Nanoscale GaN quantum dots were fabricated in AlxGa1-xN confined layer structures via metalorganic chemical vapor deposition (MOCVD), by using a ‘surfactant’ which can modify the GaN growth mode on AlGaN surfaces. A two dimensional growth mode (step-flow-like) of GaN films on AlxGa1-xN (x=0∼0.2) surfaces, that is energetically commenced under the conventional growth conditions, was intentionally changed into a three dimensional mode by adding tetraethyl-silane (TESi) used as a surfactant onto the AlGaN substrate surface prior to the GaN deposition. The surfactant is believed to inhibit the GaN film from wetting the AlGaN surface due to the change in surface free energy. The resulting morphological structures of GaN dots were found to be sensitive to; the doping rate of TESi, the Al content (x) of the AlxGa1-xN layer, and the growth temperature. A very intense photoluminescence (PL) emission was observed from the GaN dots embedded in the AlGaN layers. The quantum size effect in terms of the blue-shift in a PL peak position was verified using the GaN dot samples having different dot sizes.

Layers of AIBN were grown on sapphire by organometallic vapor phase epitaxy (OMVPE) at 1050°C using triethylboron (TEB), trimethylaluminum (TMA) and ammonia as precursors. Boron is readily incorporated into the layers and its concentration in the solid phase can be made fairly high, up to at least 40%. However, single phase AIxB1–xN films can only be grown for compositions not exceeding x=0.01. For higher B concentrations a second B-rich phase is formed. This phase is shown to be most probably wurtzite BN based on the results of transmission electron microscopy and x-ray diffraction. The growth of this phase occurs within the framework of wurtzite AIN islands providing the sites for lateral growth of wurtzite BN. This leads to formation of columnar structure of AIN and BN crystallites oriented in the basal plane and existing side by side. This is one of the first observation of purely thermal growth of sp3 bonded BN.

We report on essential aspects of the growth of InGaN / GaN p-n junctions by gas-source molecular beam epitaxy (MBE) and present the first blue and green electroluminescence from such structures grown entirely by MBE.

A study of the growth conditions for a GaN nucleation layer on sapphire and for the subsequent growth of undoped GaN points out the necessity for Ga-stabilized growth. Unintentionally doped GaN grown at 1 μm/h shows background doping levels below 1017 cm-3 and mobilities up to 320 cm2/Vsec (at 300K). Narrow photoluminescence with very low intensity in the yellow spectral range is observed. N- and p-type doping of GaN with Si and Mg yields layers with high mobilities (220 and 10 cm2/Vsec, respectively at 300K) at carrier densities typical for devices.

Although incorporation of Indium is strongly temperature-dependent, InGaN-layers with In-contents of over 40% are obtained routinely. The optical properties of our InGaN layers typically exhibit the commonly observed, broad deep level luminescence. Finally, we present electroluminescence in the visible spectral range up to 540 nm from InGaN / GaN double-heterojunctions.

With the use of ammonia as nitrogen precursor for Group III-Nitrides in an on surface cracking (OSC) approach, MBE becomes a serious competitor to MOVPE. Homoepitaxial GaN exhibits record linewidths of 0.5 meV in PL (4K), whereas GaN grown on c-plane sapphire also reveals reasonable material properties (PL linewidth ≈ 5 meV, n ≈ 1017 cm-3 , μ 220 cm2/Vs). Beside results on hetero- and homoepitaxial growth of GaN, insights into the growth mechanisms are presented. The growth of ternary nitrides is discussed, p- and n-doping as well as first LED results are reported.

Molecular beam epitaxy (MBE) technique is a useful method to grow III-V nitrides, especially those having a metastable crystal structure, like cubic GaN (c-GaN), because of the capability of in situ observation of growing surfaces and its non-equilibrium growth mechanism. We have grown c-GaN on GaAs and 3C-SiC substrates by gas source MBE using dimethylhydrazine or activated nitrogen beam as an N source, and measured their luminescent and optical properties. This paper summarizes the MBE growth and properties of c-GaN, comparing with those of hexagonal one, and the control of the crystal structures is discussed in terms of growth method, orientation of substrate surfaces and growth conditions.

