We report the use of ScAlMgO4 as a substrate for the epitaxial growth of wurzitic GaN. The low misfit (+1.8%) allows coherent epitaxy of GaN, as observed by RHEED. The congruent melting of ScAlMgO4 makes Czochralski growth possible, suggesting that large, high quality substrates can be realized. Boules about 17mm in diameter are reported. We have used nitrogen-plasma molecular beam epitaxy to grow GaN epitaxial films onto ScAlMgO4 substrates. Band-gap photoluminescence has been observed from some of these films, depending primarily on the deposition conditions. A 3×3 superstructure has been observed by RHEED on the GaN surfaces. Structural analysis by x-ray diffraction indicates very good in-plane alignment of the GaN films. We also report thermal expansion measurements for ScAlMgO4.

Optical spectra on free exciton properties for GaN are presented and discussed, in particular the influence of epitaxial strain and temperature. The exciton-phonon coupling is also manifested via the temperature dependence of the LO phonon replicas of the free exciton.

In this paper GaN films are examined, which are grown on basal plane (0001) sapphire substrates. Growth is performed in a novel type of vertical rotating disk reactor. The effects of several growth parameters on the film quality are discussed. The results on n-type doping of GaN with SiH4 are presented. The GaN layers were evaluated by surface morphology studies, DC X-ray diffraction, electrical and optical characterisation.

Initial results on 0.25 μm gate MODFET's have yielded ft=21.4 GHz and fmax=77.5 GHz. These devices have characteristics that agree with the gradual channel model dominated by the electron mobility. The AlGaN/GaN structure, grown on sapphire substrates, are polycrystalline, and thus yield low mobility (<100cm2/Vs) at low electron sheet density. Using a simple model, design optimization predicts electron sheet density values of 7.3 × 1012 cm−2 in thin, 3 nm quantum wells for single-sided doping with 5 nm spacer for use in future high frequency Al0.4Ga0.6N/In0.25Ga0.75N/GaN MODFET's with gate lengths of 0.10 μm. Double sided doping with 5 nm spacers would yield a sheet density of 1.4 × 1013cm−2 in such 3 nm quantum wells.

GaN single crystals have been grown from Ga solution. The crystals grow in the form of platelets with their basal plane perpendicular to the c-axis. The two opposite crystal surfaces are not equivalent since one is N- and the other Ga-terminated. Atomic force microscopy has been applied to study surface morphology on both surfaces. It was found that one side is atomically flat. The other side consists of pyramid-like structures about 25 nm in size.

The influence of annealing in an NH3+H2 atmosphere in the temperature range from 600°C to 900°C was investigated. Depending on crystal face the results were drastically different. It was found that on the rough side, annealing yields an atomically flat surface with terraces of monolayer height. The size of the terraces depends on the temperature of the annealing. On the originally flat side the surface becomes rougher after annealing. The transformation of surface morphology begins at temperatures below 700°C. Preliminary results of annealing in a hydrogen atmosphere are also reported. These findings are crucial for the understanding and development of GaN homoepitaxy.

In this paper we compare gallium nitride (GaN) films grown by molecular beam epitaxy on sapphire (Al2O3), gallium arsenide (GaAs (111)B) and lithium gallate (LiGaO2) substrates. Atomic force microscopy, scanning electron microscopy, cathodoluminescence imaging and cathodoluminescence spectroscopy are used to characterise the films. From growth runs carried out to date, GaN films on GaAs substrates exhibit the best surface uniformity and the cleanest luminescence.

Undoped layers of GaN grown by MOVPE on sapphire substrates have been characterized by photoluminescence, photocapacitance and photoinduced current transient spectroscopy (PICTS). Photocapacitance reveals in all samples two specific signatures at photon energies of 1 eV and 2.5 eV. The photocapacitance decrease observed at 1 eV seems to be due to an electron capture process from the valence band, whereas the capacitance increase at 2.5 eV is related to an electron emission process. The fact that the capacitance step at 1 eV is only seen after photoionization at energies above 2.5 eV, and the observed correlation between its amplitude and the photoluminescence intensity of the “yellow band”, lead us to conclude that both transitions are linked to the same trap, which is also suggested to be responsible for the yellow band. The position of this trap, at 2.5 eV below the conduction band, is confirmed by PICTS measurements, that show a hole thermal emission activation energy of 0.9 eV at 350 K.

