Plasma-assisted Molecular Beam Epitaxy Technique Research Articles

Effect of two-step cubic GaN buffer layer on the radio-frequency plasma-assisted molecular beam epitaxy growth of cubic AlN films grown on MgO (001) substrates

Cubic AlN (c-AlN) films were grown on MgO (001) substrates through the use of a two-step cubic GaN (c-GaN) buffer layer using radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE) technique. The two-step c-GaN buffer layer was found to be crucial for the epitaxial growth of c-AlN, as it showed a specific epitaxial relationship between c-AlN and c-GaN along the MgO [100] zone axis. On the other hand, the c-AlN films grown with the one-step and nonGaN buffer layers showed columnar grain structure. The optical study found that the c-AlN films grown with the two-step c-GaN buffer layer had an indirect bandgap energy of 4.99 ± 0.02 eV, which is consistent with previous experimental data and theoretical calculations. The c-AlN films grown with the one-step and nonGaN buffer layers had indirect bandgap energies of 5.00 ± 0.02 eV and 5.18 ± 0.02 eV, respectively. These results highlight the importance of the two-step c-GaN buffer layer in enhancing the MBE growth of c-AlN on MgO (001) substrate.

Correlated carrier transport and optical phenomena in CdO layers grown by plasma-assisted molecular beam epitaxy technique

In this work, we have investigated a series of CdO layers grown by the plasma-assisted molecular beam epitaxy technique on the m-plane sapphire substrate. The relation between Cd and O2 parameters during growth influences the stoichiometry of the CdO layers and it affects morphological, optical, and electrical properties which are manifested in the roughness parameter, lattice constant, optical bandgap, and electrical parameter of the layers. The surface morphology and structural properties of CdO layers were studied using atomic force microscopy and X-ray diffraction respectively. Temperature-dependent Hall measurement revealed the maximum mobility of 352 cm2V−1s−1 achieved at room temperature with a carrier concentration of about 4.5 × 1019 cm−3. The optical properties and bandgap of CdO layers were investigated using UV–Visible spectroscopy at room temperature. It is observed that the shift of the bandgap depends upon the carrier concentration and is acquired by combining the effect of bandgap widening and bandgap narrowing.

Effective Mg Incorporation in CdMgO Alloy on Quartz Substrate Grown by Plasma-Assisted MBE

The development of CdMgO ternary alloy with a single cubic phase is challenging but meaningful work for technological advancement. In this work, we have grown a series of Cd1-xMgxO ternary random alloys with various Mg concentrations (x = 0, 30, 32, 45, and 55%) on quartz substrate by plasma-assisted molecular beam epitaxy (PA-MBE) technique. The structural investigations of alloys were performed using the X-ray diffraction (XRD) technique. The decreases in average crystallite size and lattice parameters were observed with an increase in Mg content in the alloys. XRD analysis confirms a single cubic phase is obtained for alloy compositions. The elemental and morphological studies were carried out using energy dispersive x-ray (EDX) spectroscopy and atomic force microscope (AFM) technique, respectively. The optical investigation was carried out using UV-Vis spectroscopy. The optical bandgaps were estimated using the Tauc relation and it was varied from 2.34 eV to 3.47 eV by varying the Mg content from zero to 55% in the alloys. The Urbach energy increases from 112 meV to 350 meV which suggests a more disordered localized state with an increase in Mg incorporation in the alloys.

Investigating defects in InGaN based optoelectronics: from material and device perspective

III-nitride optoelectronics have revolutionized solid-state lighting technology. However, non-radiative defects play a major bottleneck in determining the performance of InGaN-based optoelectronics devices. It becomes especially challenging when high indium is required to be incorporated to obtain emission at higher wavelength (>500 nm). In this research article, we are going to discuss our investigation on the origin of defects in InGaN-based optoelectronics devices from the material and device perspective and characterize them through various techniques. This article broadly consists of two parts. In the first part, we investigate defects in InGaN based optoelectronics from a material point of view. Here, we discuss the challenges in the growth of InGaN planar (2-dimensional) and nanowires (1-dimensional) with high indium (≥20%) incorporation using the plasma-assisted molecular beam epitaxy (PA-MBE) technique. Photoluminescence spectroscopy (PL) has been performed to characterize these grown samples to assess their optical quality. Atomic force microscopy (AFM) has been employed to characterize the surface morphology of grown InGaN layers. High-resolution transmission electron microscopy (HRTEM) and scanning electron microcopy (SEM) are also used to characterize InGaN planar and nanowire samples grown under various process conditions. In the second part, we investigate the role of defects on InGaN optoelectronics from a device point of view. Here, we discuss the fabrication of InGaN multi-quantum well-based light emitting diodes (LEDs). Temperature-dependent current versus voltage measurements are carried out to investigate the role of defects on carrier dynamics under forward and reverse bias conditions. Frequency-dependent capacitance versus voltage (CV) and conductance versus voltage (GV) techniques are employed extensively to characterize defects in fabricated InGaN LEDs.

