GaN ultraviolet (UV) detectors were fabricated on a freestanding GaN substrate with low dislocation density. The resulting dark current density was below 1 nA/cm-2 at -8 V reverse bias, which was about 3 orders of magnitude lower than that of a similar detector made on a sapphire substrate. Moreover, the ideality factor was nearer to unity than the device on a sapphire substrate. In addition, by comparing the GaN-based device to a commonly used Si photodetector, we found that the GaN device had a lower signal-to-noise ratio and greater temperature stability. Therefore, we found a drastic reduction of dark current by using GaN freestanding substrates and so the GaN substrate produced a more effective detector than the sapphire substrate.

The relationship between the optical properties and microstructure of GaN is of great interest due to the important optical and electronic applications of this material. Several different studies have been reported attempting to link the low temperature photoluminescence (PL) peak at ∼3.4 eV to the presence of various microstructural defects. However, no clear systematic studies have been reported establishing such a link for the PL peak observed at ∼3.2 eV. In this paper, we present evidence linking the ∼3.4 eV PL peak to the presence of a thin layer of cubic phase associated with basal plane stacking faults (BSF). This relationship is mainly established by studying a series of ammonothermally-grown GaN bulk crystals. The existence and strength of the ∼3.4 eV peak are found to be related to the I2 type BSF (RI2=1/3<1>) observed in these samples. To investigate the relationship between the ∼3.2 eV peak and structural defects, a series of GaN epilayers grown on either SiC or sapphire (of various off-cut angles) was investigated by TEM and PL spectroscopy. Samples grown on 3.5° off-cut SiC and 5° and 9° off-cut sapphire substrates exhibit PL peaks near ∼3.2 and ∼3.4 eV, which are absent in the on-axis SiC and sapphire cases. TEM shows that the former group of samples has defect configurations consisting of prismatic stacking faults (PSFs) folding back and forth between two different {1120} planes connected by stair rod dislocations, which in turn fold onto to I1 type BSFs again with stair rod dislocations at the fault intersections. The ∼3.2 eV PL peaks are proposed to possibly arise from transitions involving the PSFs and the stair rods associated with their mutual intersections and their intersections with the BSFs. The ∼3.4 eV peak is again attributed to the thin layer of cubic phase associated with the I1 type BSF (three bilayers as opposed to four bilayers for the I2 type BSF).

Highly n-doped InN layers are investigated by means of Raman scattering: a strong mode is evidenced near the frequency of the A1(LO) phonon, despite the high conductivity of the films. This observation is interpreted assuming the breakdown of the wave-vector conservation leading to the decoupling of the plasmon from the phonon. The lineshape of the longitudinal optical mode is simulated using the Lindhard-Mermin dielectric function for various light scattering processes: we found that the charge density fluctuation mechanism is dominant, at least in the visible excitation range.

The homoeptaxial growth of GaN based devices has advantages against the heteroepi-taxial realization on substrates such as sapphire or SiC, since heteroepitaxy implies a lot of problems like lattice mismatch, different thermal expansion coefficients, and needs an extensive optimization of the growth at the heterointerface. In this paper we will discuss GaN based light emitting devices grown by homoepitaxy in comparison to devices grown on sapphire. We will show the differences in device performance, device processing and the influence of the thermal resistivity on the devices.

We investigate the spatial distribution and emission properties of self-assembled GaN/AlN quantum dots. High-resolution transmission electron microscopy reveals near vertical correlation among the GaN dots due to a sufficiently thin AlN spacer layer thickness, which allows strain induced stacking. Scanning electron and atomic force microscopy show lateral coupling due to a surface roughness of ∼ 50–60 nm. Near-field photoluminescence in the illumination mode (both spatially and spectrally resolved) at 10 K revealed emission from individual dots, which exhibits size distribution of GaN dots from localized sites in the stacked nanostructure. Strong spatial localization of the excitons is observed in GaN quantum dots formed at the tip of self-assembled hexagonal pyramid shapes with six [101 1] facets.

Theoretical and experimental results on ultra-fast all-optical switches based on intersubband transitions for Tb/s operation are presented. Designs for engineering intersubband transitions (ISBT) in GaN/AlN quantum wells near communication wavelengths (∼1.55 μm) and for realizing all-optical switches requiring small pulse energies are discussed. Optimized designs show all-optical switching at Tb/s data rates with pulse energies as small as 200 fJ. Experimental realization of narrow line-width ISBT in GaN/AlN superlattices is also demonstrated.

To date, light emission by AlGaN-based heterostructures and LED's operating in the ultraviolet region is far less efficient than emission from longer wavelength structures based on GaInN. We have realized GaN/AlGaN quantum well structures emitting in the 360–320 nm range with peak room-temperature internal efficiencies reaching more than 20 %. From detailed studies of the temperature and excitation power dependence of the efficiency we find that excitons play a crucial role enhancing radiative recombination in such structures. Except for the peak internal efficiency, which reaches 73 % in GaInN/GaN, the overall behavior in GaN/AlGaN and GaInN/GaN is very similar, suggesting that the main difference is the nonradiative recombination mechanism.

