Basal Plane Sapphire Research Articles

Interfaces between lanthanum hexaaluminate and sapphire studied by high-resolution electron microscopy

Abstract Thin films of lanthanum hexaaluminate were produced on the basal plane of sapphire by chemical solution deposition and thermal treatment. The close-packed oxygen sublattices of the lanthanum hexaaluminate films with the magnetoplumbite structure and the sapphire are in exact topotactic alignment. The structure of the interfaces were investigated by high-resolution electron microscopy, and the images of focus series were used to invert the imaging process and finally to reconstruct the complex electron exit wave at the object plane. Simulated electron waves based on different interface models were compared with the reconstructed waves, and very good matches of the phase images were obtained. Two different types of interface were observed and were characterized in detail along [1100]HA-[21 10]s (where the subscript HA indicates the lanthanum hexaaluminate and the subscript S sapphire). Using criteria of structural chemistry, possible stacking sequences of the oxygen sublattices across the boundary ...

Effects of thin film Nb interlayer in Cu/sapphire bonds

The effects of a thin film Nb interlayer on the microstructure and fracture energies of Cu/Nb/Al 2O 3 diffusion bonds have been studied by four-point bend testing, TEM and SEM analyses. Single crystalline Cu and α-Al 2O 3 with different crystallographic orientations at the interface were used to determine the influence of the orientation relationship (OR) on fracture resistance. The Nb film interlayer deposited by electron beam evaporation on the ceramic side prior to diffusion bonding was found to be polycrystalline and fiber-textured after diffusion bonding, with the close-packed plane (110) being parallel to the (0001) basal plane of sapphire. The introduction of the thin film Nb interlayer greatly improves the Cu/Al 2O 3 bond strength, whereas diffusion bonding can be performed at a relatively low temperature (900°C). The orientation relationship between Cu and α-Al 2O 3 crystals at the interface strongly influences the fracture energy of the Cu/Nb/Al 2O 3 bonds. The bonds with the OR Cu(100)[011]∥ Al 2 O 3(0001)[11 2 ̄ 0] possess the highest fracture energies, exhibiting a ductile interface fracture mechanism. The much higher fracture energies obtained for the Cu/Nb/Al 2O 3 bonds relative to either Cu/Al 2O 3 or Nb/Al 2O 3 bonds are attributed to the strong adhesion of Nb to α-Al 2O 3 combined with a larger plastic deformation in the metal side during fracture.

Lateral epitaxial overgrowth of GaN on sapphire and silicon substrates for ultraviolet photodetector applications

Lateral epitaxial overgrowth of GaN thin films was conducted by low-pressure metalorganic chemical vapor deposition on basal plane sapphire and (111) silicon substrates. The films were characterized through X-ray diffraction, photoluminescence, scanning electron microscopy, atomic force microscopy and deep level transient spectroscopy. Schottky metal–semiconductor–metal ultraviolet photodetectors were fabricated on LEO grown GaN films for the first time. The spectral responsivity, its dependence on optical excitation power and bias voltage, and the device time decay properties were characterized. The orientation of the interdigitated fingers with respect to the LEO stripes was investigated.

Insights into MOCVD process control as revealed by laser interferometry

In-situ laser interferometry has been used to study the nucleation of CdTe on basal plane sapphire in MOCVD growth. The nucleation delay and nucleation thickness were seen to be influential in the determination of overall buffer layer quality. This nucleation process was seen to be highly complex and the nucleation delay and nucleation layer thickness were seen to vary for constant conditions of nucleation II:VI ratio and temperature. A possible link between nucleation delay and nucleation temperature has been commented on. The results for CdTe on sapphire suggest that for consistent metalorganic chemical vapor deposition (MOCVD) growth conditions from run to run during the nucleation stage, the nucleation delay and thickness may still vary as a function of subtle differences in the substrate growth surface. This argument is further substantiated by our observations using laser interferometry to study the nucleation of CdTe on Si (211) in MOCVD, where different surface preparatory treatments were employed. Laser interferometry was found to be sensitive to differences in surface treatment of the Si substrates prior to ZnTe/CdTe growth. The benefits of the use of interferometry to study and to control the nature of complex epitaxial systems is discussed.

