Cyclotron Resonance Molecular Beam Epitaxy Research Articles

Effect of Nitridation on Crystallinity of Polycrystalline GaN Grown on Silica Glass by ECR‐MBE

Polycrystalline GaN with improved c-axis orientation is successfully grown on silica glass substrates by electron cyclotron resonance molecular beam epitaxy after nitridation of the substrate surface. This substrate nitridation gives rise to the formation of a silicon nitride layer, and results in a marked improvement in the uniformity of c-axis orientation and the surface morphology of subsequently grown polycrystalline GaN.

Local Ordering in GaN-Rich Ternary GaNP Alloys

ABSTRACTUsing micro-Raman scattering we have studied atomic distribution and the ordering effects in GaN-rich GaNP layers on (0001) sapphire substrates grown by electron cyclotron resonance molecular beam epitaxy (ECR-MBE). Raman spectrum from layers grown at higher temperatures (≥700°C) shows coexistence of GaP-rich region with cubic symmetry and GaN-rich region with hexagonal symmetry. Sharp TO and LO phonon lines, indicative of {111} ordering in the GaP region are observed. Increasing phosphorous (P >1.5%) in GaNP alloy leads to phase separation that is reflected in the suppression of GaN-like Raman modes. The phase-separated region shows an additional Raman line at 384 cm−1 between the TO and LO phonon of GaP due to strongly confined LO phonon in ordered {111} (GaP)n(GaN)m nanometer size clusters. Decreasing growth temperature increases the phosphorous concentration in the GaNP layer and disorder activated acoustic modes appear in the Raman spectrum as a result of symmetry break down. When phosphorous concentration reaches 8.2 % in the layer grown at 570°C, Raman spectrum shows broad amorphous like bands indicative of short-range ordering

Mg doping studies of electron cyclotron resonance molecular beam epitaxy of GaN thin films

A modified electron cyclotron resonance plasma source, allowing optically active GaN thin films at growth rates up to 1 μm per h, was utilized in a molecular beam epitaxy environment for Mg doping studies of GaN. The effect of the growth parameters on the Mg incorporation was studied using secondary ion mass spectroscopy and Hall measurements. We found that Mg does not incorporate when GaN is grown under Ga-rich conditions, independently of the Mg flux, in the temperature range of 700–800 °C. Only N-rich growth conditions allow measurable Mg incorporation. This incorporation increases with decreasing Ga cell temperature. Furthermore, under N-rich growth conditions, the Mg content is more sensitive to the lowering of the substrate temperature than to the increase of the Mg cell temperature.

Optical properties of GaN layers grown on C-, A-, R-, and M-plane sapphire substrates by gas source molecular beam epitaxy

GaN layers were grown on C-, A-, R-, and M-plane sapphire substrates by the electron cyclotron resonance–molecular beam epitaxy technique. We addressed a combined utilization of Raman spectroscopy, photoluminescence (PL) and reflectance measurements to investigate the optical properties of these high-quality GaN layers. First order optical phonons of A1, E1, and E2 symmetries were observed in the Raman spectra and the peaks are indicative of the wurtzite crystal structure. All three intrinsic exciton transitions arising from A, B, and C interband transitions were observed in reflectance measurements. The PL spectra were dominated by A and B free exciton transitions and the recombination of an exciton bound to a neutral donor. The experimental data clearly revealed a thickness-dependent change of the biaxial strain in the GaN layers grown on (0001) C-plane sapphire. The residual strain induced in these layers was found to have a strong influence in determining the energies of the excitonic transitions. Resonant Raman scattering measurements were performed by temperature tuning of fundamental gap in 1.0 μm GaN on C-plane sapphire. The influence of epitaxial strain in free exciton properties of GaN layers grown on various orientations of sapphire has been discussed based on the PL and reflectance results. The exciton binding energies were estimated in the GaN layers grown on C-, A-, and M-plane sapphire substrates. Polarized Raman measurements were performed on GaN layers grown on various orientations of sapphire and we observed quasipolar modes of both E1 and A1 symmetries. An additional broad photoluminescence band centered around 2.74 eV was observed in the GaN layers grown on R- and M-plane sapphire substrates. The defect induced Raman scattering in resonance with this band shows strong Raman scattering peaks resulting from the transition between energy levels of donor species or defect states.

In situ surface composition and structure of InGaN and GaN thin films by time-of-flight mass spectroscopy of recoiled ions

Time-of-flight mass spectroscopy of recoiled ions (TOF–MSRI) is used to determine the surface chemical composition and termination of GaN and InGaN thin films grown by gas source and electron cyclotron resonance molecular beam epitaxy (GS–MBE and ECR–MBE). We show that using TOF–MSRI all the critical growth steps, the nitridation, the buffer layer and the epilayers can be optimized in real time. In the case of GS–MBE, the ammonia pressure can be, reproducibly and easily, adjusted to achieve the highest N surface composition at the minimum corrosive ammonia flow. For InGaN the total TOF–MSRI ion counts drop with increasing In content. Such an observation can be applied to evaluate the thin film surface morphology in addition to its surface composition. Finally, we present preliminary data showing the use of TOF–MSRI for in situ GaN surface structure determination. We also show that by using a reflectron ion analyzer, much higher ion counts and better resolution can be achieved than a conventional electrostatic sector system. With such a modification, dopant level sensitivities should be achievable and data rates compatible with closed loop process control algorithms become possible.

