Plasma-excited Molecular Beam Epitaxy Research Articles

Direct growth of GaN film on ScAlMgO4 substrate by radio-frequency plasma-excited molecular beam epitaxy

ScAlMgO4 (SAM) has attracted attention as a substrate for nitride semiconductor crystal growth owing to its small a-axis lattice mismatch with GaN and InGaN. In this study, we investigated GaN growth on a SAM substrate via radio-frequency plasma-excited molecular beam epitaxy. By optimizing the growth conditions, GaN with the following epitaxial orientation relations (0001)GaN//(0001)SAM and [11–20]GaN//[11–20]SAM was successfully grown directly on the SAM substrate. The atomically flat and abrupt interface of GaN directly grown on the SAM substrate was observed via high-resolution transmission electron microscopy, and uniform GaN growth on a two-inch SAM substrate was also demonstrated.

Repeatability and Mechanisms of Threading Dislocation Reduction in InN Film Grown with In Situ Surface Modification by Radical Beam Irradiation

The objective of this study was to investigate the repeatability of in situ surface modification by radical beam irradiation to reduce threading dislocation density in InN film. The growth of InN template and N radical irradiation processes were repeated twice in situ in the radio-frequency plasma-excited molecular beam epitaxy chamber before the regrowth of InN film on the N radical irradiated template. Transmission electron microscopy was applied to study dislocation behaviors of the InN film grown. In this letter, we show cross-sectional-view transmission electron microscopy evidence of the threading dislocation reduction from ~2.8×1010 cm-2 in the first irradiated InN layer to ~2.0×1010 cm-2 in the second irradiated InN layer, and to ~1.3×1010 cm-2 in the top regrown InN layer. The mechanisms of threading dislocation reduction were also studied.

Metal-covered van der Waals epitaxy of gallium nitride films on graphitic substrates by ECR-MBE

We demonstrated that repetition of gallium deposition and nitrogen-rich supply is a superior approach to fabricate gallium nitride (GaN) thin films on graphene by electron-cyclotron-resonance plasma-excited molecular beam epitaxy. The gallium layer, which covered the surface at the initial growth stage, reduced nitrogen-plasma-induced damage to the graphene. It thereby might suppress the nucleation of misoriented crystals and enabled the growth of c-axis-oriented GaN films. This approach was more effective on graphene transferred onto a GaN template, which suggests the possibility of remote-homoepitaxy reflecting the electric potential of the GaN template unscreened by graphene.

Reduction of Threading Dislocation Density in InN Film Grown with in situ Surface Modification by Radio-frequency Plasma-excited Molecular Beam Epitaxy

ABSTRACTThe objective of this study was to investigate the relationship between the thickness of N radical irradiated InN template with crystallographic quality and electrical properties of InN film grown with the previously proposed method, in situ surface modification by radical beam irradiation. In this study, three InN samples were grown with this method on different thickness of irradiated templates. The crystallographic quality of InN films was analyzed by X-ray diffraction and the electrical properties were studied by Hall effect measurement. InN grown on 100 nm thick irradiated template shows lower full-width at half-maximum of X-ray rocking curves and lower carrier concentration compared to InN grown on 200 nm and 450 nm thick irradiated templates. Transmission electron microscopy revealed that threading dislocation density in the InN film decreased by an order of magnitude to ∼4.6×109cm-2. These results suggest that this method is possible for reduction of threading dislocation density in InN and the thickness of irradiated template should be minimized for higher crystallographic quality and electrical properties of the entire InN film.

Radio-frequency plasma-excited molecular beam epitaxy growth of GaN on graphene/Si(100) substrates

Strong c-axis-oriented hexagonal (0001) GaN was grown on graphene/Si(100) substrates by radio-frequency plasma-excited molecular beam epitaxy. The hexagonal symmetry of graphene transferred onto the Si(100) surface enabled the growth of a highly c-axis-oriented GaN film. The GaN showed a full width at half maximum of 11.3 arcmin for a (0002) rocking curve measured by X-ray diffraction. Strong luminescence at 3.4 eV was also observed by cathodoluminescence with a luminescence at 3.2 eV, which originated from a cubic-phase inclusion. A microstructural study using transmission electron microscopy also confirmed the growth of hexagonal (0001) GaN on a graphene/Si(100) substrate.

