Thermal treatment under propane at 1300-1400 °C has been used to prepare Silicon (001) wafers for subsequent growth of cubic GaN and AlN by Electron Cyclotron Resonance Plasma Assisted Molecular Beam Epitaxy (ECRMBE). Thermal treatment of Silicon wafers under propane, used in this experiment, produced a very thin (40 Å) layer of cubic SiC on the Silicon (001) surface. Despite an extremely low thickness of as-produced SiC layer, high quality cubic GaN has been successfully grown. The cubic form of AlN grown on the SiC(40Å)/Si(001) surface has also been observed despite a very high density of stacking faults.

We report the results of photoluminescence measurements on a number of GaN thin films grown by MBE on GaAs (111)B substrates. In particular, we draw attention to a new observation of a line at approximately 3.40eV which is accompanied by complex fine structure and interpret it as due to a donor-acceptor (DA) transition. Assuming a donor energy of 30meV, we derive an acceptor binding energy of approximately 80meV which is very much smaller than the accepted value of 250meV for the well established Mg acceptor. However, our result is in agreement with a recent estimate of the hydrogenic acceptor energy as being 85meV.

Raman spectroscopy is used to study the effect of the built-in biaxial stress on the E2 and A1 (LO) q = 0 phonon modes of wurtzite GaN layers deposited by metal organic vapor phase epitaxy on (0001) sapphire substrate. By means of phonon frequency shifts, the biaxial pressure coefficients of the mode frequencies are determined and used to calculate the corresponding deformation potentials. Stress calibration has been performed using reflectance data.

In the past year, several organizations have fabricated reliable, high-brightness LEDs from III-Nitride materials that emit in the blue and green. Recently, Nichia in Japan have announced lasing action in GaN-based diodes. Quantum well structures are key to all these results, offering higher brightness, narrower EL linewidths, and a wider spectral range. In order for the III-Nitride technology to develop, the material growth technique must offer high volume at low cost in addition to the requisite device performance. To date, only MOVPE has demonstrated this capability. We have previously reported the growth of GaN, InGaN, and AlGaN layers by MOVPE in a multi-wafer, high-speed rotating disk reactor. Both n- and p-doping and high quality optical properties have been achieved. In this paper we extend this earlier work and present results of the performance of InGaN / (Al)GaN quantum well structures. Intense PL spectra were observed in the violet and blue regions. The thinnest wells show evidence from PL and DCXRD measurments of either discontinuous layers (islands) or a diffuse upper interface, with preliminary TEM results showing the latter to be the most likely. We also report excellent uniformity of these quantum well structures, and show electroluminescence from a SQW diode emitting at 473 nm.

This work presents an optical characterization by luminescence and reflectivity of GaN layers grown on sapphire using MOVPE, HVPE and GSMBE. Well resolved optical spectra are obtained for each growth technique. The luminescence of Mg doped MOVPE grown GaN is also studied. A Mg acceptor optical depth of ~ 260 meV is obtained.

For the growth of an electrically pumped lasing nitride emitter, the development of the MOCVD equipment and the process are mutually dependent. Most important is the implementation of the rapid temperature changes that are required between the growth of the different layers of a device structure. Equally important is to provide a reaction chamber that develops a stable gas phase at all growth temperatures used in the process. In this paper we will give insight in the technology and the relationship between processes and equipment. The development of the reation chamber was supported by mathematical modeling that formed the basis for the selection of appropriate process parameters for growth of group-III nitrides. The modeling consists of the numerical solution of the Navier-Stokes equations coupled with heat transfer and mass transport of the chemical species. The modeling of radiative heat transfer takes into account the effect of changing surface radiative properties. These changes result from the coating of the reactor inner surfaces during the growth run. Coupled flow dynamics and chemistry including homogeneous and heterogeneous reactions play an important role for predicting growth rate distributions on the susceptor area. At the practically used high temperatures, group-III metalorganics turn out to be almost entirely decomposed and it is the mass transport of these decomposition products to the growing layer that is assumed to control the growth rate in accordance with experimental observations.

AIXTRON GmbH

The theoretical analysis of high pressure influence on the conditions of crystal growth of GaN is presented. High pressure influence on the transport and equilibrium properties of nitrogen is estimated using scaling approach. Nitrogen gas properties are used in the finite element calculation of the thermal conditions in the high pressure chamber. The temperature distribution during GaN growth in the vertical temperature-gradient configuration is obtained.

