Group-III nitride quantum wells (QWs) and quantum dots (QDs) have been grown by molecular beam epitaxy (MBE). Their optical properties are shown to be governed by the presence of a huge internal polarization field. For example, GaN/AlN QDs emit from the blue to the orange due to the giant quantum confined Stark effect (QCSE) induced by a built- in electric field of 4 MV/cm. Another consequence of the QCSE is to drastically reduces the oscillator strength of the ground state transition and thereby to increase by several orders of magnitude its radiative lifetime. Despite the very large density of dislocations in nitride layers, which induce non-radiative recombinations, carrier localization enhances the photoluminescence (PL) efficiency. This is demonstrated by GaN/AlN QDs grown on silicon substrates exhibiting strong PL intensity at room temperature. InGaN/GaN QWs with In composition of 20% also display 300 K PL peaking through the whole visible spectrum (0.4-0.66 νm). This is achieved by varying the QW thickness from 1.5 to 5.5 nm, the red-shift resulting from an internal electric field of 2.5 MV/cm. For InGaN/GaN QWs emitting at 2.8-2.9 eV, the PL efficiency at 300 K is larger than 10 %. This is ascribed to carrier localization, which is not due to InGaN phase separation that would form QDs, as revealed by transmission electron microscopy. Another origin of the carrier localization in InGaN/GaN QWs is then discussed.

High spatial resolution cathodoluminescence (CL) studies have been carried out on GaN/InGaN/(0001)GaN single quantum well (SQW) structures in a field emission scanning electron microscope at 5kV and temperatures down to 8K. Direct comparison of QW CL maps with transmission electron microscope studies of plan-view samples showed that edge type threading dislocations act as non-radiative recombination centers. Spectra taken from extended areas showed a progressive blue shift in the QW emission from around 460nm at low beam intensities to about 445nm as the beam intensity was increased. This effect which correlated with a decrease in the spatial resolution is interpreted as due to an increase in the diffusion length of carriers in the SQW due to a combination of screening of the piezoelectric field and band filling effects.

We report the results of epitaxial growth experiments on AlxGa1−xN (0≤ x ≤ 1) on Si(111) and sapphire substrates aimed at understanding the origin and elimination of cracking. We describe growth procedures resulting in thick layers of AlxGa1−xN, grown by gas source molecular beam epitaxy with ammonia, that are free of cracks. In GaN layers with the thickness of ∼2.5 µm, we find the background electron concentration of (1-2)×1016 cm−3 and mobility of (800±100) cm2/Vs. In AlxGa1−xN (0.2 < x < 0.6) with the film thickness of 0.5-0.7 µm the electron concentration of (2-3)×1016 cm−3 is obtained. Low background concentrations in GaN allow for formation of p-n junctions by doping with Mg. Light emitting diodes with the peak emission at 380 nm have been demonstrated.

The generation of high density 2D hole gases is crucial for further progress in the field of electronic and optoelectronic nitride devices. In this paper, we present results of C-V profiling measurements for N-face GaN/AlGaN heterostructures and systematic theoretical studies of Mg- doped GaN/AlGaN gated heterostructures and superlattices. Our calculations are based on a self- consistent solution of the multiband k.p Schrödinger and Poisson equation and reveal that the hole 2D sheet density is mainly determined by the polarization induced interface charges. For an Aluminium concentration of 30%, the induced hole density in the heterostructure can reach values up to 1.5×1013 cm−2. In the GaN/AlGaN superlattices, the hole sheet density increases with the superlattice period and saturates for a period of 40 nm at a value of 1.5×1013 cm−2.

Facet structures of GaN grown by epitaxial lateral overgrowth (ELO) via low pressure-metalorganic vapor phase epitaxy (LP-MOVPE) are controlled by growth conditions such as reactor pressure and growth temperature, where this technique is called FACELO (Facet Controlled ELO). The mechanism of the morphological change is discussed based on stability of the surface atoms. The propagation mechanism of the threading dislocations for the different GaN facet structure is also investigated. The distribution and density of the threading dislocations are observed by the growth pit density (GPD) method. Two typical models employing the FACELO are proposed; in one model, the dislocation concentrates only on the window area and, in the other model, only in the coalescence region in the center of the mask. In the latter model, the dislocation density is dramatically dropped to the order of 105−6 cm−2 with good reproducibility.