Characterization of flicker noise in GaN-based MODFET's at low drain bias

We report systematic characterizations of flicker noise in GaN based MODFET's. The devices were fabricated by MBE on (0001) basal plane sapphire substrates with an 800 /spl Aring/ AlN buffer layer grown at 800/spl deg/C. Flicker noise was measured across the channel of the devices from room temperature to about 130 K. The voltage noise power spectra, S/sub V/(f), were found to be proportional to 1/f/sup /spl gamma//. The frequency exponent, /spl gamma/, of S/sub V/(f) exhibits systematic dependencies on the device temperature as well as the gate bias, V/sub G/. The variation of the noise power spectra as a function of the drain voltage, V/sub D/, and the gate bias, V/sub G/, were studied in detail and were found to vary as V/sub D//sup 2//(V/sub G/-V/sub T/)/sup /spl beta// where /spl beta/ changes with temperature from about 2.1 at room temperature to about 0.9 at 130 K. Analyses of the data show that the noise originated from the thermal activation of carriers to the localized states at the AlGaN/GaN heterointerface in the channel area. The data suggested that the trapping and detrapping of carriers did not lead to fluctuations in the carrier concentration as postulated in the McWhorter's model. However, more work is needed to determine if surface mobility fluctuations played a key role in the 1/f noise.

Gallium Nitride Growth Using Diethylgallium Chloride as an Alternative Gallium Source

Metal organic vapor phase epitaxy (MOVPE) of GaN has been carried out using diethyl gallium chloride (DEGaCl) and ammonia. The growth rate and efficiency of the DEGaCl-based growth decreases with increasing temperature when compared to trimethyl gallium (TMG)-based growth under similar conditions. Both low temperature buffer and the high temperature GaN layers were grown using the DEGaCl-NH3 precursor combination on the basal plane of sapphire and compared to similar structures grown using TMG and NH3. DEGaCl-based growth reveals an improved growth behavior under identical growth conditions to the conventional TMGa and ammonia growth. X-ray, Hall, and atomic force microscopy (AFM) measurements have been carried out on these samples providing a direct comparison of materials properties associated with these growth precursors. For the DEGaCl-based growth, the x-ray rocking curve line width, using the (0002) reflection, is as low as 300 arcsec on a 2.5-micron thick film. A RMS surface roughness of ∼0.5nm measured over a 10×10 micron area.

Characterization of the Interface Between Lanthanum Hexa-Aluminate and Sapphire by Exit Wave Reconstruction

AbstractEpitaxial thin films of rare-earth hexaaluminates on basal plane sapphire have been produced by chemical solution deposition. LaAl11O18 films with magnetoplumbite structure grow with (0001)HA||(0001)S and [1100]HA||[2110]s orientation relationshipTo investigate the stucture of the interface exit wave reconstruction of focus series was carried out using a field emission TEM. Due to the inversion of the imaging process major artefacts at the interface can be eliminated. Exit waves were simulated based on different interface models and were compared with the reconstructed waves to localize the positions of the atoms at the interface. Two different types of interfaces were observed in the samples. One of the types, in which the spinel block of hexaaluminate faces the sapphire with the mixed cation layer with occupied octahedra and tetrahedra, is characterized in detail. Face-sharing of coordination polyhedra is avoided to large extent

High-performance GaN p-n junction photodetectors for solar ultraviolet applications

The fabrication and characterization of ultraviolet photodetectors based on GaN p-n junctions is reported. The devices are grown by metalorganic vapour phase epitaxy on basal-plane sapphire substrates. These detectors are visible-blind with a sharp wavelength cut-off at 360 nm. The photocurrent is linear with incident power from up to , with a responsivity of at 360 nm. The device time response is dominated by the effective resistance-capacitance time constant, and a 105 ns response is estimated for very low load resistances. A comparison with the response of GaN photoconductor detectors is also presented. The application of these high-performance photodetectors for solar ultraviolet monitoring is described.