Pressure-dependent bandgap study of MBE grown {CdO/MgO} short period SLs using diamond anvil cell

Semiconductor superlattices (SLs) have found widespread applications in electronic industries. In this work, a short-period SL structure composed of CdO and MgO layers was grown using a plasma-assisted molecular beam epitaxy technique. The optical property of the SLs was investigated by absorption measurement at room temperature. The ambient-pressure direct bandgap was found to be 2.76 eV. The pressure dependence of fundamental bandgap has been studied using a diamond anvil cell technique. It has been found that the band-to-band transition shifts toward higher energy with an applied pressure. The bandgap of SLs was varied from 2.76 to 2.87 eV with applied pressure varied from 0 to 5.9 GPa. The pressure coefficient for the direct bandgap of SLs was found to be 26 meV/GPa. The obtained experimental result was supported by theoretical results obtained using density functional theory calculations. The volume deformation potential was estimated using the empirical rule. We believe that our findings may provide valuable insight for a better understanding of {CdO/MgO} SLs toward their future applications in optoelectronics.

Growth of uniform Mg-doped p-AlGaN nanowires using plasma-assisted molecular beam epitaxy technique for UV-A emitters

In this manuscript, we have shown the growth and extensive structural and optical characteristic of the uniformly Mg-doped Al0.23Ga0.77N (UV-A region, λ ∼ 323 nm) nanowire. The Kelvin probe force microscopy was employed to determine the profile of holes in p-type AlGaN nanowires by measuring the work function changes induced by Mg incorporation. The influence of surface band bending on doping concentration has thoroughly been discussed. Our experiment confirms the homogeneous incorporation of Mg throughout the nanowire without any top surface Mg segregation. In this work, we have also demonstrated a comprehensive analysis of acceptor states induced thermal quenching behaviour in the optical transition of Mg-doped AlGaN nanowire. We propose a phenomenological model, based on the rate equation which confirms that achieving higher ‘hole’ (p-doping) concentration in AlGaN nanowire (>1018 cm−3) is more conducive than the planar counterpart if the growth of NWs is carried out at optimized process conditions. This rate equation-based model has also demonstrated the influence of sidewall surface passivation in those AlGaN nanowires.

Ohmic-Schottky conversion of ZnO/metal contact modulated by a plasma surface treatment method

Herein, we present a systematical investigation with regard to the Ar/N2/O2 plasma surface treatments on the ZnO thin films prepared by a radio frequency plasma-assisted molecular beam epitaxy (RF MBE) technique. The as-deposited ZnO thin films are exposed to the Ar, N2 and O2 plasma atmospheres under different RF powers. The conductivity and the surface morphology of the post-treated ZnO thin films are strongly depending on the treatment species and the RF powers. Higher RF power results in lower resistance and reduces the grain boundaries of the surface. Indium and gold metals are designed as the Ohmic and Schottky contact electrodes, respectively. The ZnO/Au contacts based on both pre-treated films and Ar/N2 plasma-treated films show obvious Ohmic contact behaviors. However, In/ZnO/Au the devices fabricated on the ZnO thin film which exposes to the O2 plasma show distinct rectified behavior, indicating the conversion of ZnO/Au contact from Ohmic type to Schottky type. The O2 plasma treatment may play two key roles within the contact conversion: 1, removing the surface contaminants such as carbon, absorbed oxygen and hydroxide; 2, repairing the defects (Vo), thus reducing the near surface donor density and defect assisted hopping transportation.

A study of the GaN/Si(111) epitaxial structures grown by PA MBE via coalescence overgrowth of GaN nanocolumns

The GaN/Si(111) epitaxial structures were synthesized by coalescence overgrowth of GaN nanocolumns using the plasma-assisted molecular beam epitaxy (PA-MBE) technique. Such epitaxial structures can be used as a buffer layer for obtaining high quality GaN epilayers. Structural, electrical and chemical properties of these samples were studied. It was demonstrated that KOH etching of the grown GaN/Si(111) samples results in the separation of the GaN epilayer from the substrate.

