Electron-cyclotron-resonance Plasma-excited Molecular Beam Epitaxy Research Articles

Fabrication of a‐plane InN nanostructures on patterned a‐plane GaN template by ECR‐MBE

Abstracta‐plane InN nanostructures were fabricated on a hole‐patterned a‐plane GaN template by electron‐cyclotron‐resonance plasma‐excited molecular beam epitaxy (ECR‐MBE). The growth temperature should be optimized to realize precise nucleation at the patterned holes with sufficient In desorption and a sufficiently long In migration length. Polarity determination clearly revealed that a‐plane InN crystals have an anisotropic growth morphology. The InN growth rate in the N‐polar [000–1] direction is higher than those in the In‐polar [0001] and [1–100] directions. a‐plane InN nanowalls were fabricated by exploiting the different the growth rates in the 〈0001〉 and 〈1–100〉 directions.magnified image SEM image of position‐controlled a‐plane InN nanostructures grown by ECR‐MBE on a hole‐patterned a‐plane GaN template.

Investigations on the properties of intermittently Gd‐doped InGaN structures grown by molecular‐beam epitaxy

AbstractIntermittent Gd‐doping into InGaN has been attempted to obtain improved crystalline quality of the InGaGdN samples for spintronic device applications. Intermittently Gd‐doped InGaN samples with different layer thickness of the intercepting‐ undoped InGaN layers have been grown by the electron cyclotron resonance plasma‐excited molecular‐beam epitaxy (ECR‐MBE) technique and their structural and magnetic properties were studied. The incorporation of Gd by this intermittent doping method was confirmed by the X‐ray fine structure (XAFS) analysis. The rocking curves and the reciprocal space maps (RSM) of these samples revealed that under certain growth parameters, they exhibited formation of superlattice like structure. The superconducting quantum interference device magnetometer (SQUID) measurements revealed that they show room temperature ferromagnetism (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Transmission Electron Microscopy Characterization of Position-Controlled InN Nanocolumns

One of the ways to obtain higher quality InN crystal is through the growth of columnar nanostructure. We have successfully grown position-controlled InN nanocolumns by electron cyclotron resonance plasma-excited molecular beam epitaxy (ECR-MBE). The position-controlled InN nanocolumns were characterized by transmission electron microscope (TEM). Selected area electron diffraction (SAED) patterns showed that InN nanocolumns were epitaxially grown on the GaN template with relaxed strain. In the majority of InN nanocolumns, however, neither edge- nor screw-type threading dislocations were generated at the interface between the GaN template and InN nanocolumns. Furthermore, various behaviors of threading dislocations in GaN template at the interface were observed. Finally, we carried out polarity determination for InN nanocolumns using convergent beam electron diffraction (CBED) technique and confirmed that InN nanocolumns have In-polarity.

Microstructure of A ‐plane InN grown on R ‐plane sapphire by ECR‐MBE

We used electron cyclotron resonance plasma-excited molecular beam epitaxy (ECR-MBE) to grow A-plane (110) InN on nitridated R-plane (102) sapphire and then measured the structural properties using transmission electron microscopy (TEM). We determined the epitaxial relationship between A-plane InN and R-plane sapphire to be (110)InN // (102)sapphire and [100]InN // [110]sapphire. Moreover, the results indicated that the nitridation of the sapphire produced a (001) cubic AlN layer, and this layer caused the subsequent InN to have its a-axis normal to the interface. Also, by using two diffraction vector orientations in the TEM measurement, we found that dislocations with a screw component had a density of about 5 × 1010 cm–2, which is about ten times higher than that with an edge component. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Position-Controlled InN Nano-dot Growth on Patterned Substrates by ECR-MBE

ABSTRACTWe report on the growth of self-aligned InN nano-dots on nano-patterned GaN templates by electron cyclotron resonance plasma-excited molecular beam epitaxy (ECR-MBE). In the fabrication of the nano-dots, InN was grown on GaN templates with reticular patterns of holes, which were prepared by the focused ion beam (FIB) technique. The InN nano-dots were formed selectively at the holes, resulting in the reticular array of InN nano-dos. The size of InN dots was controlled by varying the hole-pitch and the growth temperature. Furthermore, the shape of InN dots improved by thermal annealing after the growth. We have succeeded in controlling the position and size of InN nano-dots on nano-patterned substrates. Typically, InN nano-dots with a diameter of 50 nm and a height of 10 nm were fabricated in 410°C growth.