High temperature epitaxial growth and structure of Nb films on α–Al2O3(0001)

Epitaxial Nb thin films were grown via molecular beam epitaxy (MBE) at different substrate temperatures on α–Al2O3(0001) substrates. For temperatures of 900 °C to 1100 °C, it was found that Nb grows in the Volmer–Weber growth mode (formation of three-dimensional crystallites). Depending on the growth temperature, different epitaxial orientations of Nb films can be found. At a growth temperature of 900 °C, the Nb{111} planes are parallel to the sapphire basal plane whereas at 1100 °C the Nb grows with the {110} planes on the basal plane of sapphire. These orientations are present even in the initial stages of growth at both temperatures. The formation of two different epitaxial orientations of thick Nb films can be conclusively explained only by considering both the change in the total density of Nb islands with temperature and the influence of island size on the total energy of the islands. The Nb island growth process has been investigatedin situusing reflection high energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). Scanning electron microscopy (SEM), x-ray diffraction (XRD), and transmission electron microscopy (TEM) were employed to determine the morphology and structure of Nb islands, Nb films, and Nb/α–Al2O3interfaces.

A new surface science in situ transmission and reflection electron microscope

We describe an ultrahigh vacuum instrument for transmission electron microscopy and reflection electron microscopy for the study of surfaces and thin film growth. The focus of previous experiments was on the high spatial resolution (<3 Å) generally associated with microscopy, at the cost of controlled growth and characterization. We have taken a different approach. It has been shown that most experiments using diffraction and diffraction contrast imaging can be performed well at poorer resolution (∼20 Å), including the imaging of monatomic steps and monolayer coverages. The instrument is designed for best control of growth and vacuum, with sacrifices in optical resolution, which is theoretically ∼2 nm. The instrument is called SHEBA (surface high-energy electron beam apparatus). We can examine a ∼1 cm2 sample in both transmission electron microscopy and reflection electron microscopy, in situ with well-controlled molecular beam epitaxy (MBE) growth capabilities, well characterized vacuum, and surface characterization by Auger spectroscopy and low-energy electron diffraction. Preliminary experiments of cobalt on silicon have shown MBE growth rates ranging from 1.5 monolayers per hour to 18.3 monolayers per hour depending on the temperature of the evaporation source. Using SHEBA for a reactive MBE experiment has provided the first direct evidence of the formation of aluminum nitride by nitridation of basal plane sapphire. The most serious design problems were stray magnetic fields and mechanical vibrations. These have been overcome and allow a spatial resolution of <10 nm. The instrument is expected to be very valuable in studies of film growth.

Epitaxial growth of ultrathin Pt films on basal-plane sapphire: the emergence of a continuous atomically flat film

Epitaxial Pt films of various thicknesses were grown on basal-plane sapphire using ion beam sputtering. Scanning tunneling microscope images are used to characterize the film morphologies and their dependence on deposition temperature, T D. Epitaxial characteristics are determined, in a qualitative sense, by low-energy electron diffraction (LEED). The film morphologies are very sensitive to T D — exceeding the narrow ideal temperature range of 580–590°C means the difference between a relatively flat Pt film and a surface with tall Pt mounds separated by bare sapphire patches. Within this temperature range, Pt islands are observed at 10 Å, these islands begin to coalesce at 15 Å, and continuous films are observed at 30 Å and 50 Å. The continuous films are nearly ideal atomically flat surfaces, but both show several exposed Pt layers and the 30 Å film has many pinholes. Also, images of 5 Å Pt films deposited at room temperature are used to characterize the substrate morphology.

Gallium Nitride Growth Using Diethylgallium Chloride as an Alternative Gallium Source

AbstractMetal organic vapor phase epitaxy (MOVPE) of GaN has been carried out using diethyl gallium chloride (DEGaCI) and ammonia. The growth rate and efficiency of the DEGaCl-based growth decreases with increasing temperature when compared to trimethyl gallium (TMG)-based growth under similar conditions. Both low temperature buffer and the high temperature GaN layers were grown using the DEGaCI-NH3 precursor combination on the basal plane of sapphire and compared to similar structures grown using TMG and NH3. DEGaCl-based growth reveals an improved growth behavior under identical growth conditions to the conventional TMGa and ammonia growth. X-ray, Hall, and atomic force microscopy (AFM) measurements have been carried out on these samples providing a direct comparison of materials properties associated with these growth precursors. For the DEGaCl-based growth, the x-ray rocking curve line width, using the (0002) reflection, is as low as 300 arcsec on a 2.5-micron thick film. A RMS surface roughness of ∼0.5nm measured over a 10x10 micron area.