A room temperature (RT) plasma etch process has been developed to non-selectively etch GaN/InGaN/AlGaN structures, grown on sapphire substrates, using an electron cyclotron resonance (ECR) plasma source with RIE enhancement. The process chemistry chosen was Cl2/CH4 based in order to facilitate the formation of volatile etch by-products, typically to form group III halides and group V hydrides, although indium is more likely to form an organo-metallic compound as opposed to a chloride. A characteristic of this process is the very smooth sidewall features obtained and the controllability of the etch profile via ECR power, table bias and/or gas flow ratio. Typical results obtained using a RT process were etch rate above 100 nm/min., selectivity to resist mask above 30:1 and smooth anisotropic profile at low ion-energies (below 100 eV). The process etch rate showed a characteristic increase with increasing table bias (above 130 nm/min.) with only small changes in the relative etch rate of each compound (i.e. selectivity maintained at roughly 1:1), however, this etch does rely upon competing etching and deposition mechanisms and thus too large a variation in one parameter without a corresponding compensation with another leads to a rough surface and a more selective etch. The process has also been demonstrated using a metal mask (e.g. Ni) and present work is progressing onto other gas combinations and the use of high temperature electrodes.

Wide band gap nitrides(InN, GaN, AlN) have been considered promising optoelectronics materials for many years [1]. Recently two main technological problems in the nitrides were overcome: (1)high quality layers has been grown on both sapphire and SiC substrates and(2) p-type GaN and AlGaN material has been obtained. These achievements resulted in the fabrication of bright light emitters in the violet, blue and green spectral regions [2].First injection laser has been demonstrated [3]. This paper reviews results obtained over the last few years on nitride p-n junctions, particularly on GaN based p-n junctions grown on SiC substrates. We will consider GaN p-n junctions, AlGaN p-n junctions, GaN and AlGaN p-i-n structures, and, finally, GaN/SiC p-n structures.

We present first-principles studies of the effect of biaxial (0001)-strain on the electronic structure of wurtzite GaN, AlN, and InN. We provide accurate predictions for the valence band splittings as a function of strain which greatly facilitates the interpretation of data from samples with unintentional growth-induced strain. The present calculations are based on the total-energy pseudopotential method within the local-density formalism and include the spin-orbit interaction nonperturbatively. For a given biaxial strain, all structural parameters are determined by minimization of the total energy with respect to the electronic and ionic degrees of freedom. Our calculations predict that the valence band state Γ9(Γ6) lies energetically above the Γ7(Γ1) states in GaN and InN, in contrast to the situation in AlN. In all three nitrides, we find that the ordering of these two levels becomes reversed for some value of biaxial strain. In GaN, this crossing takes place already at 0.32% tensile strain. For larger tensile strains, the top of the valence band becomes well separated from the lower states. The computed crystal-field and spin-orbit splittings in unstrained materials as well as the computed deformation potentials agree well with the available experimental data.

A theoretical model which accounts for a physisorption precursor of molecular nitrogen is proposed for the analysis of group III-nitride growth by molecular beam epitaxy (MBE). The kinetics of nitrogen evaporation are found to be an essential factor influencing the MBE growth process of group III-nitrides. The high thermal stability of nitrides is explained to be related to the desorption kinetics resulting in a low value of the evaporation coefficient. The values of the evaporation coefficients as functions of temperature are extracted from the experimental Langmuir evaporation data of GaN and AlN. Using the revised thermodynamic properties of the group III-nitrides, and the obtained values of the evaporation coefficient, the process parameter dependent growth rate and transition to extra liquid phase formation during the GaN MBE are calculated. The theoretical results are compared to the available experimental data.

Films of GaN have been grown using a modified MBE method in which the active nitrogen is supplied from an RF activated plasma source. Wurtzite films grown on (0 0 1) oriented GaAs substrates show highly defective, ordered polycrystalline growth with a columnar structure; the (0 0 0 1) planes of the layers being parallel to the (0 0 1) planes of the GaAs substrate. Films grown using a coincident As flux, however, have a single crystal zinc-blende growth mode. They have better structural and optical properties. To improve the properties of the wurtzite films we have studied the growth of such films on (1 1 1)A and (1 1 1)B oriented GaAs substrates. The improved structural properties of such films, assessed using x-ray and TEM methods, correlate with better low temperature PL performance.

GaN layers deposited by MOCVD on sapphire have been characterized by Transmission Electron Microscopy (TEM). Two substrate orientations were used, (0 0 0 1) and(2 0).We determine the crystallographic structures (defect content and layer polarity) of three different types of GaN layers with different surface morphologies. Convergent Beam Electron Diffraction studies were particularly important to determine the polarity of the GaN layers. We find that polarity and surface diffusion are the factors that control the different growth modes. Unipolarity is obtained thanks to the annealing of the low temperature buffer layer or/and thanks to the nitridation of the sapphire substrate.

Hexagonal pyramids and flat tops are formed when the material has a dominant N-polarity. The pyramids contain many tiny hexagonal columnar Inversion Domains (IDs). These pyramids are formed when the tiny Ga-polar IDs grow faster than the surrounding N-polar matrix. Flat GaN layers are unipolar, with a Ga polarity. Rough grainy layers which are unipolar (Ga-polarity) are obtained when surface diffusion is not high enough.

