Optical properties of GaN epilayers grown on Si (111) and Si (001) substrates

We report the observation of bright photoluminescence (PL) emission from two types of GaN epilayers grown by molecular beam epitaxy (MBE). Wurtzite phase GaN/Si (111) epilayers are grown by gas source MBE process, whereas cubic phase GaN epilayers are grown on (001) Si covered by thin SiC film in the process of Si annealing in propane prior to the GaN growth. PL emissions are identified based on the results of detailed PL and time-resolved PL investigations. For the wurtzite phase GaN we observe an efficient up in the energy transfer from bound to free excitons. This process is explained by a large difference in the PL decay times for two types (free and bound (donor, acceptor)) of excitonic PL emissions. For cubic phase GaN we confirm recent suggestion that acceptors have smaller thermal ionization energies than those in the wurtzite phase GaN.     

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 (1 1 1) 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.     

Size effects of nano-pattern in Si(1 1 1) substrate on the selective growth behavior of GaN nanowires by MOCVD

We report on the size effects of nano-patterned Si(1 1 1) substrates on the selective growth of GaN nanowires (NWs) using metal organic chemical vapor deposition. The nano-patterns on Si(1 1 1) substrates were fabricated by etching process of Au nano-droplets. The size of nano-patterns fabricated on Si(1 1 1) substrates were corresponding to size of Au nano-droplets, and the diameter of GaN NWs grown on nano-patterns was similar to the size of nano-pattern. Dense and well-oriented GaN NWs were grown on Si(1 1 1) substrates corresponding to the nano-patterns with an average diameter of about 50 nm. However, only a few GaN bulk grains, and mixed phase of a few NWs and bulk crystal of GaN were grown on the nano-patterned Si(1 1 1) having too small and large diameter, respectively, compare to the nano-patterns with diameter of 50 nm. Our results suggested that the selective growth of GaN NWs is strongly affected by the size of nano-patterns and its related mechanism.     

Anti-phase domain in GaP grown on Si by MBE

Anti-phase domain structures in GaP thin film grown on Si with (001) and (011) orientations by molecular beam epitaxy (MBE) have been studied using transmission electron microscopy. The structural integrety of APDs has been analyzed using a polarity of (111) diffracted discs in the convergent beam electron diffraction (CBED) pattern. The crystallographic reflections of APDs in the present non-centro symmetry crystal were then identified through an observation of the non-polarity in the diffraction intensity of ±111 discs of CBED projection. This result suggests that a CBED pattern may become a simple tool for the determination of a complex APD present in a spharelite structure.     

Study of using aqueous NH3 to synthesize GaN nanowires on Si(1 1 1) by thermal chemical vapor deposition

High-quality GaN nanowires (NWs) and zigzag-shaped NWs were grown on catalyst-free Si(1 1 1) substrate by thermal chemical vapor deposition (TCVD). Gallium (Ga) metal and aqueous NH₃ solution are used as a source of materials. Ga vapor was directly reacts with gaseous NH₃ under controlled nitrogen flow at 1050 °C. Scanning electron microscopy (SEM) images showed that the morphology of GaN displayed various densities of NWs and zigzag NWs depending on the gas flow rate, and increased nitrogen flow rate caused density reduction. The GaN NWs exhibited clear X-ray diffraction analysis (XRD) peaks that corresponded to GaN with hexagonal wurtzite structures. The photoluminescence spectra showed that the ultraviolet band emission of GaN NWs had a strong near band-edge emission (NBE) at 361–367 nm. Yellow band emissions were observed at low and high flow rates due to nitrogen and Ga vacancies, respectively. Moderate N₂ flow resulted in a strong NBE emission and a high optical quality of the NWs. This study shows the possibility of low-cost synthesis of GaN nanostructures on Si wafers using aqueous NH₃ solution.     