Tem Characterization of ZnO and AIN/ZnO Thin Films Grown on Sapphire

AbstractThin (~ 250 nm) films of ZnO grown by pulsed laser deposition on basal plane of sapphire were studied by transmission electron microscopy (TEM). Plan-view TEM study proved the films to be single crystal with the following epitaxial relationship with the substrate: (0001)znO || (0001)sap with the 30 30° in-plane rotation - [0110]ZnO || [1210]sap. Dislocations lying mostly in basal plane of ZnO and aligned along both <:1010> and <1120> directions having b=1/3[1120] were found. ZnO films were found to have layered growth morphology contrary to columnar morphology of III-nitrides. Consequently, the threading dislocation density in ZnO films (opposing to the AIN and GaN) drops very fast with the thickness: down to 107cm-2 at ~ 250 nm. The effect of post-annealing (which caused significant improvement in electrical and optical properties) on the microstructure of ZnO films was also studied. Contrary to the atomically sharp and clean interface in the as-deposited films, the post-annealed ZnO/sapphire interface contained reacted layer of 30 - 60 A thickness. The structure of the interlayer was determined to be ZnAl2O4 (spinel). The formation of this single crystal spinel layer did not cause deterioration of the ZnO film structure or properties. We have also explored the possibilities of using ZnO as a buffer for III-nitride growth. The epitaxial AIN films were grown on top of the ZnO layer by pulsed laser deposition. Thin (20 -60 A) interfacial reaction layer (also spinel ZnAm2O4) was observed between AIN and ZnO. Formation of this interlayer is studied in conjunction with the AIN epitaxy and the characteristics of defects and interfaces.

Ultraviolet Photodetectors Based on AlxGa1-xN Schottky Barriers

Schottky barrier photovoltaic detectors based on Si-doped AlxGa1-xN (0 ≤ × ≤ 0.22) have been fabricated and characterized. Samples were grown on basal plane sapphire by LP-MOVPE. Schottky contacts were made with Au. Responsivities are independent of the diode size and of the incident power in the range measured (10mW/m2 to 2KW/m2). The spectral response shows an abrupt cutoff that shifts linearly to higher energy with increasing Al content. A visible rejection of 3 to 4 orders of magnitude is observed in AlxGa1-xN Schottky photodiodes. Device time response is RC-limited, and a minimum decay time as short as 15ns have been estimated in unbiased Al0.22Ga0.28N diodes. This time response can be further reduced by reverse biasing.

Photoluminescence characterization of GaN thin film grown by atmospheric pressure organometallic vapor phase epitaxy

Thin films of gallium nitride (GaN) were successfully prepared by our atmospheric pressure (AP) organometallic vapor phase epitaxy (OMVPE) system using GaN buffer layer over basal plane (0001) sapphire substrates. In the present study, strong band-edge photoluminescence (PL) in GaN films is observed. The variations of the growth temperature and ammonia/trimethylgallium (NH 3/TMG) mass flow ratio with the full width at half maximum (FWHM) of band-edge luminescence ( I BE, respectively, indicate that we could obtain ranges of the growth temperature and the NH 3/TMG ratio to have high quality GaN single crystal. The band-edge to deep-level luminescence ( I DL) ratio ( I BE/ I DL) also reveals that we could obtain optimized appropriate growth temperature and the supply of active nitrogen of excellent optical properties.

GaN epitaxial growth on sapphire (0 0 0 1): the role of the substrate nitridation

The nitridation of sapphire substrates was monitored in situ by reflection high-energy electron diffraction. The evolution of the lattice-mismatch evidences the formation of an AlN relaxed layer when exposing the sapphire surface heated at 850°C to an ammonia flow. GaN thin films were grown by gas-source molecular beam epitaxy on variously nitridated sapphire substrates. High-resolution transmission electron microscopy study reveals the existence of two different epitaxial relationships between GaN and sapphire (0 0 0 1). The well-known orientation with the c-axis of the GaN crystal perpendicular to the Al 2O 3 surface is observed when starting the growth on a nitridated substrate. On the other hand, growing GaN directly on bare Al 2O 3 surfaces results in a different crystallographic orientation where the c-axis is tiled by about 19° with respect to the sapphire basal plane. Photoluminescence measurements show that both the intensity of the yellow-band emission (∼ 2.2 eV) and the residual donor-acceptor pair recombinations are affected by the nitridation state of the starting surface. The dependence of the optical properties of GaN thin films versus the NH 3 exposure time is then used to optimize the nitridation step.