We have studied the nonlinear elasticity effects in III-N compounds. Particularly, we have determined the pressure dependences of elastic constants in wurtzite and zinc-blende InN, GaN, and AlN by performing ab-initio calculations in the framework of plane-wave pseudopotential implementation of the density-functional theory. We have found that C11, C12 in zinc-blende phase and C11, C12, C13, C33 in wurtzite phase depend significantly and almost linearly on hydrostatic pressure, for all considered nitrides. Much weaker dependences on pressure have been observed for C44 in both wurtzite and zinc-blende phases. Further, we have examined the influence of pressure dependence of elastic constant on the pressure coefficient of light emission, dEE / dP, in wurtzite and cubic, InGaN/GaN and GaN/AlGaN quantum wells. We show that the pressure dependence of elastic constants results in significant reduction of dEE / dP in nitride quantum wells and essentially improves the agreement between experimental and theoretical values.

InGaN/GaN multiple quantum wells (MQWs) with [0001], <11.2>, and <11.0> orientations have been fabricated by means of the re-growth technique on patterned GaN templates with striped geometry, normal planes of which are (0001) and {11.0}, on sapphire (0001) substrates. It was found that photoluminescence intensity of the {11.2} QW is the strongest among the three QWs, and its internal quantum efficiency was estimated to be as large as about 40% at room temperature. The radiative recombination lifetime of the {11.2} QW was about 0.39 ns at 14 K, which was 3.8 times shorter than that of conventional c-oriented QWs emitting at a similar wavelength. These findings are well explained by the high internal quantum efficiency in the {11.2} QW owing to the suppression of piezoelectric fields.

III-nitride films were grown on the corrugated interface substrate using a metalorganic chemical vapor deposition system to increase the optical power of white LEDs. The patterning of a substrate for enhancing the extraction efficiency was processed using an inductively coupled plasma reactive ion etching system and the surface morphology of the etched sapphire wafer and that of the non-etched one were investigated using an atomic force microscope. The structural and optical properties of GaN on CIS were characterized by a high-resolution x-ray diffraction, transmission electron microscopy, AFM and photoluminescence. The roughness of the etched sapphire wafer was higher than that of the non-etched one. The GaN layer didn't grow locally over the surface of the cone shape pattern. The reason is that (0001) c-plane which is favor for GaN growth doesn't exist on the cone shape patterned region. The lateral growth of the GaN layer that was initially grown on the (0001) c-plane among pattern regions, was enhanced by raising the growth temperature and lowering the reactor pressure, resulting in the smooth surface over the pattern region. The (102) FWHM of GaN layer on the patterned substrate was better than that of GaN on the conventional substrate and no defect was detected at the interface of the cone shape pattern. The optical power of the LED on the patterned substrate was 20% higher than that on the conventional substrate due to the increased extraction efficiency.

The effect of H2-preannealing of sapphire substrate on low-temperature (LT- ) AlN buffer layer deposited by metalorganic vapor phase epitaxy is investigated. Crystallinity of LT-AlN drastically changes with preannealing temperature variation. It is found that H2-preannealing of sapphire substrate is a requisite to get a better quality LT-AlN and as a consequence it leads to growth of better quality GaN epi-layer on it.

The incorporation of Mn into GaMnN epilayers by MOCVD growth was investigated. Samples with high Mn concentrations lead to room temperature ferromagnetism. In addition an absorption band around 1.5 eV was observed. Intensity and linewidth of this band scaled with the Mn concentration and with the room temperature (RT) saturation magnetization. This band is assigned to the internal Mn3+ transition between the 5E and the partially filled 5T2 levels of the 5D state. The broadening of the absorption band is introduced by the high Mn concentration. Recharging of the Mn3+ to Mn2+ was found to effectively suppress these transitions resulting also in a significant reduction of the RT magnetization. The pronounced sensitivity of the relative position of the Fermi level and 1.5 eV absorption band can be used to predict the magnetization behavior of the Ga1−xMnxN epilayers. The absence of doping-induced strain was observed by Raman spectroscopy. The structural quality, the presence of Mn2+ ions were confirmed by EPR spectroscopy, meanwhile no Mn-Mn interactions were observed.

Recently, wide band gap II-IV-N2 semiconductors such as ZnSiN2, and ZnGeN2 and ZnSiGeN2 have been synthesized, but very little is known about their band structure, optical properties, or electronic properties. Bulk crystals are hard to synthesize because high temperatures and pressures are required. The success in growing II-IV-N2 films epitaxially by MOCVD creates interesting opportunities. The crystal structure of II-IV-N2 compounds is orthorhombic, and when grown on r-plane sapphire can provide a suitable template for GaN growth. Optical transmission studies of the band edge of ZnSiN2 and ZnSiGeN2 with varying Si and Ge percentages were conducted. The indirect nature of the band gap was investigated, and prism coupling was used to obtain the refractive indices in the visible and NIR portion of the spectrum. Although the crystal symmetry was orthorhombic, the refractive indices indicated uniaxial optical properties. Optical loss measurements indicate that the films are suitable for waveguides and novel devices based on birefringent optical effects.