We report the growth of InGaN/AIGaN MQWs on c-plane sapphire by electron cyclotron resonance assisted molecular beam epitaxy (ECR-MBE). Two types of structures were investigated; one employing a GaN and the other a A1GaN barrier layer. The first structure consists of five periods of 80 Å thick In0.09Ga0.91N wells separated by 90 Å thick GaN barriers. The second structure consists of|seven periods of 120 Å thick In0.35Ga0.65N wells and Al0.1Ga0.9N barriers. The substrate temperature was kept constant during the growth of both the wells and the barriers, thus avoiding the need for any temperature cycling during the growth, which may lead to interfacial contamination. The films were characterized by cross sectional transmission electron microscopy (TEM), room temperature photoluminescence (PL) and sub-micron resolution luminescence microscopy. TEM images show sharp and abrupt interfaces, thus confirming the high interfacial quality of the MQW structures. Both structures exhibit strong RT luminescence emission peaking at 387 nm (FWHM = 16nm) for the In0.09Ga0.91N/GaN structure and at 463 nm (FWHM = 28nm) for the In0.35Ga0.65N/A10.1Ga0 9N structure. The high resolution luminescence microscopy studies reveal that the radiative recombination for the InGaN quantum wells is 60–70 times more efficient than for the underlying GaN film.

AIN films were grown on Si (111) substrates using a PSMBE deposition system. Two types of growth methods were used: (i) a continuous growth at one temperature and (ii) a two stage growth where a buffer layer at low temperature is followed by deposition at higher temperature. The structure and morphology of the films grown at different temperatures are compared. The evolution of surface structure and morphology of the film during the growth were studied using in-situ RHEED, X-ray diffraction, and AFM.

Nucleation and growth of GaN under Ga-rich conditions by molecular beam epitaxy using a nitrogen rf plasma source is shown to result in both a smoother GaN surface and a reduced inversion domain content. In addition, preliminary results of the dramatic effect of atomic hydrogen on growth kinetics for Ga-rich growth are presented.

We report a study of N incorporation in GaAs and InP by gas-source molecular beam epitaxy using a N radical beam source. For GaNAs grown at high temperatures, phase separation was observed, as evidenced from the formation of cubic GaN aside from GaNAs. By lowering the growth temperature, however, GaNAs alloys with N as high as 14.8% have been obtained without showing any phase separation. For InNP, no phase separation was observed in the temperature range studied (310 – 420 °C). Contrary to GaNAs, incorporating N in InP is very difficult, with only less than 1% N being achieved. Optical absorption measurement reveals strong red shift of bandgap energy with direct-bandgap absorption. However, no semimetallic region seems to exist for GaNAs and a composition-dependent bowing parameter has been observed.

A set of GaAsN/GaAs strained layer superlattices was grown on GaAs (001) substrates by electron cyclotron resonance microwave plasma assisted molecular beam epitaxy. An ex-situ high resolution X-ray diffraction was employed to characterize interface quality and determine the effective nitrogen content, as a function of growth temperature, of buried GaAsN layers. The interface quality, which was assessed through FWHM’s and intensities of superlattice peaks, and nitrogen content were found to change dramatically in the temperature range from 540°C to 580°C. A first order kinetic model was introduced to quantitatively explain this dependencies, in terms of energetically favorable N for As anion exchange and thermally activated N desorption and segregation processes. The strong nitrogen surface segregation process, which acts concurrently with nitrogen desorption is found to be responsible for both degradation of interface quality and reducing nitrogen content at elevated temperatures. The predicted nitrogen profile smearing of (2 – 3)nm, obtained from the model, is found to be in good agreement with cross-sectional transmission electron microscopy observations.