Highly oriented star-like patterns observed on GaSe epilayers grown on Si(111)

Epitaxial lamellar gallium selenide (GaSe) semiconductors have been grown on trench-patterned silicon (Si) substrates by molecular beam epitaxy. An intriguing star-like patterned morphology was identified by atomic force microscopy on these epilayers. This non-trivial feature can be correlated with the accumulation of stacking faults of two concurrent epitaxial domains around self-oriented triangular pits developed earlier on the Si(111) surface by the chemical etching. Crystallographic considerations show how the stars can be formed.     

Effect of nitridation on crystallinity of GaN grown on GaAs by MBE

GaN films are grown on [0 0 1] GaAs substrates by plasma-assisted molecular beam epitaxy using a three-step process that consists of a substrate nitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. Films are evaluated by X-ray diffraction and the dependence of crystalline quality on the nitridation temperature is studied. It is demonstrated that nitridation has to be performed at low-temperature to achieve c-oriented α-GaN. Higher nitridation temperature promotes formation of mis-oriented domains and β-GaN inclusions     

Near-infrared photoluminescence of vertically aligned InN nanorods grown on Si(111) by plasma-assisted molecular-beam epitaxy

We demonstrate that vertically aligned InN nanorods have been grown on Si(111) substrates by plasma-assisted molecular-beam epitaxy (PA-MBE) at low and high growth temperatures (LT- and HT-InN nanorods). High-resolution scanning electron microscopy images clearly show that InN nanorods grown on Si(111) are hexagonal in shape, vertically aligned, well separated and densely distributed on the substrate. The size distribution of LT-InN nanorods is quite uniform, while the HT-InN nanorods exhibit a broad, bimodal distribution. The structural analysis performed by Raman scattering indicates that PA-MBE grown InN nanorods have the wurtzite-type InN single-crystal structure with the rod axis (growth direction) along the c-axis. In addition, both types of nanorods contain high concentrations of electrons (unintentionally doped). Compared to the HT-InN nanorods and the PA-MBE-grown InN epitaxial film, the LT-grown InN nanorods have a considerable number of structural defects. Near-infrared photoluminescence (PL) from LT- (∼ 0.77 eV) and HT-InN (∼ 0.70 eV) nanorods is clearly observed at room temperature. In comparison with the LT-InN nanorods, the PL efficiency of HT-InN nanorods is better and the PL peak energy is closer to that of InN-on-Si epitaxial films (∼ 0.66 eV). We also find that the PL band at low temperatures from nanorods is significantly weaker (compared to the InN film case) and exhibits anomalous temperature effects. We propose that these PL properties are results of considerable structural disorder (especially for the LT-InN nanorods) and strong surface electron accumulation effect (for both types of nanorods).     

Self-assembled Al(x)Ga(1-x)N nanorods grown on Si(001) substrates by using plasma-assisted molecular beam epitaxy

Hexagonal Al(x)Ga(1-x)N nanorods were grown by plasma-assisted molecular beam epitaxy (PAMBE) on Si(001) substrates. The Al mole fraction was determined from x-ray diffraction (XRD) measurement and its value was varied from 0 to 15. It is found that, under group III-rich conditions, the growth rate of the Al(x)Ga(1-x)N nanorods decreases and the diameter increases due to the possibility of incorporation of aluminium and gallium. In order to study structural and optical properties, x-ray diffraction and cathodoluminescence (CL) measurements were carried out. The Al content (x) is calculated from these measurements and their values are compared.     

The effect of nitridation parameters and initial growth conditions on the polarity of GaN epitaxial layers grown by plasma-assisted molecular-beam epitaxy on Si(111) substrates

The dependence of the crystallographic polarity of GaN epitaxial layers produced by nitrogen plasma-enhanced molecular-beam epitaxy on Si(111) substrates on the nitridation parameters and initial growth conditions has been studied. A rapid procedure for determining the polarity of GaN epitaxial layers was developed. It was found experimentally that the nitridation parameters of the silicon substrate have no effect on the polarity of a GaN layer. It was shown that the substrate temperature in the stage of nucleation of a GaN epitaxial layer is one of the factors determining its polarity.     

