In this work we evaluate the strategy of using 3C-SiC as a substrate for III-V nitrides heteroepitaxy (AlN, GaN…). Our methodology is based on the elasticity theory of strained interfaces and involves not only geometric parameters of host materials but also parameters related to their elastic properties. The basic physics involved in the theory correlates lattice dynamics and strain gradients via effective elastic constants associated with the host materials forming the heterosystem (S factor). Within this approach, the optimization of the IIIV/ 3C-SiC interface is achieved by applying, at the interface, continuity conditions to the host material S factors and the related geometric features. An alloyed layer, i.e. AlxGa1−xN, simulates the III-V compound. We find out that the optimizing composition of this layer is x=1 corresponding to a stoechiometric AlN layer. This is consistent with the result showing that AlN presents the closest structural characteristic to SiC. Our results also predict that, when used as a buffer layer, AlN may provide a mean to optimize the GaN/SiC interface.

We report on a comparative study of the growth of GaN in an arsenic free MBE system using either the method of plasma activation of molecular nitrogen or catalytic decomposition of ammonia on a heated substrate. We find that while growth with a plasma source leads to smooth films only under Ga- rich conditions, growth with ammonia leads to smooth films under ammonia-rich conditions. In both cases we find a 2×2 surface reconstruction when using an AlN buffer, which is evidence that material grown with this buffer layer has the Ga-polarity. In the case of plasma growth we also investigated the use of a GaNbuffer and found that at the growth temperature the surface is unreconstructed, however it undergoes 3×3 reconstruction upon cooling to 300 °C. This observation is evidence that material grown on a GaN buffer has the N-polarity.

Erbium (Er) ions were implanted into gallium nitride (GaN) and the temperature-dependent photoluminescence (PL) and PL lifetime were characterized. Thermal quenching of the Er3+ luminescence was restrained by using GaN as a host material and PL was observed from Er3+ at room temperature. We prepared a flat depth profile of the density distribution of Er by ion-implanted into GaN and monitored the thermal quenching process due to the residual defects after implantation and annealing. We also monitored the auger effect that is believed to be the main cause of the thermal quenching process and concluded that, in the temperature range 15 K to 100 K, the thermal quenching process is dominated by nonradiative recombination from the first excited state (4I13/2) to the ground state (4I15/2) of Er3+.

The morphology and composition of AlN crystals on 6H-SiC (0001) at the initial stage of crystal growth by sublimation re-condensation technique were investigated by SEM and SAM. Discontinuous AlN coverage occurred after 15 minutes growth. The AlN nuclei size, and growth rate increased as temperature increased or pressure decreased. The SiC substrate decomposed leaving hexagonal hillocks; simultaneously, the AlN nucleated on these SiC hillocks apparently rotated by 15° to 30°. The chemical composition of the substrate and different AlN crystal facets were characterized by SAM. The bare substrate area was stoichiometric SiC with insignificant conversion to silicon nitride, while Si and C preferentially incorporated in the AlN at the initial stages of growth on specific crystal planes.

Step-doped structures of both magnesium and beryllium were grown in GaN and analyzed using secondary ion mass spectrometry. Dopant incorporation was studied as a function of substrate temperature and dopant flux for Ga-polarity and N-polarity GaN. Incorporation is different for each polarity, with Mg incorporating by up to a factor of 20 times more (30 times more with atomic hydrogen) on the Ga-face, while Be incorporates more readily on the N-face. The effect of atomic hydrogen on the incorporation kinetics of both Mg and Be is also discussed. Mg and Be both undergo surface segregation during growth. Photoluminescence measurements suggest that Be is a p-type dopant with an optical activation energy of approximately 100 meV.

We apply pyrometric interferometery to control the growth of GaN, AlN, and AlxGa1−xN on Si (111) by molecular beam epitaxy. We show that the period of intensity oscillations in the temperature measured by the pyrometer is caused by optical interference in the growing nitride layer. The period of oscillations is inversely proportional to the growth rate. This effect is used for in-situ measurement of the growth rate of GaN, AlN, and AlxGa1−xN (for 0<×<1). We also show that pyrometric interferometery can be used for in-situ determination of the composition of the growing layer.

