InP/InGaAlAs distributed Bragg reflectors grown by low-pressure metal organic chemical vapor deposition

Long-wavelength vertical cavity surface emitting lasers (VCSELs) are considered the best candidate for the future low-cost reliable light sources in fiber communications. However, the absence of high refractive index contrast in InP-lattice-matched materials impeded the development of 1.3–1.5 μm VCSELs. Although wafer fusions provided the alternative approaches to integrate the InP-based gain materials with the GaAs/AlAs materials for their inherent high refractive index contrast, the monolithic InP-based lattice-matched distributed Bragg reflectors (DBRs) are still highly attractive and desirable. In this report, we demonstrate InP/InGaAlAs DBRs with larger refractive index contrast than InP/InGaAsP and InAlAs/InGaAlAs DBRs. The switching between InP and InGaAlAs layers and growth rate control have been done by careful growth interruption technique and accurate in situ optical monitoring in low-pressure metal organic chemical vapor deposition. A 35 pairs 1.55 μm centered InP/InGaAlAs DBRs has the stopband of more than 100 nm and the highest reflectivity of more than 99%. A VCSEL structure incorporating 35 pairs InP/InGaAlAs DBR as the bottom mirror combined with a 2λ thick periodic gain cavity and 10 pairs SiO₂/TiO₂ top dielectric mirrors was fabricated. The VCSELs lased at 1.56 μm by optical pumping at room temperature with the threshold pumping power of 30 mW.     

Investigation of characteristics of laterally overgrown GaN on striped sapphire substrates patterned by wet chemical etching

The new developed maskless lateral-epitaxial-overgrowth technique, in which the striped substrates are patterned by wet chemical etching, is systematically investigated using scanning electron microscopy, X-ray diffraction, and atomic force microscopy (AFM). Wing tilt is measured for the GaN films on patterned substrates with a range of “fill factor” (ratio of groove width to stripe period) and for the GaN in different growth time. It is found that changes in these parameters have a significant effect on the extent and distribution of wing tilt in the laterally overgrown regions relative to the GaN directly on the sapphire substrate. Increasing the thickness of GaN films is benefit to reduce wing tilt. The tilt is avoided in the GaN films with 4.5 μm thickness and fill factor for 0.46. The full-width at half-maximum of X-ray rocking curves of the asymmetric diffraction peaks and the image of AFM both show that the threading dislocations in the developed lateral epitaxial overgrowth of GaN films are reduced sharply. The GaN template could be used as an excellent substrate to fabricate high-performance optoelectronic devices.     

Stacking faults blocking process in (1 1 −2 2) semipolar GaN growth on sapphire using asymmetric lateral epitaxy

We have succeeded in effectively stopping the propagation of basal stacking faults in (1 1 −2 2) semipolar GaN films on sapphire using an original epitaxial lateral overgrowth process. The growth conditions were chosen to enhance the growth rate along the [0 0 0 1] inclined direction. Thus, the crystal expands laterally until growth above the a-facet of the adjacent crystal seed, where the basal stacking faults emerge. The growth anisotropy was monitored using scanning electron microscopy. The faults filtering and improvement of crystalline quality were attested by transmission electron microscopy, X-ray diffraction and low temperature photoluminescence, which exhibits high intensity band-edge emission with low stacking faults related emission.     

Heteroepitaxy of CdTe on tilting Si(2 1 1) substrates by molecular beam epitaxy

CdTe(2 1 1)B epilayers were grown on 3 in Si(2 1 1) substrates which misoriented 0–10° toward [1 1 1] by molecular beam epitaxy (MBE). The relationship of X-ray double-crystal rocking curve (XRDCRC) FWHM and deflection angle from CdTe(2 1 1) to Si(2 1 1) was studied. For 4.2–4.5 μm CdTe, the best value of FWHM 83 arcsec was achieved while deflection angle is 2.76°. A FWHM wafer mapping indicated a good crystalline uniformity of 7.4 μm CdTe on tilting Si(2 1 1), with FWHM range of 60–72 arcsec. The shear strains of these epilayers were analyzed, using reciprocal lattice points of symmetric and asymmetric reflections measured by high-resolution multi-crystal multi-reflection X-ray diffractometer (HRMCMRXD). It was found that the shear strain angle of epilayer is effectively reduced by using proper tilting Si(2 1 1) substrate. It was also proved that the lattice parameter of CdTe(2 1 1)B is affected by the shear strain and thermal strain.     

Selective-area MOCVD growth for distributed feedback lasers integrated with vertically tapered self-aligned waveguide

The growth pressure and mask width dependent thickness enhancement factors of selective-area MOCVD growth were investigated in this article. A high enhancement of 5.8 was obtained at 130 mbar with the mask width of 70 μm. Mismatched InGaAsP (−0.5%) at the maskless region which could ensure the material at butt-joint region to be matched to InP was successively grown by controlling the composition and mismatch modulation in the selective-area growth. The upper optical confinement layer and the butt-coupled tapered thickness waveguide were regrown simultaneously in separated confined heterostructure 1.55 μm distributed feedback laser, which not only offered the separated optimization of the active region and the integrated spotsize converter, but also reduced the difficulty of the butt-joint selective regrowth. A narrow beam of 9° and 12° in the vertical and horizontal directions, a low threshold current of 6.5 mA was fabricated by using this technique.     