TEM study of the structure of GaAs on vicinal Si (001) surface grown by MBE

Molecular beam epitaxy (MBE) grown GaAs films on Si substrates (0 0 1) 4° off towards 1 1 1 A and towards 1 1 1 B, were examined by means of transmission electron microscopy (TEM). The results indicate that in both samples, threading dislocations in the GaAs epilayer are blocked mainly in a thin layer near the GaAs-Si interface. This thin layer is like an inner interface, consisting of pyramidal islands and is flatter on the As growth surface than that on the Ga growth surface. In the type B sample, the density of dislocations is lower, the inner interface is flatter and the number of twins is much larger than that in the type A sample.     

Abnormal photoluminescence properties of GaN nanorods grown on Si(111) by molecular-beam epitaxy

We have studied the photoluminescence properties of GaN nanorods grown on Si(111) substrates by radio-frequency plasma-assisted molecular-beam epitaxy. The hexagonal shaped nanorods with lateral average diameters from 30 to 150?nm are obtained by controlling the Ga flux with a fixed amount of nitrogen. As the diameters decrease, the main emission lines assigned as donor bound excitons are blueshifted, causing a spectral overlap of this emission line with that of the free exciton at 10?K due to the quantum size effect in the GaN nanorods. The temperature-dependent photoluminescence spectra show an abnormal behaviour with an 'S-like' shape for higher diameter nanorods. The activation energy of the free exciton for GaN?nanorods with different diameters was also evaluated.     

Polarized infrared reflectance studies for wurtzite InN epilayers on Si(111) grown by molecular beam expitaxy

Room temperature polarized infrared reflectance technique is employed to study the optical properties of wurtzite InN epilayers on Si(111) grown by molecular beam expitaxy. The reflection spectra are compared to the calculated spectra generated based on the anisotropic dielectric function model. Good agreement between the measured and calculated spectra is obtained. From the fit of the experimental curve, the reststrahlen parameters at the center of Brillouin zone, the carrier concentration and mobility as well as the epilayers thicknesses are determined. The values of the carrier concentration and mobility are in good agreement with the results obtained from the Hall effects measurements.     

Effect of layer thickness on structural quality of Ge epilayers grown directly on Si(001)

A study of Ge epilayer growth directly on a Si(001) substrate is presented, following the two temperature Ge layer method. In an attempt to minimize the overall thickness while maintaining a good quality Ge epilayer, we have investigated the effect of varying the thickness of both the low and high temperature Ge layers, grown at 400 °C and 670 °C, respectively, by reduced pressure chemical vapor deposition. We find that the surface of the low temperature (LT) seed layer has a threading dislocation density (TDD) to the order of 10¹¹ cm^− 2. On increasing the LT layer thickness from 30 nm to 150 nm this TDD decreases by a factor of 2, while its roughness doubles and degree of relaxation increases from 82% to 96%. Growth of the high temperature (HT) layer reduces the TDD level to around 10⁸ cm^− 2, which is also shown to decrease with increasing layer thickness. Both the surface roughness and degree of relaxation reach stable values for which increasing the thickness beyond about 700 nm has no effect. Finally, annealing the HT layer is shown to reduce the TDD, without affecting the degree of relaxation. However, unless a thick structure is used the surface roughness increases significantly on annealing.     

Piezoelectric field in highly stressed GaN-based LED on Si (1 1 1) substrate

Stress states in GaN epilayers grown on Si (1 1 1) and c-plane sapphire, and their effects on built-in piezoelectric field induced by compressive stress in InGaN/GaN multi-quantum well (MQW) light-emitting diodes (LEDs) were investigated using the electroreflectance (ER) spectroscopic technique. Relatively large tensile stress is observed in GaN epilayers grown on Si (1 1 1), while a small compressive stress appears in the film grown on c-plane sapphire. The InGaN/GaN MQWs of LED on c-plane sapphire substrate has a higher piezoelectric field than the MQWs of LEDs on Si (1 1 1) substrate by about 1.04 MV/cm. The large tensile stress due to lattice mismatch with Si (1 1 1) substrate is regarded as external stress. The external tensile stress from the Si substrate effectively compensates for the compressive stress developed in the active region of the InGaN/GaN MQWs, thus reducing the quantum-confined Stark effect (QCSE) by attenuating the piezoelectric polarization from the InGaN layer.     