We show theoretically that disorder-induced scattering of light between « bright » and « dark » exciton-polariton modes governs the coherent optical response of GaN/AlGaN quantum wells (QWs) on a femtosecond scale. The incident light essentially excites the “bright” modes, they emit the most part of light radiatively during first few hundreds of femtoseconds. The rest of the coherent signal, i.e. the temporal behaviour on a scale from 1 to few picoseconds, comes from the “dark” modes, populated due to the elastic scattering processes in the system, and mixed so that they form two bulk-like polariton branches. The pico-second scale optical response of GaN/AlGaN QWs is essentially incoherent because of a very efficient scattering of exciton-polaritons by a structural disorder.

GaN-based electroluminescence devices (ELDs) were fabricated using a GaN powder as an emission layer. The electroluminescence spectra of the GaN ELDs under AC operation were observed at room temperature. The emission characteristics of GaN-based ELDs were studied to compare the EL spectra and the cathodoluminescence (CL) spectra. It was clarified that the EL spectra were similar to the CL spectra of a GaN emission layer. The emission peaks in the EL spectra were shifted toward the high-energy side with increasing operation frequency.

GaN in its hexagonal Wurtzite crystal has 3.4 eV bandgap and is capable of withstanding over 3 MV/cm electric field intensity. It is grown on either sapphire or SiC and the lattice mismatches cause threading dislocations. If it is grown along the c-axis on its Ga face, a thin pseudomorphic AlxGa1−xN barrier layer grown on top causes a substantial electrical polarization, which induces a substantial 2DEG in the GaN [1]. With x =.3 this 2 DEG is about 1 × 1013/cm2 without any doping impurities. The ∼ 109/cm2 threading dislocations accept electrons from the low density ambient donors, yielding very low net electron densities (< 1014/cm3) in the GaN buffer layer used as the HEMT channel [2]. The 2DEG electron mobility can be 1500-1700 cm2/V-s with ≥ 1 × 1013/cm2 electron sheet density. This 2DEG sheet density charge is less than the electrical polarization charge of 1.7 × 1013/cm2 for Al.3Ga.7N/GaN, due to some surface depletion and due to the fact that each dislocation traps ≥ 2 × 103 electrons. The design, processing, and measured performance of such undoped, polarization-induced HEMT's are presented below.

The underlying GaN layers on which laser diodes are fabricated have been improved through two steps. In the first step, GaN single layer on sapphire was investigated. The residual strain and etch pit density were measured. We found that they reflect the optical quality. We found that the threading dislocation can be reduced to 4 × 108 cm−2. The optical quality depends on the residual strain and dislocation density. In the next step, we have utilized epitaxial lateral overgrowth ELO) technique. The optimized GaN layer on sapphire with the smallest dislocation density was used as seed layer. In the wing region of ELO-GaN, the threading dislocation density was reduced to 1 × 106 cm−2. On the other hand, in the seed region, dislocation density remained 4 × 108 cm−2. Photoluminescence intensity in the wing region was three times as large as that in the seed. The laser diodes were fabricated on the ELO-GaN layer, so that the ridge stripe was fabricated over the wing region, and its properties were compared with those of laser diodes on sapphire. It was found that the lifetime can be increased by using the ELO-GaN layer as the underlying layer.

We report on UV Raman spectroscopy of InxGa1−xN thin films grown on (0001) sapphire substrates using a specially designed metal-organic chemical vapor deposition (MOCVD) reactor. Eight films were examined in the compositional range 0<×<0.50. Mid and deep-UV Raman spectroscopy was done with the 325.2 nm line of the HeCd laser and the 244 nm line of a double frequency Ar ion laser. The mode behavior of the A1(LO) and E2 phonons was also investigated. We have found compelling evidence for one-mode behavior for the A1(LO) phonon mode, while our data for the E2 mode deviates from the predictions for one-mode behavior. The results for the A1(LO) mode are consistent with the previously reported phonon mode behavior in AlGaN alloys. Also, evidence regarding the presence of compositional inhomogeneities and spinodal decomposition in InGaN thin films is presented.