Epitaxial Growth of AlInGaN Quaternary Alloys by RF-MBE

AlInGaN quaternary alloys were successfully grown on sapphire substrate by radio-frequency plasma-excited molecular beam epitaxy (RF-MBE). AlInGaN quaternary alloys with different compositions were acquired by changing the Al cell's temperature. The streaky RHEED patterns were observed during AlInGaN quaternary alloys growth. Scanning Electron Microscope (SEM), Rutherford back-scattering spectrometry (RBS), X-Ray diffraction (XRD) and Cathodoluminescence (CL) were used to characterize the structural and optical properties of the AlInGaN alloys. The experimental results show that the AlInGaN quaternary alloys grow on the GaN buffer in the layer-by-layer growth mode. When the Al cell's temperature is 920 degrees C, the Al/In ratio in the AlInGaN quaternary alloys is about 4.7, and the AlInGaN can acquire better crystal and optical quality. The X-ray and CL full-width at half-maximum (FWHM) of the AlInGaN are 5arcmin and 25nm, respectively.

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)

The growth of high-Al-content InAlGaN quaternary alloys by RF-MBE

High-Al-content InxAlyGa1−x−yN (x = 1–10%, y = 34–45%) quaternary alloys were grown on sapphire by radio-frequency plasma-excited molecular beam epitaxy. Rutherford back-scattering spectrometry, high resolution x-ray diffraction and cathodoluminescence were used to characterize the InAlGaN alloys. The experimental results show that InAlGaN with an appropriate Al/In ratio (near 4.7, which is a lattice-match to the GaN under-layer) has better crystal and optical quality than the InAlGaN alloys whose Al/In ratios are far from 4.7. Some cracks and V-defects occur in high-Al/In-ratio InAlGaN alloys. In the CL image, the cracks and V-defect regions are the emission-enhanced regions.

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.

Growth of polycrystalline InN on silica glass by ECR‐MBE

InN films are grown on amorphous silica glass by electron cyclotron resonance plasma-excited molecular beam epitaxy. InN films grown directly on the substrate without a buffer layer are polycrystalline with no preferred growth orientation and a three-dimensional grainy morphology, whereas films grown on a low-temperature InN buffer layer are polycrystalline with uniform c-axis orientation and random a-axis orientation. Substrate nitridation prior to the growth is found to improve the surface morphology and uniformity of c-axis orientation, similar to the case for poly-GaN growth on silica glass. Notably, this poly-InN on silica glass exhibited room-temperature photoluminescence emission at ∼0.75 eV, with an optical absorption edge at almost the same position as the emission peak. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and characterization of InN on sapphire substrate by RF-MBE

Indium nitride (InN) films were grown on sapphire substrates by radio-frequency plasma-excited molecular beam epitaxy (RF-MBE). Atomic force microscopy (AFM), reflection high-energy electron diffraction (RHEED), double-crystal X-ray diffraction (DCXRD) and photoluminescence (PL) spectroscopy were used to characterize the InN films. The results show that the InN films have good crystallinity, with full-width at half-maximum (FWHM) of InN (0 0 0 2) DCXRD peak being 14 arcmin. At room temperature, a strong PL peak at 0.79 eV was observed. At 1.9 eV or so, no peak was observed. In addition, it is found that the InN films grown with low-temperature (LT) InN buffer layer are of better quality than those without LT-InN buffer layer.

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.

