Electron Cyclotron Resonance Molecular Beam Epitaxy Research Articles

Raman scattering in resonance with yellow luminescence transitions in GaN layers grown on sapphire by molecular beam epitaxy

We have carried out defect-induced Raman scattering in resonance with yellow-luminescence (YL) transitions in GaN layers grown on sapphire by electron cyclotron resonance-molecular beam epitaxy technique. The photoluminescence (PL) data shows strong excitonic peaks in the near-band-edge region apart from the weak and broad yellow-luminescence transitions. Raman spectra from various GaN samples prepared by this technique show strong and distinct Raman peaks in the low-energy region (90–260 cm −1). These peaks are attributed to electronic excitations of donors, which are involved in the yellow-luminescence transitions. Resonant enhancement of these peaks is seen as a function of temperature.

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

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.

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.

Operation of a compact electron cyclotron resonance source for the growth of gallium nitride by molecular beam epitaxy (ECR-MBE)

The operation of an ASTeX compact electron cyclotron resonance plasma source and its effect on the growth of GaN thin films by electron cyclotron resonancemolecular beam epitaxy has been investigated. The role a flow limiting orifice plays in increasing plasma stability as well as reducing ion damage and impurities in resultant films has also been studied. Both optical emission spectroscopy as well as electrostatic (Langmuir) probe studies have been employed to elucidate the generation and transport of charged and neutral species. With the introduction of the flow orifice, a substantial decrease in ion induced damage as well as surface roughening in the films is observed. This can be accounted for in terms of a collisionally induced relaxation of the grad-B acceleration of charged species toward the substrate in plasma sources employing axial solenoidal fields.

The optical properties and electronic transitions of cubi and hexagonal GaN films between 1.5 and 10 eV

The optical properties and electronic transitions of cubic (β) and hexagonal (α) GaN films grown by electron-cyclotron resonance molecular beam epitaxy were investigated in the energy region from 1.5 to 10 eV with conventional and synchrotron radiation ellipsometry. This is the first direct observation of the dielectric function of both polytypes in this energy region. The fundamental absorption of β-GaN is located at a lower energy than that of α-GaN. The observed structures are different for both phases and are analysed in the temperature range 80–650 K. With reference to electronic band structure calculations, the transitions are assigned to specific regions in the respective Brillouin zones. Finally, the reflectivities of α-GaN are β-GaN are compared with contradictory results in the literature.

Interfacial characteristics of AlGaAs after in situ electron cyclotron resonance plasma etching and molecular beam epitaxial regrowth

Regrown/processed AlGaAs interfaces using secondary ion mass spectrometry, cross section transmission electron microscopy (TEM), and reflection high energy electron diffraction have been characterized. Two sets of samples, GaAs/Al0.4Ga0.6As (with GaAs on top) and Al0.4Ga0.6As/GaAs (with Al0.4Ga0.6As on top), are used as starting materials. For the GaAs/Al0.4Ga0.6As samples that are first exposed to atmosphere, the experiment is performed in an integrated processing system where etching and regrowth chambers are linked together by ultrahigh vacuum transfer modules. The etching process includes electron cyclotron resonance (ECR) hydrogen plasma cleaning of GaAs native oxides, ECR SiCl4 plasma anisotropic deep etching into Al0.4Ga0.6As, and an optional, brief Cl2 chemical etching. Regrowth is carried out using solid-source molecular beam epitaxy (MBE). Despite the in situ processing, significant amounts of C, Si, and O impurities at the 10, 5, and 50×1012 cm−2 levels exist at the interfaces. However, the impurity level is one order of magnitude smaller than that in air-exposed, ECR plasma etched and MBE regrown Al0.4Ga0.6As/GaAs of the set 2 samples. As revealed using TEM, isolated small particles (presumably correlated to aluminium oxides) exist at the regrown/processed interface of the set 1 samples, but no other defects such as dislocation are seen. Impurities and defects are mainly caused by the high reactivity of AlGaAs during ECR plasma etching.