Metal-insulator-semiconductor structures are fabricated by depositing SiO2 films on an MOCVD-grown n-type GaN epitaxial layer and a GaN/Al0.4Ga0.6N/GaN double heterojunction. The SiO2 films are grown by plasma-enhanced chemical vapor deposition. High-frequency C-V characteristics show the agreement of the measured C-V curve of SiO2/n-GaN with an ideal curve in deep depletion and the very small hysteresis, which indicates that the interface traps concentration in the sample is low. However, for SiO2/GaN/Al0.4Ga0.6N/GaN, the measured C-V curves show a notable flat-band shift of about 9.2 V and a typical polarization hysteresis window. These show the influence of the polarization charges in this structure. The capacitance on SiO2/GaN/Al0.4Ga0.6N/GaN reaches a minimum value under around –5V bias. The saturation at a minimum value of the C-V curve indicates the presence of holes accumulation in the MIS structure. These results imply that the piezoelectric effect in GaN/Al0.4Ga0.6N/GaN play an important role for the formation of the p-channel.

We have investigated relationships between the microscopic structure and optical property of polycrystalline GaN grown by (electron-cyclotron-resonance plasma-excited molecular beam epitaxy) ECR-MBE on silica glass substrates, using scanning electron microscope (SEM) and cathodoluminescence (CL). It was found that CL intensity was stronger for the samples with a large columnar domain size. These individual columnar domains showed clear luminescence. It was found that the origin of strong luminescence from polycrystalline GaN is due to such a columnar domain. That luminescence was closely related to the morphology of the columnar domains. It was revealed that the columnar domain with a homogeneous and hexagonal shape showed clear luminescence.

Time-resolved photoluminescence experiments at varying temperature are performed on a series of InxGa1−xN/GaN quantum well and quantum box samples of similar compositions (0.15 < x < 0.20). The results are analyzed by using envelope-function calculations of transition energies and oscillator strengths, accounting for internal electric fields. The respective influences of localization and electric fields on radiative and nonradiative lifetimes and on the Stokes shift are deduced. The results indicate that the spatial extension of localization centers is much smaller than the size of the quantum boxes (∼10 × 3 nm, typically). The room-temperature radiative efficiency of both quantum well and quantum box samples is enhanced by replacing the topmost GaN barrier by an AlGaN one.

Most of the work done on GaN has taken into account layers grown on the (0001) sapphire. However one would expect the growth on (1120) to lead to different structural defects. As has been shown, in one direction, the mismatch is rather small. In this work, we have carried out structural analysis of layers and interfacial relationship. Inside the layers, the density of defects is comparable to that found conventionally in layers grown on top of (0001) sapphire. The growth mode is also mosaic with a grain size of a few microns. One interesting result is the interface structure, which differs from conventional growth where a flat or stepped interface is formed with a large distance between steps. In this case, the interface is found to be rough at the atomic scale so that this roughness has a random distribution. Moreover, the misfit dislocation spacing is 1nm which is only half the dislocation spacing found in GaN growth on (0001) sapphire.

p-InGaN/n-GaN heterojunction diodes were grown by metalorganic vapor phase epitaxy and characterized using current-voltage (I-V) and capacitance-voltage (C-V) measurements. We changed the In mole fraction in p-InGaN from 0 to 25% to investigate diode characteristics. All the diodes showed rectified I-V characteristics at room temperature. The ideality factors obtained from forward I-V characteristics were around 2, meaning that the recombination current is dominant instead of the tunneling current through the defects in depletion layers of the diodes. The breakdown voltage in reverse I-V characteristics depends on the net donor concentration (ND - NA) in n-GaN instead of the In mole fraction in p-InGaN. This result also means that the defects in p-InGaN do not influence the breakdown voltage. The built-in potential from C-V measurements decreases with the In mole fraction in p-InGaN, meaning that the valence band discontinuity increases with the In mole fraction. This valence band discontinuity realizes the hole confinement in an HBT with an p-InGaN base. Using these InGaN/GaN heterojunction diodes, an InGaN/GaN double heterojunction bipolar transistor was fabricated for the first time. The maximum current gain of 1.2 was obtained at room temperature.