We present a theoretical analysis of the electronic structure of GaN quantum dots (QD) with an adjacent threading dislocation. The QD carrier spectra and wavefunctions are calculated using a plane-wave expansion method within an 8-band k.p model. The method is very efficient, because the strain and built-in electric fields can be included through their discrete Fourier transforms. The QD structures considered have been analysed experimentally by other groups. The GaN QDs are truncated hexagonal pyramids on a wetting layer with an edge dislocation adjacent to each dot. The built-in piezoelectric potential strongly influences the localisation of the carrier wavefunctions. This potential pushes the electrons to the top of the dot, the holes to the bottom and, additionally, causes strong lateral confinement of the carriers. The effect of the dislocation strain field at the dot edge on the carrier states in each GaN/AlN QD is shown to be insignificant. Results are presented for the confined state energies and optical matrix elements for a range of different sized dots with and without dislocations. The size of the dot influences the energies and overlaps, but the presence of the dislocation has minimal effect. The dependence of the ground state optical transition energy on the size of the dot is in good agreement with experimental data.

We report on a high transparency low resistance contact to p-GaN composed of a thin oxidized Ni/Au bilayer overcoated with indium tin oxide (NiO/Au/ITO). The NiO/Au/ITO layer shows a specific contact resistivity, c, of 1.8 × 10−3 Ωcm2 that is nearly ten times lower than conventional Ni/Au annealed under N2. Measurements on fully processed LEDs with a NiO/Au/ITO current spreading layer (CSL) show an operating voltage of around 4 V at 20 mA that is comparable to LEDs fabricated with a conventional Ni/Au CSL and a dramatic improvement over the previous ITO data. LED top surface light emission through the NiO/Au/ITO CSL is shown to be greater than that from LEDs with a conventional semi-transparent Ni/Au CSL. Taken together, these results demonstrate the feasibility of using NiO/Au/ITO as a CSL for high performance GaN LEDs.

Room temperature and low temperature photoluminescence studies of AlxGa1−xN/GaN superlattices reveal a red shift of the dominant transition band relative to the bulk GaN bandgap. The shift is attributed to the quantum-confined Stark effect resulting from polarization fields in the superlattices. A theoretical model for the band-to-band transition energies based on perturbation theory and a variational approach is developed. Comparison of the experimental data with this model yields a polarization field of 4.6 × 105 V/cm for room temperature Al0.1Ga0.9N/GaN and 4.5 × 105 V/cm for room temperature Al0.2Ga0.8N/GaN. At low temperatures the model yields 5.3 × 105 V/cm for Al0.1Ga0.9N/GaN and 6.3 × 105 V/cm for Al0.2Ga0.8N/GaN. The emission bands exhibit a blue shift at high excitation densities indicating screening of internal polarization fields by photo-generated free carriers.

Near-band-edge cathodoluminescence emission for some nominally undoped GaN samples decreases with time during electron bombardment in a scanning electron microscope. The rate of decrease depends on the incident beam voltage and current and on the size of the area being scanned, which is determined by the magnification used for the scanning electron microscope. Faster intensity decreases occur with lower beam voltages (10kV versus 20kV), higher beam currents (600nA or 200nA versus 35nA) and smaller scanned areas (120K magnification versus 15K magnification). For larger scanned areas, the maximum luminescence intensity occurs 10's or 100's of seconds after the start of electron bombardment, and the intensity then decreases with further bombardment. In some cases the size of the degraded region is larger than the bombarded area. These observations suggest that the degradation is caused by electromigration of impurities or other charged defects within the GaN resulting from local charging by electron bombardment.

Lateral epitaxial overgrowth (LEO), pendeo-epitaxy (PE), and solid-phase epitaxial recrystallization (SPER) are discussed as three approaches for reducing the defect density in group III nitride based heterostructures. Studies of the LEO GaN and PE GaN revealed, that a major factor for the defect reduction in the laterally overgrown regions is the change of the dominant growth direction - from vertical in the window regions to lateral in the regions over the mask or over the trenches, and the related threading dislocations lines redistribution. The mechanisms of defect reduction in LEO GaN and PE GaN are similar, although they arise through different process routes, and are related to the free-standing (PE) or quasi-free-standing (LEO) growth of GaN, and the associated stress redistribution. The stress distributions in the LEO and PE GaN heterostructures are calculated and compared with finite element modeling. Another approach for reduction of the defects is the SPER process and the related thermal activation for dislocation reactions and grain boundary mobility and migration. This approach is shown in the example of annealed AlN films.