Thin films of AlxGa1−xN (0.05 ≤ x ≤ 0.96) having smooth surfaces were deposited directly on both vicinal and on-axis 6H-SiC(0001) substrates. Cross-sectional TEM of Al0.13Ga0.87N revealed stacking faults near the SiC/Nitride alloy interface and numerous threading dislocations. EDX, AES and RBS were used to determine the compositions, which were paired with their respective CL near band-edge emission energies. A negative bowing parameter was determined. The CL emission energies were similar to the bandgap energies obtained by SE. FE-AES of the initial growth of Al0.2Ga0.8N revealed an aluminum rich layer near the interface. N-type (silicon) doping was achieved for AlxGa1−xN for 0.12 ≤ x ≤ 0.42. Al0.2Ga0.8N/GaN superlattices were fabricated with coherent interfaces. Additionally, HEMT structures using an AlN/GaN/AlN buffer structure were fabricated.

Optically detected electron nuclear double resonance (ODENDOR) was measured in the 2.2 eV ‘yellow’ luminescence band associated with the residual donor in n-type undoped GaN. The ODENDOR lines are due to gallium and show a quadrupole splitting which can be described with an axial tensor. The quadrupole parameter was estimated to be q(69Ga) = 1/2 Qzz = 0.22 MHz. A hyperfine interaction for 69Ga of about 0.3 MHz for the isotropic and of about 0.15 MHz for the anisotropic part was estimated from the width of the ODENDOR lines. It is tentatively suggested that a Ga interstitial is the residual donor.

We report the growth of AlxGa1−xN alloys on (0001) sapphire by the method of Electron Cyclotron Resonance-assisted Molecular Beam Epitaxy (ECR-MBE). The films were doped n-type with silicon at carrier concentration levels from 1016 to 1019 cm−3. SEM studies reveal smooth surface morphology consistent with the observed 3×4 surface reconstruction in the RHEED pattern. Independent determination of the Al-concentration and the lattice constant of the alloys shows that Vegard’s rule is obeyed in the pseudo-binary GaN-AlN system. The bandgap of the alloys, determined by transmission and photoluminescence measurements, was found to depend linearly on Al-concentration.

Electroluminescence spectra of superbright blue and green LEDs based on epitaxial InxGa1−xN/AlyGa1−yN/GaN heterostructures with thin quantum well active layers [1] were studied at currents J = 0.01-20 mA. Spectral maxima of blue and green LEDs are ωmax = 2.58-2.75 eV and ωmax = 2.38-2.45 eV, dependent on the active layer In content. The low energy tails of the spectra are exponential with the parameter E0 = 42-50 meV almost independent of the temperature. The high energy tails of the spectra are exponential with a temperature dependent parameter E1= 20-40 meV. Both parameters (E0, E1) are current independent at J > 0.5 mA. The spectral band can be described by taking into account quantum size effects, impurities and electron-phonon interactions in active layers. A structure in the spectra was detected which can be described by the influence of light interference in the GaN layer on the sapphire substrate. Light intensity was a linear function of the drive current over the interval J = 1-20 mA, and was slightly temperature dependent. In the blue LEDs, the efficiency fall off at low currents (J < 0.7 mA) had a I ~ J4-5 dependence at room temperature. The green LEDs showed no such dependence. The influence of tunnel effects on the efficiency at low currents is discussed. Tunnel radiation spectra with maxima moving with the voltage were detected at low currents in III-N structures.

GaN(0001) has been grown on Al2O3 (0001) by molecular beam epitaxy where NH3 was used as the nitrogen precursor. Desorption mass spectroscopy and reflection high energy electron diffraction (RHEED) were used to monitor the relationship between growth rate and the incident fluxes during growth. Excess surface Ga decreases the GaN formation rate when the substrate temperature is too low or the Ga flux is too high. A simple rate equation is used to describe the observed behavior.

GaN epitaxial layers on GaN single crystals were grown using molecular beam epitaxy with an NH3 source. The deposited layers were examined by high resolution x-ray diffraction and photoluminescence (PL) spectroscopy. We observed strong and extremely narrow (half-widths of 0.5 meV) lines related to the bound excitons. In the higher energy range we observed three strong lines. Two of them are commonly attributed to free exciton transitions A (3.4785 eV) and B (3.483 eV). Their energetic positions are characteristic of strain-free GaN material.