Optical emission spectroscopy during InN growth by ECR–MBE

We observed emissions from excited indium (In *) by optical emission spectroscopy (OES) during InN growth by electron–cyclotron–resonance plasma-excited molecular beam epitaxy (ECR–MBE). The emission intensity of In * was found to increase with the increase in substrate temperature. Considering the decrease in growth rate by increasing substrate temperature, it is suggested that some In which reached the growing surface from the In cell were thermally desorbed and excited when passing through the nitrogen plasma. It was also found that as the positive bias to the substrate increased, the emission intensity of In * decreased. The growth rate of InN with a deeper positive bias was higher than that without bias. These results were successfully explained by considering that effective In supplied to the surface increased by the decrease in In desorption when deeper positive bias was applied. From these observations, it is confirmed that the desorption of In could be characterized by monitoring In * emission intensity. Thus, OES is demonstrated to be useful as a monitoring tool also for InN growth by MBE.

Effect of hydrogen on morphological changes in columnar structure of GaN grown by ECR–MBE

We investigate the effect of hydrogen on morphological changes in GaN columns grown by electron–cyclotron-resonance plasma-excited molecular beam epitaxy on sapphire (0 0 0 1) substrates. In GaN growth with a hydrogen flow of 6 sccm, hexagonal columnar structure was obtained. Morphological changes of the GaN columns from hexagonal to triangle were observed by increasing the hydrogen flow rate. It is considered to be caused by preferential growth along three of the six 〈1 0 1 ̄ 0〉 directions. The surface of the triangle columns consisted of {1 0 1 ̄ 3} and {1 0 1 ̄ 1} faceted planes. Formation mechanism of the triangle columnar structure is discussed.

Structural characterization of GaN grown by hydrogen-assisted ECR-MBE using electron microscopy

Structural changes in GaN layers grown by electron–cyclotron-resonance plasma-excited molecular beam epitaxy using hydrogen–nitrogen mixed plasma were investigated. A columnar structure with a faceted surface and stacking faults was observed in a GaN layer grown with hydrogen, indicating the growth mode of GaN changed from two-dimensional layer growth to three-dimensional columnar growth by using hydrogen–nitrogen mixed plasma. At the early stage of the GaN growth, island growth with a mesa shape occurred. The effects of hydrogen on the structure of GaN are discussed and the growth mechanism of the columnar structure in the GaN layers grown with hydrogen is proposed.

Cathodoluminescence and Micro-Structure of Polycrystalline GaN Grown on ZnO/Si

ABSTRACTWe have investigated relationships between microscopic structure and cathodoluminescence (CL) property of polycrystalline (poly-) GaN grown by electron-cyclotron-resonance plasma-excited molecular beam epitaxy (ECR-MBE) on ZnO/Si substrates. Very strong CL with a peak of 3.45 eV was observed from the poly-GaN, which mainly showed a columnar structure with a size of 50-100 nm. On the other hand, the intensity of CL from the poly-GaN with few columnar domains was weaker than that of the poly-GaN with the columnar structure. The CL image from the poly-GaN, in which the columnar domains were locally observed, showed a strong contrast between bright domains and a dark background. It is confirmed that these bright regions in the CL images are corresponding to the columnar domains of the poly-GaN, by comparing with the SEM images. These results suggest that the columnar domains are responsible for the strong CL from the poly-GaN grown on the ZnO/Si substrates. Cross-sectional transmission electron microscope (TEM) observation revealed that the columnar domains had high quality crystallinity with few defects.

ECR-MBE growth of GaN on LiGaO2

GaN thin films were grown at relatively low temperatures on multi-domain LiGaO2 substrates by electron-cyclotron-resonance plasma-excited molecular beam epitaxy (ECR-MBE). LiGaO2 is one of the most promising substrates for GaN epitaxial growth because of its small lattice mismatch of around 0.9%. The multi-domain LiGaO2 substrate has two different polarity domains in the (001) surface. Two different GaN surface morphologies depending on the surface polarities were observed by atomic force microscopy (AFM). The difference was also observed in the shift of photoluminescence (PL) spectra. Each peak corresponding to two inversion domains was shifted in parallel around 2nm. One possible explanation for the cause of the peak wavelength shift is due to the difference of the strains in GaN thin films.

