Modified Molecular Beam Epitaxy Research Articles

Investigation of MBE grown polycrystalline CdTe films on the Medipix readout chip

Cadmium Telluride (CdTe) films are directly deposited on a CMOS (complementary metal-oxide-semiconductor) based readout chip as sensor layer for X-ray detection. This is performed by using a modified Molecular Beam Epitaxy (MBE) setup with a carbon collimator enabling growth rates up to 10µm/h. To obtain a good contacting behaviour of the 25–50µm thick CdTe films, Te and Sb2Te3 are additionally evaporated during the process. The investigation of polycrystalline sensor layers deposited at 400°C with SEM (scanning electron microscopy) and XRD (X-ray diffraction) reveals a columnar growth of the individual grains oriented predominantly in (111). By PES (photoelectron spectroscopy) measurements the chemical composition of the different layers is identified in a depth profile and changes in work function along the contact structure are observed. Detector properties reveal a linear behaviour of the count rate with increasing radiation intensity as well as sensibility to holes and electrons. Spatial resolution measurements result in a resolution of 5 lp/mm, which is a mandatory requirement for medical applications.

Synthesis of the ferroelectric solid solution, Pb(Zr1−xTix)O3 on a single substrate using a modified molecular beam epitaxy technique

High-throughput synthesis of the ferroelectric solid solution Pb(Zr1−xTix)O3 (PZT) on single Pt∕Ti∕SiO2∕Si substrates was demonstrated using a modified molecular beam epitaxy (MBE) system. The PZT films exhibited a phase transition from rhombohehdral to tetragonal symmetry as a function of Zr:Ti ratio, across the substrate diagonal. This was consistent with the presence of a morphotropic phase boundary at Zr:Ti ratio of 0.64:0.36, different from the value of 0.53:0.47 observed for bulk ceramics. All points on the films exhibited ferroelectric hysteresis. Results demonstrate the feasibility of high-throughput MBE for deposition of complex ferroelectric oxides, and pave the way for further materials discovery.

Synthesis of the Ferroelectric Solid Solution, Pb(Zr1−xTix)O3 on a Single Substrate Using a Modified Molecular Beam Epitaxy Technique

AbstractHigh-throughput synthesis of the ferroelectric solid solution Pb(Zr1−xTix)O3 (PZT) on single Pt/Ti/SiO2/Si substrates was demonstrated using a modified molecular beam epitaxy (MBE) system. The PZT films exhibited a phase transition from rhombohehdral (R) to tetragonal (T) symmetry as a function of Zr:Ti ratio, across the substrate diagonal. This was consistent with the presence of a morphotropic phase boundary (MPB) at a Zr:Ti ratio of 0.64:0.36, different from the value of 0.53:0.47 observed for bulk ceramics. All points on the films exhibited ferroelectric hysteresis loops. The results demonstrate the feasibility of high-throughput MBE for deposition of complex ferroelectric oxides, and pave the way for further materials discovery, in particular Pb-free piezoceramics.

Atomic force microscopy study of sapphire surfaces annealed with a H[sub 2]O flux from a baffled molecular-beam epitaxy effusion cell loaded with Al(OH)[sub 3

We present an atomic force microscopy (AFM) study of sapphire surfaces that contain scratches with various severities. The objective was to observe the effects of substrate annealing at 850°C for 200min with a H2O-based overpressure resulting from an Al(OH)3 powder that was thermally cracked at 1200°C. The Al(OH)3 was decomposed into Al2O3 and H2O according to a partial Bayer process in a modified molecular-beam epitaxy (MBE) effusion cell, which was equipped with homemade baffles placed at its outlet. These homemade, simple-to-construct tantalum baffles allow for the selective outfluxing of gaseous species, from those that are solid based. A UTI™ 100C-model mass spectrometer was used to monitor the species present at the sapphire surface during annealing. Any aluminum-based solid species from the Al(OH)3 were not observed in the mass spectrum, although the H2O-based species were. The sapphire substrates were annealed in a Varian Gen II™ MBE system, with H2O beam equivalent pressures (BEPs) of 5×10−6 and 2×10−5Torr, as well as with no H2O flux at all. The AFM images show that the samples annealed with a higher H2O BEP of 2×10−5Torr had noticeably less severe surface scratches than the samples that were annealed with lower H2O BEPs.

Photoluminescence and optical quenching of photoconductivity studies on undoped GaN grown by molecular beam epitaxy

Deep levels in undoped GaN materials grown by modified molecular beam epitaxy (MBE) are investigated by photoluminescence (PL) and optical quenching of photoconductivity measurements. A broad band which extends from 2.1 to 3.0 eV with a maximum at about 2.7 eV is observed, and four prominent quenching bands were found located at 2.18, 2.40, 2.71, and 2.78 eV above the valence band, respectively. These levels are attributed to four holes trap levels existence in the material. The defects cannot be firmly identified at present.

Studies of Mg-GaN grown by MBE on GaAs(111)B substrates

This paper discusses the growth of Mg-doped GaN samples using a modified Molecular Beam Epitaxy (MBE) method. Our results suggest that the dopant is incorporated from a surface population maintained by the incident Mg flux by a rapid diffusion mechanism. It follows that the chemical concentration will increase with time of growth and that the p-doping level will also increase progressively with film thickness for a given Mg flux. Increasing the Mg flux to the surface results at first in a higher doping density, but this saturates when the Mg surface concentration reaches a finite value.