The effect of high temperature, high pressure annealing on morphology, optical and structural properties of free-standing GaN films grown by hydride vapor phase epitaxy is studied. The annealing is found to change the intensities of the photoluminescence peaks as a result of a redistribution of the impurities and native defects in the thick GaN films. A positron annihilation study shows a decrease of the Ga vacancy-related defects below the detection limit after the annealing. The defect redistribution is correlated with a flattening of the stress distribution across the thickness, as revealed by micro Raman study, and with a decrease of the curvature of the annealed free-standing films.

We report on p-type AlGaN/GaN and GaN/InGaN superlattice (SL) designs with significantly improved vertical and lateral electrical conductivity (σV and σL). Composition-graded p–AlGaN layers were used to produce eight fold reduction in barrier height and a ∼40% increase in the sheet hole density in the p-GaN wells compared to typical SL structures. Thirteen orders of magnitude and 35 times improvement is obtained for σV and σL compared to typical SL and bulk p-GaN, respectively. A similar approach for p-GaN/InGaN SL resulted in seven fold reduction in barrier height and a ∼30% increase in the sheet hole density in the p-InGaN wells compared to a typical SL structures. σL is strongly dependent on hole mobility in the well and about 19 times improvement is obtained for the optimized design, SL-B, with μp=30 cm2 V-1s-1 compared to bulk-InGaN. More than 10 orders of magnitude improvement in σV is obtained for SL-B compared to modulation doped SL.

Photoreflectance (PR) modulation spectroscopy was performed to investigate surface properties of GaN films grown on sapphire substrates. From the period of the Franz-Keldysh oscillations, the surface electric field across the GaN space charge region was found to be (197 ± 11) kV/cm, which corresponds to a surface state density of 1.0×1012 cm−2. A surface barrier height of 0.71 eV was determined by fitting the dependence of the PR intensities on pump beam power density. We suggest that a deep level is formed at 2.68 eV above the GaN valence band edge due to the large density of surface states.

We have developed the deposition and studied the electrical characteristics and thermal reliability of Ru-based contacts on n- type GaN as well as on AlGaN/GaN heterostructure. Amorphous, conducting and transparent RuSiO4 contacts with their extremely low reverse currents and thermal stability up to 900°C, show great potential for use as Schottky contacts to n-type GaN and gate electrodes for AlGaN/GaN HEMT in high temperature, high power applications as well as in UV detectors.

In order to reduce vertical leakage in III-nitride detectors, we have grown a patterned array of hexagonal GaN columns on masked heteroepitaxial GaN template layers using a-plane sapphire substrates. In addition to eliminating cracking, we have found that for GaN columns tens of microns in diameter and several microns high, the dislocation density is also significantly reduced. We have developed a simple closed-form analytical model for predicting the critical column height for the onset of the reduction in the dislocation density. Among the predictions of this model is that the critical column height for the onset of dislocation density reduction is proportional to the product of column width and the grain size of the GaN film.

A method for making AlGaN with a high AlN molar fraction and low dislocation density is needed for fabricating deep ultraviolet emitters and detectors. In this study, we reduced the dislocation density in AlGaN over a large surface area by using low-pressure MOVPE on a continuously rugged epitaxial AlN substrate. The AlN molar fraction of the AlxGa1-xN was × = 0.51, and atomic steps in the surface were clearly observed with an atomic force microscope (AFM). In addition, the dislocation density was estimated to be 8.8 × 107 cm−2, which is two orders of magnitude lower than that of AlGaN grown on a flat AlN epitaxial layer. Our results indicate that the dislocation density of AlGaN can be greatly reduced by using a rugged AlN epitaxial substrate with continuously inclined facet.

The reduction of threading dislocations (TDs) in GaN is very important as TDs act as non-radiative recombination centres and generally reduce the luminescence efficiency of GaN-based optoelectronic devices, particularly UV emitting devices. In order to reduce the density of TDs, we have used an in-situ ELO technique by depositing a non-uniform SiNx mask (as revealed by STEM-EDX) using simultaneous flow of silane and ammonia gas, either on a sapphire substrate or on a GaN pseudo-substrate. The effect of several growth parameters, such as V/III ratio, duration of Si/N treatment, pressure and temperature, on the morphology, structural quality and reduction mechanisms of TDs in GaN have been investigated and the effects of the first two parameters are reported here. To start the epilayer growth with a low V/III ratio gives rise to predominantly 3D growth, forming islands with multiple inclined facets and a vanishing top (0001) facet. A high V/III ratio enhances the lateral growth, and islands with inclined {-2112} facets together with wide flat top (0001) facets are formed. The duration of the Si/N treatment and the low V/III ratio growth regime are optimised in order to reduce the TD density down to 8×107 cm−2, as measured from plan-view TEM images. It is observed that TDs bend through 90° at inclined side facets. The role of atomic ledges moving across inclined facets which are responsible for bending the line direction of the TDs is briefly discussed.