GaN epitaxial films were analyzed by Secondary Ion Mass Spectrometry (SIMS). Standard implanted samples were used to determine the appropriate analytical conditions for analysis of impurities. The dose and energy of implantation for selected elements (Mg, Al, Si, Zn, Cd, H, C and O) were chosen so the maximum impurity concentration was not more than 1020 atoms/cm3. The optimum analysis conditions were ascertained from the standards for each element, and the detection limits were deduced from the background levels of the implantation profiles. We demonstate that lower detection limits of 1015 atoms/cm3 with a dynamic range 103 − 105 are possible. Zn and Cd have low ion yields, so the minimum detection level for these elements is the background level of the detector. The detection limits of the other elements are determined by the contamination of an initial GaN matrix.

The luminescent properties of AlGaN epitaxial layers with AlN mole fractions up to 30% and various types of AlGaN/GaN-based heterostructures have been studied. The structures were grown on 6H-SiC substrates by MOCVD. The structures' cathodoluminescence and electroluminescence were measured. A “blue” shift of the edge luminescent peak position for AlGaN alloys was measured to be a non-linear function on the AlN mole fraction. For p-AlGaN/n-GaN double heterostructures (DH), the edge peak position was detected at 365 nm (300K). For a p-Al0.05Ga0.95N/n-Al0.03Ga0.97N heterostructure, the electroluminescent edge peak was observed at 355 nm (300K). The effects of temperature and forward current on the edge electroluminescence of theAlGaN/GaN DH's were investigated.

Undoped AlxGa1−xN layers with good surface morphology and very low electron concentration have been grown by MOCVD on sapphire substrates. The observed electrical and optical properties depend strongly on the growth temperature. Layers grown at 1000 °C exhibited low resistivity and strong optical absorption below the bandgap. In contrast,layers grown at 1050 °C had low carrier concentrations and good mobilities. Virtually no optical absorption near the band edge was observed as opposed to the usual situation in AlxGa1−xN. The electrical properties of these layers can be explained by the presence of donor centers whose energy increases with composition, and deeper lying compensating defects. The interaction of these centers renders the samples with x<0.2 highly resistive, with room temperature resistivity higher than 106 ohm-cm. SIMS data strongly suggest that the electrically active centers in our AlGaN layers are native defect-related. Implantation of Si ions into Al0.12Ga0.88N, and subsequent annealing at 1140 °C resulted in layers with electron concentration of 4.6 × 1017cm−3.

We have studied GaInN/GaN quantum well structures grown by LP-MOVPE by picosecond time-resolved photoluminescence spectroscopy. For the quantum wells we find rather long PL decay times of up to 600 ps at low temperature. At temperatures higher than about 100 K, the decay time decreases rapidly, reaching about 75 ps at room temperature. From measurements of the integrated PL intensity, we conclude that this decrease of the decay time is due to nonradiative recombination processes. By combining our data for the lifetime and the intensity, we derive the radiative lifetime, which is constant at low temperature and increases at elevated temperatures. We explain this behavior on the basis of the interface roughness at low temperature and thermal dissociation of excitons at higher temperatures.

We report on nickel based technology for the fabrication of GaN mesa structures. Ti/Ni ohmic contacts for n-doped GaN with contact resistivity Rc ~2×10−5  Ω×cm2 and Ni ohmic contacts for p-doped GaN with Rc ~ 4×10−2 Ω×cm2 were formed. Both types of contacts were used as masks for GaN reactive ion etching (RIE) in a CCl2F2/Ar gas mixture. Maximum etch rates of ~ 40 nm/min were obtained. Mesa structures up to 3 μm in height were formed.

A new heterobipolar transistor was made with the wide bandgap semicon-ductors gallium nitride (GaN) and silicon carbide (SiC). The heterojunction allows high injection efficiency, even at elevated temperatures. A record current gain of ten million was obtained at room temperature, decreasing to 100 at 535°C. An Arrhenius plot of current gain vs 1/T yields an activation energy of 0.43 eV that corresponds to the valence band barrier blocking the escape of holes from the base to the emitter. This activation energy is approximately equal to the difference of energy gaps between emitter and base. This Transistor can operate at high power without cooling. A power density of 30 KW/cm2 was sustained.

We report on a comprehensive study of the defect structure in GaN grown on c-oriented sapphire by gas source molecular beam epitaxy and metal organic vapour phase epitaxy. Transmission electron microscopy is used to investigate the defect structures which are dominated by threading dislocations perpendicular to the sapphire surface and stacking faults. Additionally, dislocation densities are determined. For determination of dislocation densities by x-ray diffraction we employ a model that uses the linewidth of x-ray rocking curves for this purpose. Finally, Rutherford backscattering spectrometry is performed to complement the structural investigation.