Anisotropic X-Ray diffraction (XRD) and transport properties of cubic GaN grown on GaAs substrates correspond to the features of low-temperature grown GaN (LT-GaN) buffer layer. When the LT-GaN layer is grown on the surface tilted from (001) to [1-10] with annealing in arsenic ambient, the macroscopic step edges along [1-10] direction are modified by either the ambient of thermal annealing, or substrate misorientation. A parallel conduction in GaN, GaAs, and GaN/GaAs hetero-interface was observed by photoconductivity measurements. Transmission electron microscope (TEM) observation shows that self-annihilations for (-111) B stacking faults are preferentially occurred near GaAs interface when GaN film grown on the surface tilted from (001) toward [1-10] (As step edge) is annealed in arsenic ambient. TEM observation also shows that stacking faults and dislocations are preferentially generated near GaN/GaAs interface. It is suggested that anisotropic transport properties correspond to the well-like potential generated by band bending at GaN/GaAs interface. The nearly isotropic mobility of 3,000 cm2/Vsec at 77K is obtained by improving interface property.

Photoenhanced electro-chemical (PEC) wet etching has been shown to be suitable for dislocation-density estimation in n-GaN films as well as for GaN-based device fabrication. We report on PEC etching of n-GaN samples grown by MBE and HVPE methods in unstirred aqueous KOH solution under He-Cd laser illumination. Characterization of the etched samples was carried out using atomic force microscopy (AFM) in both cross-sectional and plan-view configurations and scanning electron microscopy (SEM). At moderate illumination densities, the SEM and AFM analyses reveal sub-100 nm scale threading vertical wires on the etched surfaces. The calculated density (∼1×10 9cm−2) is in agreement with dislocation density found by transmission electron microscopy. Using cross-sectional AFM, we find that these vertical wires are ∼1[.proportional]m high and are perpendicular to the sapphire surface. Applying a higher illumination density or an external voltage, we obtain a higher etch rate with a smooth free-feature etched surface. Some highly resistive samples that cannot be etched under normal conditions because the band bending is too small to confine the holes to the surface for them to participate in the PEC process, can be etched with the application of a voltage to the sample. In this case, the etch rate depends on both the polarity and the magnitude of the voltage applied. In an MBE-grown sample with an AlN/GaN superstructure inside, we report on high selectivity between AlN and GaN (AlN is an etch stop); the selectivity is due to the etching mechanism of the PEC process.

In this report two different aspects in the development of AlGaN/GaN power amplifiers will be discussed. In the first part of this paper we report on the optimization of the Ti/Al/Ni/Au metallization scheme on a doped AlGaN/GaN FET structure. By a systematic investigation we were able to reduce the contact resistance to 0.2 ωmm (7.3×10×7ωcm2). The Al/Ti thickness ratio for this contact was 6, which according to the Al-Ti binary phase diagram, does not result in excess Ti which should react with nitrogen in the AlGaN layer to render the surface heavily doped. Preliminary results on Schottky contacts indicate an improvement in the reverse leakage current if a RIE oxygen plasma in combination with a NH4OH dip is performed prior to metallization.

Coplanar waveguides on AlN are discussed in the second part of this paper. These transmission lines can be used in AlGaN/GaN power amplifiers if no via-hole technology is available or if a hybrid solution is pursued. The signal line should have a large metal cross- sectional area (> 5 × 50 [.proportional]m2) in order to carry enough current in the output stage of an amplifier. It is shown that CPWs with large dimensions show non-quasi TEM behavior related to propagation of parallel plate modes.

We present the results of the experimental and theoretical studies of the low field mobility of two-dimensional electrons in the homoepitaxial AlGaN/GaN heterostructures and in the AlGaN/GaN heterostructures grown on SiC. We show that, at cryogenic temperatures, the temperature dependence of the mobility is primarily determined by the deformation potential scattering and that most of other important scattering mechanisms are temperature independent. We show also that two-dimensional (2D) and three-dimensional (3D) mobility models yield very close results. The analysis of the mobility dependence on the electron sheet density ns shows two possible explanations of the non monotonic mobility versus carrier density dependence: i) the alloy/interface scattering and ii) transfer of the 2D electrons into 3D states in GaN. We present experimental data suggesting that for high 2D gas densities in the investigated structure grown on SiC the 2D-3D transition should take place and might be responsible for the mobility decrease at high electron sheet densities.

Site-selective photolumiescence (PL) and photoluminescence excitation (PLE) spectroscopies have been carried out at 6 K on the ∼1540nm 41/32 → 4I1/52 Er3+ transition in in-situ-doped GaN:Er which have detected nine differenct Er3+ sites and associated PL spectra. Three distinct Er3+ sites are identified in the in-situ-doped samples. For the in-situ-doped samples, the concentration of the various Er+3 sites are comparable while for the ion-implanted sample, the concertration of one Er3+ site was higher than the concentration of the other sites. In-situ-doped samples grown with different Ex-cell temperatures were considered, and the width of the PLE spectrum appears to be a function of the Er-cell temperature.