Optical properties of doped GaN grown by a modified molecular beam epitaxial (MBE) process on GaAs substrates

Both zinc-blende and wurtzite GaN layers were grown by a modified molecular beam epitaxy (MBE) method. These layers were grown on semi-insulating GaAs(100) substrates and doped with silicon and beryllium as n- and p-type dopants. The structure of the materials was confirmed using X-ray diffraction (XRD). Photoluminescence (PL) measurement at 77 K shows a single sharp emission at 3.473 eV for the wurtzite GaN doped with silicon, and this peak is shifted downwards in energy by about 200 meV in the zinc-blende GaN doped with silicon. The donor ionisation energy is estimated to be ∼ 30 meV. For beryllium in GaN, PL shows a broad emission at about 2.55 eV for the wurtzite material, while two lines at 2.90 eV and about 2.2 eV appear in the zinc-blende sample. An acceptor ionisation energy for zinc-blende GaN of 200 meV is estimated for the first time.

The growth and properties of group III nitrides

We have studied a novel material system (AlGa)(AsN), which can be lattice matched to GaP (or more importantly Si), grown using a low temperature modified molecular beam epitaxy (MBE) technique to reduce the density of native defects. Active nitrogen is provided by an Oxford Applied Research, RF activated plasma source. This source has enabled us to prepare binary films of GaN and InN and alloy layers of Ga(AsN) and In(AsN) at growth rates of approximately 0.3 monolayers/s. The films have been studied using in-situ reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES) and ex-situ using X-ray diffraction, C–V profiling and photoluminescence/excitation measurements. We have obtained clear evidence for the existence of films with significant concentrations of N (∼ 20%), using appropriate growth conditions.

Group IV element (Si, Ge and α-Sn) superlattices — low temperature MBE

Si/Ge and Sn/Ge heterostructures and short-period superlattices are grown by a modified molecular beam epitaxy (MBE) technique on Si and Ge substrates. Low energy electron diffraction and Auger electron spectroscopy are used in order to optimize the growth conditions with respect to interface roughness, segregation and intermixing. Transmission electron micrographs reveal that the interface quality of short period superlattices is improved by substrate temperature modulation during deposition. For Si/Ge superlattices we use a temperature range between 250 and 400°C. However, due to the extreme tendency of Sn to segregate on the film surface, it is necessary to further decrease the substrate temperature during overgrowth of the Sn layers. The optimized growth conditions for high-quality Sn/Ge superlattices were found to be in the temperature range between 45 and 300°C. It is demonstrated that low temperatures, low growth rates, and temperature variations during deposition of the individual layers in MBE open the possibility to synthesize structures far beyond thermodynamic equilibrium conditions.

Comparison of Growth and Strain Relaxation of Si/Ge Superlattices Under Compressive and Tensile Strain Field

ABSTRACTOptimization of growth parameters of short period Si/Ge superlattices (SLs) has been achieved via in situ low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) measurements during homo- and heteroepitaxy on Si (001) and Ge (001) substrates. Transmission electron microscopy (TEM) reveals that pseudomorphic SimGe12-m (m = 9 and 3 for growth on Si and Ge, respectively) SLs with extended planar layering can be prepared almost defect-free by a modified molecular beam epitaxy (MBE) technique. Whereas the SLs on Ge can be deposited at a constant substrate temperature, high-quality growth on Si demands for temperature variations of more than 100°C within one superlattice period. Strain relaxation of these SLs with increasing number of periods has been directly compared by means of TEM. For the compressively strained structures grown on Si we found misfit dislocations of the type 60° (a/2)<110>. Under opposite strain conditions i.e. for growth on Ge, strain relief occurs only by microtwin formation through successive glide of 90° (a/6)<211> Shockley partial dislocations. This is consistent with a calculation of the activation energy for both cases based on a homogeneous dislocation nucleation model.

Dynamical X-Ray Diffraction Studies of Interfacial Strain in Superlattices Grown by Molecular Beam Epitaxy

ABSTRACTDynamical X-ray diffraction studies have been carried out for lattice-matched InGaAs/InP superlattices grown by modified molecular beam epitaxy (MBE) techniques. The (400) X-ray satellite pattern, which is predominantly affected by the strain modulation, was analyzed. The strain and thickness of the actual layers including the presence of strained interfacial regions were determined.

Growth of cubic phase gallium nitride by modified molecular-beam epitaxy

Gallium nitride is a compound semiconductor with a wide direct band gap (3.45 eV) and a large saturated electron drift velocity. Nearly all single-crystal thin films grown to date have been wurtzite (hexagonal) structure. Cubic GaN has the potential for higher saturated electron drift velocity and somewhat lower band gap. These properties could increase its applicability for high-frequency devices (such as impact ionization avalanche transit time diodes) as well as short-wavelength light emitting diodes and semiconductor lasers. This paper reports the growth of cubic phase single-crystal thin-film GaN using a modified molecular-beam epitaxy technique. A standard effusion cell was used for gallium, but to activate nitrogen gas prior to deposition, a microwave glow discharge was used. Auger electron spectroscopy showed a nominally stoichiometric GaN film. Transmission electron microscopy with selected area diffraction indicated the crystal structure to be zinc blende.