The pressure variation of the bandgap, at the isostructural phase transition of wurtzite InN, is determined theoretically, using the first-principles total-energy pseudopotential method. It is found that the bandgap (as well as the structural parameters) exhibit three different types of behavior, in three regions of pressure. Optical experiments at low temperatures could then be employed to directly identify the two different wurtzite phases of InN.

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.

We have used plasma molecular beam epitaxy on (0 0 0 1) and (0 0 0 ) ZnO substrates to induce epitaxial growth of GaN of a known polarity. The polarity of the ZnO substrates can be easily and unambiguously determined by measuring the sign of the piezoelectric coefficient. If we assume that N-face GaN grows on O face ZnO and that Ga-face GaN grows on Zn face ZnO, then we can study the growth of both Ga and N faces. The most striking difference is the doping behavior of the two faces. Growth on the Ga-face is characterized by a higher carrier concentration and a lower threshold for Ga droplet formation.

In this paper, we address two different aspects relevant to the growth of GaN. The first part concerns alternative nitrogen source whereas in the second part, we report experimental results on Mg doping. Several nitrogen precursors have been used for the growth of GaN in MOVPE. To produce active species from N2 or NH3, a remote Plasma Enhanced Chemical Vapour Deposition (RPECVD) process has been implemented. In addition, nitrogen organic precursors, triethylamine and t-butylamine were also used. To accurately control the critical parameters of the MOVPE of GaN, we have implemented a laser reflectometry, which allows a real time in situ monitoring of the different steps of the growth. MeCp2Mg was used as Mg precursor for the p doping study. The dependence on the partial pressure of Mg precursor of dopant incorporation, electrical activity and growth rate are reported.

Undoped and Zinc-doped GaN films have been grown using TMGa, DEZn and Ammonia by MOVPE. The GaN blue-green LEDs of m-i-n structure have been fabricated. They can be operated at forward bias less than 5 volts. The EL peak wavelength was from 455 nm to 504 nm.

We investigate the structure, growth morphology and the related electro-optical properties of gallium nitride (GaN) films deposited on (0001) sapphire substrates by gas source molecular beam epitaxy (GSMBE) and use transmismission electron microscopy, atomic force microscopy and scanning tunneling microscopy, photoluminescence (PL) and cathodoluminescence (CL). We find two types of specimens: one type which shows a strong UV luminescence (band-to-band transition at 358nm/3.46eV) in CL and PL and only faint yellow luminescence (Gaussian shaped CL/PL peaks at around 528nm/2.35eV), specimen ‘B’, and another type, which shows a strong UV and a comparably strong yellow luminescence, specimen ‘Y’. These two types of specimens have a rough layer surface, specimen ‘Y’ even an islanded one with, facetted hexagonal islands with a width of 1-2μm at a height of 50nm. A correlation of spectrally resolved CL images to the observed defect structure shows: (i) the yellow luminescence is homogeneously distributed over the complete specimen for ‘B’ and ‘Y’ specimens. Our investigations strongly suggest the yellow luminescence to be related to screw dislocations with , which are found randomly distributed in ‘B’ and ‘Y’ specimens with a high density of 1.3·109cm−2; (ii) the strong UV luminescence in ‘Y’ specimens is located in the troughs between adjacent surface islands, where dislocations essentially in small angle grain boundaries of edge type, i.e. with or are located; (iii) in the case of the ‘B’ specimens these dislocations are randomly distributed and so is the luminescence.

We present recent results on bulk GaN crystallization. The best quality GaN crystals grown from the solution at high N2 pressure without an intentional seeding are single crystalline platelets of stable morphology reaching dimensions up to 10 mm. The fastest growth direction for such crystals is [1 0 0], perpendicular to the GaN c-axis. The maximum stable growth rate perpendicular to crystal c-axis is determined from the experiment and used for an estimate of the effective supersaturation for the {10 0} face assuming two dimensional layer growth. The heat of GaN disssolution, determined from experimental solubility data, is used for the estimation of the edge energy of 2-D nuclei on the growing {10 0} face. Bulk crystal growth seeded by a single hexagonal needle with well developed {10 0} faces is also reported. The crystallization mechanisms and morphological stability in seeded growth of GaN are discussed on the basis of experimental results. The physical properties of the GaN crystals and homoepitaxial layers grown on them are briefly reviewed.