The smallest PPC effect was observed for the GaN film with the best interface properties grown on SiC.