Determining the polarity of droplet epitaxy grown AlGaN nanorods using piezoresponse force microscopy

Al0.97Ga0.03N nanorods are grown by the plasma-assisted molecular beam epitaxy technique under extreme group III rich condition. Droplet epitaxy process is employed for the formation of well-faceted nanorods with smooth morphology. We report the polarity of Al0.97Ga0.03N nanorods oriented in different crystallographic directions by the non-destructive technique of piezoresponse force microscopy (PFM). The phase formation is investigated by the Raman spectroscopic analysis. Spatial distribution of piezoresponse of Al0.97Ga0.03N nanorods is mapped, and the polarities of the different crystalline planes of m and c are determined. The corresponding piezoresponse is explained on the basis of spontaneous polarization in a unit cell of the hexagonal crystal, which is further confirmed using piezoresponse spectroscopy. Piezoelectric coefficient (d33) is measured to be ∼9.0 ± 1 pm/V in Al0.97Ga0.03N nanorods using the PFM technique.

Highly efficient p-type doping of GaN under nitrogen-rich and low-temperature conditions by plasma-assisted molecular beam epitaxy

Highly efficient and reproducible p-type doping of GaN under nitrogen-rich and low-growth-temperature conditions was demonstrated with the plasma-assisted molecular beam epitaxy technique. The low-temperature range is approximately below 650 °C and refers to growth temperatures at which the thermal desorption of any excess Ga is negligibly slow. The Mg and hole concentrations obtained with the N-rich condition were more than one order of magnitude higher than those obtained with the Ga-rich condition while keeping all other conditions identical. The Mg doping under such N-rich conditions was also found to show Mg-mediated suppression of background impurities, good epitaxy quality on GaN templates, and relatively low surface roughness. Over the investigated growth temperature range from 580 °C to 650 °C, the Mg incorporation efficiency under the N-rich condition was found to be close to unity (70%-80%) and independent of the growth temperature. High hole concentrations of up to 2×1019 cm-3 and activation efficiencies of up to 16.6% were obtained. The result rules out the Mg surface sticking probability as the limiting mechanism for Mg incorporation in this temperature range, as it would be temperature dependent. Instead, the Mg incorporation rate was more likely governed by the availability of substitutional sites for Mg on the surface, which should be abundant under the N-rich growth conditions. Excellent diode characteristics and electroluminescence results were observed when this p-type doping method was employed in the growth of full device structures.

A Method of Combining the Increased Density of Acceptors with Restrained Density of Oxygen Vacancies to Fabricate p-Type Single-Crystalline ZnO Films

Single-crystalline ZnO films with good crystal quality were grown by plasma-assisted molecular beam epitaxy (MBE) technique on c-plane sapphire substrates and implanted with fixed energy of 180-keV P and 100-keV O ions at 460°C. The implanted single-crystalline ZnO films exhibit p-Type characteristics with hole concentration in the range of 5.3 × 1017–1.5 × 1018 cm−3, hole mobilities between 1.4 cm2V−1 s−1 and 2.1 cm2V−1 s−1, and resistivities in the range of 0.672–1.832 Ωcm, as confirmed by Hall-effect measurements. The x-ray diffraction pattern of the implanted single-crystalline ZnO films exhibits (002) orientation (c-plane), with no other secondary phase appearing after ion implantation and dynamic annealing. It is deduced from x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy results that P ions were effectively implanted and formed acceptor complex PZn–2VZn, acting predominantly in all acceptors and achieving the goal of increasing the density of acceptors. Raman spectra and XPS results reflect that the enhanced solubility and stability of acceptor complexes in implanted single-crystalline ZnO films are related to the reduction of the concentration of oxygen vacancies by O ion implantation, achieving the goal of restraining the density of oxygen vacancies. It is concluded that the method of combining the increased density of acceptors and the restrained density of oxygen vacancies is meaningful and feasible, and afforded excellent p-type characteristics.

P-type single-crystalline ZnO films obtained by (Na,N) dual implantation through dynamic annealing process

Single-crystalline ZnO films were grown by plasma-assisted molecular beam epitaxy technique on c-plane sapphire substrates. The films have been implanted with fixed fluence of 130 keV Na and 90 keV N ions at 460 °C. It is observed that dually-implanted single crystalline ZnO films exhibit p-type characteristics with hole concentration in the range of 1.24 × 1016–1.34 × 1017 cm−3, hole mobilities between 0.65 and 8.37 cm2 V−1 s−1, and resistivities in the range of 53.3–80.7 Ω cm by Hall-effect measurements. There are no other secondary phase appearing, with (0 0 2) (c-plane) orientation after ion implantation as identified by the X-ray diffraction pattern. It is obtained that Na and N ions were successfully implanted and activated as acceptors measured by XPS and SIMS results. Also compared to other similar studies, lower amount of Na and N ions make p-type characteristics excellent as others deposited by traditional techniques. It is concluded that Na and N ion implantation and dynamic annealing are essential in forming p-type single-crystalline ZnO films.