Experimental results obtained in two different HVPE reactors are analyzed and compared to the theoretical curves, taking into account the surface kinetics, the mass transfer, and parasitic deposition on the glass walls before the substrate. Unexpected high growth rate values, up to 50νm/h, were measured in conditions of expected substrate etching by HCl. A systematic experimental study of this new phenomenon is presented together with a theoretical analysis. This analysis suggests a new mixed general mechanism of growth.

Amorphous thin films of AlN doped with Cu (blue luminescence), Tb (green luminescence) and Cr (red luminescence) were deposited on p-doped Si (111) substrates using RF magnetron sputtering in a nitrogen atmosphere and made luminescent active by heat treatment at 1000°C. Single layered amorphous AlN films deposited with Tb and Cu showed cathodoluminescence from only the Tb3+ ions. Presumably, energy is transferred from Cu luminescence centers to Tb centers in close proximity. In contrast to this, double layered amorphous AlN films doped with Cu (∼200 nm) on top of Tb (∼50 nm) emits from both ions. This behavior is observed in a double layered amorphous AlN film doped with Cu (∼200 nm) on top of a Cr (∼200 nm) doped film. Secondary-ion mass spectrometry depth profiling revealed that the incorporated metals moved from one layer into the other during the heat treatment step necessary for luminescence activation.

One of the applications of wurtzite gallium based nitride compounds will be to provide optoelectronic devices from 6.2eV (AlN) to 1.89eV (InN). This will depend on the possibility to grow wurtzite AlGaN and InGaN ternary alloys. As expected, the most difficult region is InGaN due to the large misfit between GaN and InN (= 10%), in this case, ordering, phase separation and growth instabilities have been reported. In the case of AlN and GaN, the misfit is smaller (∼ 2.5%) and one would expect more stable growth. However, it was in this system that ordering along the c axis between AlN and GaN was reported for the first time.

In this work, we have found that the growth of AlGaN may be more complicated. Not only the wurtzite lattice can be decreased to simple hexagonal by AlN/GaN ordering along the c axis, but the growth can lead to other types of stackings. Even in the low Al composition range, 10 - 15%, we have found that three processes can operate:

1. Ordering into AlN/GaN as two simple hexagonal sublattices.

2. The 3:1 ordering which has been recently reported to occur in InGaN.

3. A new type of ordering where diffraction experiments (XR and electron diffraction) detect superlattice reflection with a period close to 3 nm. The most adequate model which was found to take this into account shows that, in these growth conditions, the system has preferred to form one AlN cell in between 5 GaN cells, leading to a 5:1 ordering.

We present first-principles calculations of structural and electronic properties of heterovalent SiC/AlN and SiC/GaN heterostructures with wurtzite AlN and GaN films pseudomorphically grown on the 6H-SiC and 3C-SiC substrates along the c-axis. We have investigated reconstructed stoichiometric interfaces consisting of one mixed layer with various lateral arrangements. The preferred bonding configurations of the reconstructed interfaces are found to be Si-N and Ga-C. The calculated valence band discontinuities for SiC/AlN and SiC/GaN heterostructures lie in the range of 1.5 - 2.3 eV and 0.4 - 1.4 eV, respectively. The SiC/AlN heterostructures are predicted to be of type I, whereas SiC/GaN heterostructures can be of type I or II. The polarization induced interface charges are of the order of 4.8 × 1012cm−2 and 0.7 × 1012 cm−2 in SiC/AlN and SiC/GaN junctions, respectively.