Reactive N and H, created using rf plasma sources, were used to grow undoped GaN along with p-type GaN:Mg and p-type GaN:Mg:H thin films. By comparing the optical emission spectra from several rf sources with observed GaN grow rates, we deduce that nitrogen atoms and I stpositive series nitrogen molecules (3.95 eV binding energy) are the reactive species responsible for GaN film growth. A Mg ground state acceptor binding energy of about 224 meV was determined from low temperature photoluminescence (PL) experiments for both p-type GaN:Mg and p-type GaN:Mg:H films.

GaN films were grown on sapphire substrates at temperatures below 1000 K utilizing a Hollow Anode nitrogen ion source. A Ga flux limited growth rate of ~ 0.5 µm/h is demonstrated. Active utilization of strain and the assistance of a nitrogen partial pressure during buffer layer growth are found to be crucial issues that can improve the film quality. The best films exhibit a full width at half maximum of the x-ray rocking curves of 80 arcsec and 1.85 meV for the excitonic photoluminescence measured at 4 K. A Volmer-Weber three dimensional growth mode and the spontaneous formation of cubic GaN inclusions in the hexagonal matrix are observed in the investigated growth temperature range. It is argued that this growth mode contributes to a limitation of the carrier mobility in these films that did not exceed 120 cm2/Vs though a minimum carrier concentration of ~ 1015 cm−3 was achieved.

GaN films grown on sapphire at different temperatures are investigated. A Volmer-Weber growth mode is observed at temperatures below 1000K that leads to thin films composed of oriented grains with finite size. Their size is temperature dependent and can actively be influenced by strain. Largest grains are observed in compressed films. It is argued that diffusing Ga ad-atoms dominate the observed effects with an activation energy of 2.3 ± 0.5 eV. Comparably large grain sizes are observed in films grown on off-axes sapphire substrates and on bulk GaN. This assures that the observed size limitation is a consequence of the 3D growth mode and not dependent on the choice of the substrate. In addition, the grain size and the surface roughness of the films depend on the nitrogen partial pressure in the molecular beam epitaxy (MBE) chamber,most likely due to collisions between the reactive species and the background gas molecules. This effect is utilized to grow improved nucleation layers on sapphire.

AlxGal−xN films (x≤0.60) were grown on c-plane sapphire and (0001) 6H-SiC substrates using ECR plasma assisted Molecular Beam Epitaxy. Evidence of long range ordering in the investigated AlxGal−xN films is presented. Without intentional dopants the films are semi-insulating with resistivities ranging from 103 to 105 Ω.cm. The films were doped n-type with Si and p-type with Mg. The carrier concentration in the Si doped films, as determined by Hall effect measurements, was between 1016 to 1019 cm−3. At constant Si cell temperature, the carrier concentration was found to be reduced with AlN mole fraction, consistent with the observation that the donor ionization energy increases with Al content. Correspondingly, the electron mobility decreases with Al concentration, a result attributed to alloy scattering. The Mg doped films were found to exhibit p-type conductivity by thermoelectric power measurements with resistivities varying from 3 to 30 Ω-cm.

Ill-nitride semiconductors are emerging as highly promising candidates for the fabrication of wide band-gap electronic and opto-electronic devices. Sapphire ((α-A12O3) is currently one of the primary substrates of choice for the growth of GaN despite a large lattice mismatch. Significant improvements in the quality of III-nitride layers have been demonstrated by exposure of the substrate to reactive nitrogen species followed by deposition of a low temperature AIN or GaN buffer layer. In this paper we present a study of the evolution of the surface topography and defect microstructure of nitrided α-A12O3 substrates and AIN buffer layers deposited by reactive molecular beam epitaxy (RMBE). Their influence on the morphology and properties of GaN layers is also discussed. Both nitridation time and AIN deposit thickness were varied systematically, at different temperatures and buffer growth rates. The microstructures were characterized using the atomic force microscope (AFM) and transmission electron microscope (TEM). Initial growth studies are ideally suited to in-situ experiments, and further investigations are also in progress using a unique UHV TEM with the facility for in-situ RMBE.