Radio frequency-molecular beam epitaxial growth of InN epitaxial films on (0001) sapphire and their properties

Growth of InN by radio frequency plasma-excited molecular beam epitaxy on (0001) sapphire was systematically studied. To improve the crystalline quality of InN, the following growth conditions were found to be essential: (1) nitridation of sapphire, (2) two-step growth, (3) precise control of V/III ratio, and (4) selection of optimum growth temperature. Results of structural characterization using x-ray diffraction, transmission electron microscopy, and extended x-ray absorption fine structure have clearly demonstrated that InN grown in this study had single crystalline with ideal hexagonal wurtzite structure. It is confirmed, however, that the InN had a threading dislocation density on the order of 1010/cm2 and large twist distribution. Photoluminescence studies on these well-characterized InN clearly demonstrated that band-gap energy of InN should be less than 0.67 eV at room temperature.

Single Crystalline InN Films Grown on Si Substrates By Using A Brief Substrate Nitridation Process

ABSTRACTInN films were grown on Si (111) substrates by radio-frequency plasma-excited molecular beam epitaxy. InN films highly oriented to the c-axis were obtained by optimizing growth conditions in the direct growth on Si. Growth of single crystalline InN films was realized on Si substrates with substrate nitridation for 3 min. On the other hands, when the substrate nitridation was lasted over 30 min, obtained InN films were polycrystalline due to the amorphous SiNx layer formed on a substrate surface. We also studied the local atomic structure in the single crystalline InN film using extended X-ray absorption fine structure measurements.

Electricaland Optical Properties of InN/Si Heterostructure

ABSTRACTSingle crystalline InN films were grown on Si substrates by radio-frequency plasma-excited molecular beam epitaxy. Electrical property of InN/Si heterojunction was investigated. We obtained rectifying characteristics in n-InN/p-Si heterostructure for the first time. Forward I-V characteristics were affected by both the buffer layer deposition and the nitridation process. Strong photoluminescence peaks for both single crystalline and polycrystalline InN films grown on the Si substrates were observed at around 0.8 eV, which were smaller than the previous reported PL emission peak of around 1.9 eV.

Study on Cubic GaN Growth on (001) Rutile TiO2 Substrates by ECR-MBE

ABSTRACTGaN layers were grown on a (001) rutile TiO2 substrate by electron cyclotron resonance plasma-excited molecular beam epitaxy. For the first time, c-GaN with a preferential growth orientation was obtained. Based on the results from electron diffraction and X-ray diffraction analysis, we found that c-GaN with the growth direction of [110] was grown on the TiO2 substrate. The formation of c-GaN was also confirmed by cathodoluminescence, in which a luminescence peak was observed at 3.24eV.

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.

Effect of sapphire substrate nitridation on determining rotation domain in GaN growth

GaN films were grown at various V/III ratios on sapphire (0 0 0 1) substrates with and without nitridation process by radio-frequency plasma-excited molecular beam epitaxy. Based on the results of X-ray diffraction pole-figure and φ scan measurements, the GaN film grown without nitridation process has a tendency to form the metastable in-plane rotation domain by ∼11° from the substrate, in addition to the main domain rotated by 30° from the substrate. In suppressing the formation of the metastable in-plane rotation domain, it is found to be very effective to perform the nitridation process. In addition, even in GaN films grown without nitridation process, decrease in V/III ratio enables us to suppress the formation of the metastable domain. The rotation domains are considered to have the epitaxial relationship of [1 2 3 ̄ 0] GaN ∥[1 1 2 ̄ 0] sapphire and [ 1 ̄ 2 ̄ 3 0] GaN ∥[1 1 2 ̄ 0] sapphire .

Growth of high-quality InN using low-temperature intermediate layers by RF-MBE

InN with a film thickness of 600 nm was grown on a (0 0 0 1) sapphire substrate using low-temperature-grown intermediate layers by radio frequency plasma-excited molecular beam epitaxy (RF-MBE). From SEM observation, it was found that InN films with uniform surface morphology were grown. The electron mobility at room temperature obtained in this study was 830 cm 2 /V s and the corresponding carrier density was 1.0×10 19 cm −3. To our knowledge, this electron mobility is the highest value ever reported for single-crystal InN films.