Photoluminescence (PL) spectra of GaN epitaxial layers grown on vicinal (0001) GaN substrates (i.e. N-polar) demonstrate high optical quality showing free exciton transitions in addition to narrow bound-exciton peaks (line width 1-2 meV). In contrast, N-polar films deposited on exactly oriented (0001) substrates exhibit a broad PL emission resulting from high free carrier concentrations. As derived from Secondary Ion Mass Spectroscopy (SIMS), exact (0001) epilayers reveal very high concentrations of oxygen (1.5×1019 atom/cm3), which are about one order of magnitude higher then in the layers grown on a 4°-off substrate. It is therefore concluded that oxygen is responsible for the high free carrier concentration and could be a dominant shallow donor in the homoepitaxial N-polar films. It is also shown that incorporation of oxygen strongly depends on the polarity of the films (Ga or N). Concentrations of other impurities, such as Si, C and H are found to be similar for homoepitaxial films of both polarities being consistent with the results of exciton-related PL.

The low temperature photoluminescence (PL) of Mg-doped GaN co-doped with oxygen is investigated. Two PL bands are observed at low temperature at 2.5 and 2.8 eV in the p-type oxygen co-doped samples. Both the 2.5 eV and 2.8 eV bands are attributed to donor-acceptor (DA) transitions involving deep donors of different origin and a shallow MgGa acceptor. The integrated intensity of the 2.8 eV band in p-type GaN decreases as the oxygen partial pressure is increased. The decrease is attributed to reduced compensation by deep native donors. The two bands are not formed in n-type (n > 1019 cm−3) GaN:Mg over-doped with oxygen indicating that the deep donors responsible for these transitions have high formation energies in n-type material.

In epitaxial GaN layers on (0001) sapphire, the distance between edge dislocations in sub-grain boundaries is variable. The strain state in such sub-grains is quantitatively measured by processing of HRTEM images. The dislocation core distribution maps and in plane Burgers vectors components are derived from experimental strain tensor by applying the continuum dislocation theorem. For image simulationsthe atomic models of edge dislocations are calculated using a modified Stillinger-Weber potential. It is shown that the strain field extracted from simulated images matches that from experimental ones. The corresponding dislocation density peaks are larger and more splitted than those from simulated images.

We report on the Raman analysis of the phonon lifetimes and phonon decay channels of the A1(LO), E1(LO) and E2(high) phonons of single crystalline bulk AlN grown by the sublimationrecondensation method. The temperature dependence of the phonon lifetimes was investigated from 10K to 1275K. Our experimental results show that amongst the various possible decay channels, the A1(LO) phonons of AlN decay primarily into two phonons of equal energy (Klemens model), most likely longitudinal-acoustic (LA) phonons. AlN is therefore in great contrast to GaN, where a symmetric decay of the A1(LO) phonons is not possible due to a large energy gap between the acoustic and optical phonon branches. For the E2(high) phonon, we find an asymmetric phonon decay into a high-energy and a low-energy phonon. Possible decay channels of the E2(high) phonon include combinations of E2(low) and acoustic phonons. For the E1(LO) phonons, the temperature dependence of the phonon lifetime is consistent with a symmetric phonon decay. Phonon lifetimes of the A1(LO), the E1(LO) and the E2(high) phonon of 0.75ps, 1.2ps and 2.9ps, respectively, were measured at 10K.

We have investigated relationships between microscopic structure and cathodoluminescence (CL) property of polycrystalline (poly-) GaN grown by electron-cyclotron-resonance plasma-excited molecular beam epitaxy (ECR-MBE) on ZnO/Si substrates. Very strong CL with a peak of 3.45 eV was observed from the poly-GaN, which mainly showed a columnar structure with a size of 50-100 nm. On the other hand, the intensity of CL from the poly-GaN with few columnar domains was weaker than that of the poly-GaN with the columnar structure. The CL image from the poly-GaN, in which the columnar domains were locally observed, showed a strong contrast between bright domains and a dark background. It is confirmed that these bright regions in the CL images are corresponding to the columnar domains of the poly-GaN, by comparing with the SEM images. These results suggest that the columnar domains are responsible for the strong CL from the poly-GaN grown on the ZnO/Si substrates. Cross-sectional transmission electron microscope (TEM) observation revealed that the columnar domains had high quality crystallinity with few defects.