Optical emission spectroscopy as the monitoring tool in ECR-MBE growth of GaN

We demonstrated an observation of excited gallium (Ga*) emission by optical emission spectroscopy (OES) during GaN growth. It was also demonstrated that crystal structure of GaN could be controlled by changing bias conditions during electron-cyclotron-resonance plasma-excited molecular beam epitaxy (ECR-MBE). Change of GaN crystal structure from hexagonal to cubic (c-) by applied positive bias voltage to the substrate was observed for the first time. Growth of c-GaN and decrease in Ga* emission intensity in deeper positive bias conditions were successfully explained by suppression of Ga desorption. From these observations, OES is demonstrated to be useful as the monitoring tool in ECR plasma-excited MBE.

Electrical properties of GaAs selectively grown by electron cyclotron resonance plasma-excited molecular beam epitaxy

Electrical properties of GaAs layers selectively grown by electron cyclotron resonance plasma-excited molecular beam epitaxy (ECR-MBE) have been studied. Exposing a Si3N4 mask to ECR plasma introduces Si from the mask to the selectively grown layer and increases the carrier concentration of the undoped layer. By doping Si into this selectively grown layer, carrier concentrations as high as 5×1018 cm−3 could be obtained. It was also found that the carrier concentration profile did not show any decrease at the interface between the substrate and the grown layer. These results suggest that selective area growth of a heavily doped n-type layer makes ECR-MBE a very promising method to form low-resistance ohmic contacts as a regrowth process.

Plasma-controlled selective area growth of GaAs by electron-cyclotron resonance plasma-excited molecular-beam epitaxy (ECR-MBE)

Selective growth of GaAs on a GaAs substrate with a Si 3N 4-patterned mask has been controlled at 630°C by plasma conditions, using the electron-cyclotron resonance plasma-excited molecular-beam epitaxy (ECR-MBE) method. Increasing the microwave power or negative DC bias applied to the substrate, indicative of an increase in the impinging energy of charged particles on the substrate surface, improved the selectivity of the growth. The ratios of the widths of unmasked and masked areas did not affect the growth rate in the unmasked areas. Increasing the microwave power reduced the growth rate in the unmasked areas. We discuss the growth kinetics and show that these results support our contention that a plasma-enhanced desorption process of impinging atoms from the Si 3N 4 surface plays an important role in the selective growth of GaAs.

Improvements of electrical and optical properties of GaAs by substrate bias application during electron-cyclotron-resonance plasma-excited molecular beam epitaxy

Electrical and optical properties of GaAs layers are improved by applying substrate bias during electron-cyclotron-resonance plasma-excited molecular beam epitaxy (ECR-MBE). The highest mobility obtained without substrate bias is 4500 cm2 V−1 s−1. By applying positive bias, the mobility of the grown layer is increased to as high as 6800 cm2 V−1 s−1. Intense photoluminescence spectra comparable to that of a high quality MBE-grown layer are also observed. The suppression of high-energy ion impingement on the growing surface plays an important role in obtaining high quality crystal.

Selective Area Growth of GaAs on Si by Electron-Cyclotron-Resonance Plasma-Excited Molecular-Beam-Epitaxy (ECR-MBE)

AbstractSuccessful selective area growth of GaAs on Si following low-temperature plasma cleaning of the Si surface is carried out by ECR plasma-excited MBE. Both cleaning and selective area growth are carried out at 630° C. GaAs selectively grows only in Si windows patterned in the SiN mask. Neither deposition on the SiN mask nor anomalous growth at the mask edge is observed. GaAs thickness is independent of the SiN mask width, which implies that the plasma-enhanced desorption of migrating atoms from the SiN mask is essential in this selective area growth. Furthermore, results for the helium-diluted arsine imply that the plasma-enhanced desorption is mainly caused by physical bombardment.