Field emission properties of GaN films on Si(111)

GaN thin films were grown by electron cyclotron resonance molecular beam epitaxy on Si(111) wafers. X-ray diffraction and transmission electron microscopy revealed that the thin films were single crystals with a hexagonal symmetry and a clear textured structure. The average column size was determined to be close to 100 nm in diameter. Despite the large defect density, a strong room temperature photoluminescence signal with a full width at half maximum of 138 meV was observed from these samples. The surface exhibited random array of sharp tips at the microscopic level with about 5×109 tips/cm2 density. The field emission characteristics of the as-grown thin films were measured, and a threshold electric field as low as 30–40 V/μm and an emission current density of more than 100 mA/cm2 were obtained.

Very low resistance ohmic contacts to n-GaN

Ohmic contacts with low resistance are fabricated on n-GaN films using Al/Ti bilayer metallization. GaN films used are 0.3 µm thick layers with carrier concentrations of 1 × 1019 cm−3 grown on the c-plane sapphire by ion-removed electron cyclotron resonance molecular beam epitaxy. The lowest value for the specific contact resistivity (ρc) of 1.2×10−8 Ω·cm2 was obtained with furnace annealing at 500°C for 60 min. This result shows the effectiveness of high carrier concentration GaN layers and the low temperature annealing for the realization of low resistance ohmic contacts. Sputtering Auger electron spectroscopy analysis reveals that Al diffuses into Ti layer and comes into contact with the GaN surface.

GaN-Rich Side of GaNAs Grown by Gas Source Molecular Beam Epitaxy

A large variation in wavelength from the ultraviolet to longer than 2 µm could be achieved in the GaN-rich side of the GaN1-xAsx alloy due to the large bowing of bandgap energy. Layers of GaN1-xAsx are grown on (0001) sapphire substrates by electron cyclotron resonance molecular beam epitaxy (ECR-MBE) using an ion-removed ECR radical cell after the growth of GaN buffer layers. During the growth of GaN1-xAsx layers, a streaky reflection high-energy electron diffraction (RHEED) pattern was observed. The excitonic photoluminescence (PL) peak from the GaN-rich side of the GaN1-xAsx layer shows a large red shift as the As content changes. When an As content of up to x=0.009 is attained, a bandgap bowing parameter of 19.6 eV is experimentally obtained. Such a large value of the bowing parameter is promising for applications to optical devices operating over wide range of wavelength.

Time of Flight Mass Spectroscopy of Recoiled Ions Studies of Gallium Nitride Thin Film Deposition by Various Molecular Beam Epitaxial Methods

Gallium Nitride (GaN) thin films were successfully grown by electron cyclotron resonance molecular beam epitaxy (ECR-MBE), gas source MBE (GSMBE), and chemical beam epitaxy (CBE). Time of flight mass spectroscopy of recoiled ions (TOF-MSRI) and reflection high energy electron diffraction (RHEED) were used in-situ to determine the surface composition, crystalline structure, and growth mode of GaN thin films deposited by the three MBE methods. The substrate nitridation and the buffer layers were monitored and optimized by TOF-MSRI and RHEED. For GSMBE, the gallium to nitrogen ratio is found to correlate well with ex-situ optical properties. In the case of CBE, carbon incorporation determines the surface morphology, crystalline quality and optical activity of the epilayers.

Gas Source Molecular Beam Epitaxy Growth of GaN-Rich Side of GaNP Alloys and Their Observation by Scanning Tunneling Microscopy

The GaN-rich side of a GaNP alloy exhibits a potentially large variation in band-gap energy with P content due to its large bowing. To study the phase separation observed in GaNP grown on a (0001) sapphire substrate with a P content larger than 0.015, GaNP layers are grown on (111)A substrates by electron cyclotron resonance molecular beam epitaxy (ECR-MBE). During the growth of GaNP layers, reflection high energy electron diffraction (RHEED) exhibits additional spotty patterns indicating phase separation as well as (1×1) streaks. Scanning tunneling microscopy (STM) images on the phase-separated samples show the bright clusters with about 3 nm in size, which correspond to the phase-separated GaP-rich region. I-V curves on the bright clusters are quite different from those on other areas indicating a lower band-gap energy.

Gas source MBE growth of GaN rich side of GaN1 − P using ion-removed ECR radical cell

Abstract GaN-rich side of GaN 1 − x P x alloy are grown on (0 0 0 1) sapphire substrates by electron cyclotron resonance molecular beam epitaxy (ECR-MBE) using an ion removed ECR radical cell after the growth of high-quality GaN layers. GaN 1 − x P x exhibits potentially large variation of band-gap energy with P composition due to its large bowing. Maximum P composition of x = 0.015 is obtained. However, at the growth condition of high PH 3 flow rate, phase separation into GaN-rich GaN 1 − x P x and GaP-rich GaP 1 − y N y is observed. Near band edge excitonic photoluminescence (PL) peak from GaN-rich side of GaN 1 − x P x layers shows large red shift with P composition. It suggests that the direct band edge of GaNP alloy shows large bowing.

Optical and structural properties of GaN films grown on c-plane Al2O3 by electron cyclotron resonance molecular beam epitaxy

Optical and structural properties of undoped GaN films grown on Al2O3 (0 0 0 1) substrates under various nitrogen pressure and plasma power by electron cyclotron resonance molecular beam epitaxy were investigated by photoluminescence (PL) spectroscopy and X-ray diffraction (XRD). It was found that the optimum nitrogen pressure determined by full width at half maximum of XRD peak was in just agreement with that by PL spectra, and the GaN film grown under the optimum nitrogen pressure contains high density of dislocations. These results suggest that the optical quality is sensitive to the stress in the film and that the relaxation of stress depends on V/III ratio. We also found that the effects of increasing nitrogen pressure are not equivalent to the effects of increasing plasma power though they have been regarded as having the same effects.