Synthesis and Structure of a Novel Lewis Acid-Base Adduct, (H(3)C)(3)SiN(3).GaCl(3), en Route to Cl(2)GaN(3) and Its Derivatives: Inorganic Precursors to Heteroepitaxial GaN.

The formation of a novel Lewis acid-base complex between the silyl azide Si(CH(3))(3)N(3) and GaCl(3) having the formula (H(3)C)(3)SiN(3).GaCl(3)()()(1) is demonstrated. The X-ray crystal structure of 1 shows that the electron-donating site is the nitrogen atom directly bonded to the organometallic group. Compound 1 crystallizes in the orthorhombic space group Pnma, with cell dimensions a = 15.823(10) Å, b = 10.010(5) Å, c = 7.403(3) Å, and Z = 4. Low-temperature decomposition of 1 via loss of (H(3)C)(3)SiCl yields Cl(2)GaN(3) (2), which serves as the first totally inorganic (C,H-free) precursor to heteroepitaxial GaN by ultrahigh-vacuum chemical vapor deposition. A volatile monomeric Lewis acid-base adduct of 2 with trimethylamine, Cl(2)GaN(3).N(CH(3))(3) (3), has also been prepared and utilized to grow high-quality GaN on Si and basal plane sapphire substrates. The valence bond model is used to analyze bond lengths in organometallic azides and related adducts.

Stimulated emission at 300 K from photopumped GaN grown by plasma-assisted molecular beam epitaxy with an inductively coupled plasma source

GaN epilayers have been grown on basal plane (0001) sapphire by plasma-assisted molecular beam epitaxy (MBE) with a novel, inductively coupled nitrogen plasma source. Films grown at 700 °C generate stimulated emission at 300 K when optically pumped in vertical geometry with ∼3.5 eV (λ=355 nm) photons. The extrapolated pump power threshold is ∼3.6 MW cm−2 which corresponds to an absorbed value of 700 kW cm−2 and a peak carrier number density of ∼4×1019 cm−3.

Optical and magnetic resonance characterization of undoped and doped wurtzite GaN films deposited on sapphire substrates

We have used a combination of defect-sensitive techniques to study native defects and impurities in GaN films deposited on the basal plane of sapphire. The optical and electronic properties of undoped samples are dependent on the degree of compensation between shallow donor and acceptors levels. The paramagnetic studies of undoped films reveal resonances from effective-mass and deep-donor states. Photoluminescence studies of films doped with Mg-acceptors indicate a possible dependence of the localization of the Mg-related center with the increasing concentration of activated acceptors. The optically detected magnetic resonance measurements of Mg-doped samples show that the Mg-acceptor is perturbed from its effective-mass state.

Effect Of Mg, Zn, Si, And O On The Lattice Constant of Gallium Nitride Thin Films

AbstractThis study analyzes the impact of most common impurities and dopants on the c lattice parameter for thin films of Gallium Nitride (GaN) deposited on basal plane sapphire. Both Mg (∼1017 cm-3) and Zn (∼3 × 1020 cm-3) doping were found to expand the c lattice parameter as much as +0.38% and +0.62%, respectively. On the contrary, Oxygen up to concentrations 9 1021 cm-3 is shown to replace N in GaN thin films reducing the c parameter only by a small amount. Incorporation of Si leads to a large decrease of the c parameter which can not be attributed to the different size of Ga and Si atom. It is suggested that doping alters the film stoichiometry by a predicted Fermi level dependence of defect formation energies. The impact of stoichiometry on c lattice parameter and the effect of hydrostatic strain on resistivity in undoped and doped GaN is discussed.