Nanoindentation-induced phase transformation in (1 1 0)-oriented Si single-crystals

Pressure-induced plastic deformation and phase transformations manifested as the discontinuities displayed in the loading and unloading segments of the load–displacement curves were investigated by performing the cyclic nanoindentation tests on the (1 1 0)-oriented Si single-crystal with a Berkovich diamond indenter. The resultant phases after indentation were examined by using the cross-sectional transmission electron microscopy (XTEM) technique. The behaviors of the discontinuities displayed on the loading and re-loading segments of the load–displacement curves are found to closely correlate to the formation of Si-II metallic phase, while those exhibiting on the unloading segments are relating to the formation of metastable phases of Si-III, Si-XII, and amorphous silicon as identified by TEM selected area diffraction (SAD) analyses. Results revealed that the primary indentation-induced deformation mechanism in Si is intimately depending on the detailed stress distributions, especially the reversible Si-II ? Si-XII/Si-III phase transformations might have further complicated the resultant phase distribution. In addition to the frequently observed stress-induced phase transformations and/or crack formations, evidence of dislocation slip bands was also observed in tests of Berkovich nanoindentation.     

Self-assembled InN quantum dots grown on AlN/Si(111) and GaN/Al2O3(0001) by plasma-assisted molecular-beam epitaxy under Stranski–Krastanow mode

We demonstrate that InN quantum dots (QDs) can be spontaneously formed on AlN and GaN surfaces by plasma-assisted molecular-beam epitaxy under the Stranski–Krastanow (S–K) mode. Both Si(111) wafers and metal–organic chemical vapor deposition grown GaN/Al₂O₃(0001) templates were used as substrates in this work. Silicon is particularly interesting as a substrate for InN QD applications because of its electrical conductivity and transparency in the near-infrared. By using reflection high-energy electron diffraction (RHEED), the formation process of InN QDs can be monitored in situ. We observed the 2D–3D transition of S–K growth mode and the lattice constant varied dramatically at the 2D–3D transition point from AlN to InN lattice constant. Furthermore, from the ex situ atomic force microscopy and scanning electron microscopy measurements, we directly imaged InN QDs on the AlN surface with an average diameter of ∼ 14 nm and high areal density of ∼ 1.6 × 10¹¹ cm^− 2.     

Carrier dynamics and activation energy of CdxZn(1-x)Te/ZnTe quantum dots on GaAs and Si substrates

We have investigated the carrier dynamics and activation energy of CdxZn(1-x)Te/ZnTe quantum dots (QDs) on GaAs and Si substrates. The carrier dynamics of QDs on GaAs and Si substrates is studied using time-resolved photoluminescence (PL) measurements, revealing shorter exciton lifetimes of QDs on Si substrate. In particular, the activation energy of electrons confined in QDs on the GaAs substrate, as obtained from temperature-dependent PL spectra, is higher than that of electrons confined in QDs on the Si substrate. Both results confirm that defects and dislocations in QDs on the Si substrate provide nonradiative channels.     

MBE方法在GaAs(001)衬底上外延生长GaSb膜

利用分子束外延方法(MBE)在GaAs(001)衬底上外延生长了GaSb薄膜,利用高分辨透射电子显微镜(HRTEM)、原子力显微镜(AFM)、Hall效应(HallEffect)和低温光荧光谱(LTPL)等手段对薄膜的晶体质量、电学性能和光学质量进行了研究。发现直接生长的GaSb膜表面平整,空穴迁移率较高。引入GaSb/AlSb超晶格可有效阻断进入GaSb外延层的穿通位错,对应的PL谱强度增强,材料的光学质量变好。