P-type single-crystalline ZnO films obtained by (Li, N) dual implantation through dynamic annealing process

Single-crystalline ZnO films were grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy technique. The films have been implanted with fixed fluence of 30 keV Li and 80 keV N ions at 460 °C. It is shown that dually-implanted single crystalline ZnO films exhibit p-type characteristics with hole concentration in the range of 2.5 × 1015–2.3 × 1016 cm−3, hole mobilities between 0.8 and 1.04 cm2 V−1 s−1, and resistivities in the range of 328.9–2081 Ω cm by Hall-effect measurements. The ZnO film exhibit (0002) (c-plane) orientation with no secondary phase appearing after implantation as identified by the X-ray diffraction pattern. It is confirmed that Li and N ions were effectively implanted and successfully behave as acceptor measured by XPS and SIMS results. Also compared to other similar studies, lower amount of Li and N ions make p-type characteristics excellent. It is concluded that Li and N ion implantation and dynamic annealing play a critical role in forming p-type single-crystalline ZnO films.

P-type single-crystalline ZnO films obtained by (N,O) dual implantation through dynamic annealing process

Single-crystalline ZnO films were grown on a-plane sapphire substrates by plasma-assisted molecular beam epitaxy technique. The films have been implanted with fixed fluence of 120 keV N and 130 keV O ions at 460 °C. Hall measurements show that the dually-implanted single-crystalline ZnO films exhibit p-type characteristics with hole concentration in the range of 2.1 × 1018–1.1 × 1019 cm−3, hole mobilities between 1.6 and 1.9 cm2 V−1 s−1, and resistivities in the range of 0.353–1.555 Ω cm. The ZnO films exhibit (002) (c-plane) orientation as identified by the X-ray diffraction pattern. It is confirmed that N ions were effectively implanted by SIMS results. Raman spectra, polarized Raman spectra, and X-ray photoelectron spectroscopy results reflect that the concentration of oxygen vacancies is reduced, which is attributed to O ion implantation. It is concluded that N and O implantation and dynamic annealing play a critical role in forming p-type single-crystalline ZnO films.

Structural and optical properties of β-Ga2O3 thin films grown by plasma-assisted molecular beam epitaxy

Epitaxial beta-gallium oxide (β-Ga2O3) has been deposited on c-plane sapphire by plasma-assisted molecular-beam epitaxy technique using two methods. One method relied on a compound Ga2O3 source with oxygen plasma while the second used elemental Ga source with oxygen plasma. A side-by-side comparison of the growth parameters between these two methods has been demonstrated. With various substrate temperatures, pure phase (2¯01) oriented β-Ga2O3 thin films were obtained using both sources. Reflection high energy electron diffraction patterns displayed a threefold reconstruction during the growth. X-ray photoelectron spectroscopy analysis showed a shift in the binding energy of the Ga 2p peaks consistent with a Ga being in a +3 oxidation state. For transparent oxide like β-Ga2O3, it is important to determine the index of refraction (n) and its functional dependence on the wavelength. The Cauchy dispersion relation was employed to evaluate the refractive index, film thickness, roughness values, and extinction coefficient. The band gap energies of the thin films were calculated to be ∼5.02 eV by extrapolating the linear portion of Tauc-plot analysis and the refractive index is ∼1.89 at the wavelength (λ) of 6328 Å, suggesting high structural quality and packing density of the oxide films.

Deep traps and photo-electric properties of p-Si/MgO/n-Zn1−xMgxO heterojunction

In the paper, the photoluminescence (PL) measurements, current–voltage–temperature (I-V-T) measurements, space charge techniques (C-V and deep level transient spectroscopy (DLTS)), and photocurrent spectral characteristics have been applied to investigate defects in p-Si/MgO/ n-Zn1−xMgxO heterojunction (HJ). The HJ structure was grown on p-type Si (111) substrate with resistivity equal to 0.1 Ω cm by the plasma-assisted molecular beam epitaxy technique. A radio-frequency cell was used for the generation of oxygen plasma. PL spectrum let us determine the Mg content ∼10%. Besides the excitonic Zn0.9Mg0.1O line, the PL spectrum also contains green and yellow emission bands indicating the presence of defect states in the investigated structures. I-V measurements reveal the rectifying properties of the HJ and the current thermally activated with a trap with the activation energy equal to 0.42 eV. DLTS studies yield the majority trap of the activation energy 0.42 eV, confirming the result obtained from the I-V measurements. It was found that the defects related to this trap have a point like behaviour. A spectral characteristic of the photocurrent shows that the p-Si/MgO/n-Zn1−xMgxO HJ may be applied as a photodiode operating within the wavelength range of 300 nm-1100 nm. The dark current transport and photocurrent spectrum were explained using the Anderson model of a HJ.