AIN/SiC structures are of current importance for the development of high temperature electronic devices. A systematic study of the growth and structure of AIN on Lely grown 6H-SiC was performed. The SiC substrates were chemically etched followed by thermal desorption at 875 °C under UHV conditions. AFM and RHEED images show the relative improvement in surface morphology following the cleaning procedure. AIN films were grown on 6H-SiC substrates using plasma source molecular beam epitaxy (PSMBE). Substrate temperatures were varied from 400 °C to 800 °C and deposition energies from 1 eV to 25 eV during AIN growth. The AIN crystal structure and topology were analyzed by RHEED and AFM. The AIN/SiC film microstructure are correlated with the quality of the SiC surface and the parameters of the AIN deposition process. Epitaxial growth of AIN(0002) was strongly dependent on the SiC surface quality and deposition energy. The highest quality AIN was achieved at growth temperatures of approximately 600 °C and 15 eV ion energy. Higher ion energy resulted in amorphous film growth.

High resolution electron microscopy has been applied to characterize the structure of β-GaN epilayers grown on (001) GaAs substrates by plasma-assisted molecular-beam-epitaxy. The rf plasma source was used to promote chemically active nitrogen. The layer quality was shown to depend on growth conditions (Ga flux and N2 flow for fixed rf power). The best quality of GaN layers was achieved by “stoichiometric” growth; Ga-rich layers contain a certain amount of the wurtzite phase. GaN layers contain a high density of stacking faults which drastically decreases toward the GaN surface. Stacking faults are anisotropically distributed in the GaN layer; the majority intersect the interface along lines parallel to the “major flat” of the GaAs substrate. This correlates well with the observed anisotropy in the intensity distribution of x-ray reflexions. Formation of stacking faults are often associated with atomic steps at the GaN-GaAs interfaces.

Surface reconstructions for MBE grown GaN are identified. Different cases are considered according to the type of substrate or crystal symmetry and surface phase diagrams are obtained. Through different examples, it is shown how growth monitoring can be efficiently achieved through the use of surface reconstructions. Finally, from the observation that a residual arsenic overpressure in the MBE chamber changes the surface reconstructions of cubic (001) GaN grown onto 3C-SiC (001) substrates to that commonly observed for GaN growth on (001) GaAs, it is proposed that arsenic might be a surfactant for nitride growth.

A newly developed Metalorganic Chemical Vapor Deposition (MOCVD) reactor for processing batches of seven 50mm wafers per run at deposition temperatures up to 1600°C is introduced. The substrates are individually rotated by means of gas bearings utilizing high purity gas for operation. In order to achieve the maximum uniformity on a wafer the uniformity of the growth temperature was maximized. Pyrometric measurements revealed a temperature uniformity across the susceptor of ±4°C at a temperature of 1300°C. Growth of GaN and GaInN produced uniform layers regarding composition and thickness. On a 50mm diameter wafer the standard deviation for the thickness of a GaN layer is 6% and the standard deviation for the composition of a GaInN layer as determined by photoluminescence is 2%.

MBE growth of III-V nitrides is being studied at NCSU using MOVPE grown GaN buffer layers on SiC as substrates. Rf plasma sources are being used for the generation of active nitrogen during MBE deposition. Through the use of multiple rf plasma sources, sufficient active nitrogen is generated in order to examine the properties of III-V nitride layers grown at higher substrate temperatures and growth rates. The resulting MBE-grown GaN films exhibit remarkably intense photoluminescence (PL) dominated by a sharp band-edge peak at 3.409 eV having a FWHM of 36 meV at 300K. No deep level emission is observed. AlGaN and InGaN films and quantum well structures have also been prepared using multiple sources. A modulated beam MBE approach is used in conjunction with the multiple rf plasma sources to grow InGaN. RHEED and TEM studies reveal flat 2D InGaN quantum well structures. Depending on the indium content, GaN/InGaN single-quantum-well structures exhibit electroluminescence at 300K peaked in the blue-violet to the green spectral region.