Properties of ZnO single quantum wells in ZnMgO nanocolumns grown on Si (1 1 1)

In this paper, we report a method of growing of the catalyst-free self-organized ZnMgO nanocolumns with single quantum well on Si (111) substrates by plasma-assisted molecular beam epitaxy technique (PA-MBE). The structures were grown without buffer layers. Optical properties of the ZnMgO/ZnO/ZnMgO quantum wells were studied by photo (PL)- and cathodoluminescence (CL) spectroscopy. A detailed analysis of the optical properties has been carried out, including quantum confinement effect and temperature dependence of excitonic emission. The structures reveal intense near band edge emission in PL as well as in CL. Blue shift of excitonic emission from the wells in comparison to bulk ZnO due to the quantum confinement effect is observed. Cross-sectional SEM–CL mapping shows that the ZnO/ZnMgO single quantum wells with different well widths are located in ZnMgO nanocolumns. The crystalline quality of the heterostructure was characterized by X-ray diffraction (XRD). No phase separation in ZnO/ZnMgO quantum structures was found.

Properties of Pt Schottky Contact on Porous In<sub>0.27</sub>Ga<sub>0.73</sub>N Thin Film Revealed from <i>I-</i><i>V</i> Measurements

The electrical properties of a Pt Schottky contact on porous In0.27Ga0.73N/GaN/AlN/Si (111) thin film that was grown via the plasma-assisted molecular beam epitaxy technique were reported. Porous film nanostructure was synthesized using the electrochemical etching technique at a current density of 25 mA/cm2. The formed pores were dissimilar in terms of size and shape. The effects of annealing temperature and applied bias on Schottky contact for porous sample were investigated by current-voltage (IV) measurements in ambient illumination. The barrier height and ideality factor were determined. The Pt Schottky contact exhibits thermal stability during annealing. Schottky barrier height increased when each of the annealing temperature and bias voltage were increased.

Deep traps in n-type GaN epilayers grown by plasma assisted molecular beam epitaxy

In this study, we present the results of investigations on Schottky Au-GaN diodes by means of conventional DLTS and Laplace DLTS methods within the temperature range of 77 K–350 K. Undoped GaN layers were grown using the plasma-assisted molecular beam epitaxy technique on commercial GaN/sapphire templates. The quality of the epilayers was studied by micro-Raman spectroscopy (μ-RS) which proved the hexagonal phase and good crystallinity of GaN epilayers as well as a slight strain. The photoluminescence spectrum confirmed a high crystal quality by intense excitonic emission but it also exhibited a blue emission band of low intensity. DLTS signal spectra revealed the presence of four majority traps: two high-temperature and two low-temperature peaks. Using the Laplace DLTS method and Arrhenius plots, the apparent activation energy and capture cross sections were obtained. For two high-temperature majority traps, they were equal to E1 = 0.65 eV, σ1 = 8.2 × 10−16 cm2 and E2 = 0.58 eV, σ2 = 2.6 × 10−15 cm2 whereas for the two low-temperature majority traps they were equal to E3 = 0.18 eV, σ3 = 9.7 × 10−18 cm2 and E4 = 0.13 eV, σ4 = 9.2 × 10−18 cm2. The possible origin of the traps is discussed and the results are compared with data reported elsewhere.

Molecular beam epitaxial growth of ZnCdTeO epilayers for intermediate band solar cells

We report the growth and characterization of the lattice-matched Zn1-xCdxTe1-yOy (ZnCdTeO) layers on ZnTe substrates by radio frequency plasma-assisted molecular beam epitaxy technique. The Cd composition increases linearly with increasing Cd/(Zn+Cd) flux ratio, indicating a controllability of Cd composition by Cd flux. Introduction of O radical during the growth of ZnCdTe resulted in the formation of ZnCdTeO layer. At particular O and Cd compositions lattice-matched ZnCdTeO epilayers on ZnTe substrate were obtained. Photoreflectance (PR) spectroscopy on the lattice-matched ZnCdTeO layer revealed two distinct PR features in the energy regions at 2.2–2.5eV and 1.5–1.8eV, which can be attributed to transitions from the valence band to the two conduction subbands, E+ and E−, respectively.