AFM investigation of step kinetics and hillock morphology of the {1 0 0} face of KDP

Step velocities and hillock slopes on the {1 0 0} face of KDP were measured over a supersaturation range of 0<σ<0.15, where σ is the supersaturation. The formation of macrosteps and their evolution with distance from the hillock top were also observed. Hillock slope depended linearly on supersaturation and hillock geometry. The two non-equivalent sectors exhibited different slopes and step velocities. AFM shows an elementary step height of 3.7 Å, or half the unit cell height, whereas previous interferometric experiments assumed the elementary step was a unit cell. Values of the step edge energy (), the kinetic coefficients for the slow and fast directions (β_S and β_F), and the activation energies for slow and fast step motion (E_a,S and E_a,F) were calculated to be 24.0 erg/cm², 0.071 cm/s, 0.206 cm/s, 0.26 eV/molecule, and 0.21 eV/molecule, respectively. Analysis of macrostep evolution including the dependence of step height on time and terrace width on distance were performed and compared to predictions of published models. The results do not allow us to distinguish between a shock wave model and a continuous step-doubling model. Analysis within the latter model leads to a characteristic adsorption time for impurities (λ⁻¹) of 0.0716 s.     

AlGaN photodetectors grown on Si(1 1 1) by molecular beam epitaxy

The fabrication and characterisation of Al_xGa_1−xN (0x0.35) photodetectors grown on Si(1 1 1) by molecular beam epitaxy are described. For low Al contents (<10%), photoconductors show high responsivities (10A/W), a non-linear dependence on optical power and persistent photoconductivity (PPC). For higher Al contents the PPC decreases and the photocurrent becomes linear with optical power. Schottky photodiodes present zero-bias responsivities from 12 to 5 mA/W (x=0−0.35), a UV/visible contrast higher than 10³, and a time response of 20 ns, in the same order of magnitude as for devices on sapphire substrate. GaN-based p–n ultraviolet photodiodes on Si(1 1 1) are reported for the first time.     

Influence of cooling rate on the liquid-phase epitaxial growth of Zn3P2

We report the liquid-phase epitaxial growth of Zn₃P₂ on InP (1 0 0) substrates by conventional horizontal sliding boat system using 100% In solvent. Different cooling rates of 0.2–1.0 °C/min have been adopted and the influence of supercooling on the properties of the grown epilayers is analyzed. The crystal structure and quality of the grown epilayers have been studied by X-ray diffraction and high-resolution X-ray rocking measurements, which revealed a good lattice matching between the epilayers and the substrate. The supercooling-induced morphologies and composition of the epilayers were studied by scanning electron microscopy and energy dispersive X-ray analysis. The growth rate has been calculated and found that there exists a linear dependence between the growth rate and the cooling rate. Hall measurements showed that the grown layers are unintentionally doped p-type with a carrier mobility as high as 450 cm²/V s and a carrier concentration of 2.81×10¹⁸ cm⁻³ for the layers grown from 6 °C supercooled melt from the cooling rate of 0.4 °C/min.     

Shape transition from InAs quantum dash to quantum dot on InP(3 1 1)A

We report on the shape transition from InAs quantum dashes to quantum dots (QDs) on lattice-matched GaInAsP on InP(3 1 1)A substrates. InAs quantum dashes develop during chemical-beam epitaxy of 3.2 monolayers InAs, which transform into round InAs QDs by introducing a growth interruption without arsenic flux after InAs deposition. The shape transition is solely attributed to surface properties, i.e., increase of the surface energy and symmetry under arsenic deficient conditions. The round QD shape is maintained during subsequent GaInAsP overgrowth because the reversed shape transition from dot to dash is kinetically hindered by the decreased ad-atom diffusion under arsenic flux.     

Self-assembled InAs quantum dots with two different matrix materials

The effects of matrix materials on the structural and optical properties of self-assembled InAs quantum dots (QDs) grown by a molecular beam epitaxy were investigated by atomic force microscopy, cross-sectional transmission electron microscopy (TEM), and photoluminescence (PL) spectroscopy. Cross-sectional TEM image indicated that the average lateral size and height of InAs QDs in a GaAs matrix on a GaAs substrate were 20.5 and 5.0 nm, respectively, which showed the PL peak position of 1.19 μm at room temperature. The average lateral size and height of InAs QDs buried in an InAlGaAs matrix on InP were 26.5 and 3.0 nm, respectively. The PL peak position for InP-based InAs QDs was around 1.55 μm at room temperature. If we only consider the size quantization effects, the difference in PL peak position between two QD systems with different matrices may be too large. The large difference in peak position can be mainly related to the QD size as well as the strain between the QDs and the matrix materials. The intermixing between the QDs and the matrix materials can partially change the In composition of QDs, resulting in the modification of the optical properties.     

Crystal growth of Ce: PrF3 by micro-pulling-down method

Micro-pulling-down (μ-PD) growth apparatus was modified for fluoride crystals. PrF₃ was grown with various concentrations of Ce³⁺ from 0–100%. The crystals were transparent and colorless (CeF₃) or greenish and 3 mm in diameter and 15–50 mm in length. Neither visible inclusions nor cracks were observed. Radioluminescence spectra and decay kinetics were measured for the sample set at room temperature. In comparison to the Czochralski or Bridgman method, the μ-PD method allows to produce single crystalline material in a faster thus more economic way. Once it is established for the fluoride crystals, it is an efficient tool for exploring the field of new functional fluorides.     

Floating zone growth of high-quality SrTiO3 single crystals

Strontium titanate single crystals 15–20 mm in diameter and 40–80 mm in length were grown by a floating zone method with radiation heating. Additional crystal heating just below the molten zone by an in-growth annealing furnace was applied in order to lower the temperature gradients and to achieve slower cooling of the grown crystal. The crystal perfection was studied with X-ray topography and double-crystal diffractometry. The most perfect crystals were grown in [0 0 1] direction with single grain rocking curve widths of about 30″ and subgrain misorientations of 1′–3′ over 10×10 mm² areas of the boule cross-section for both (0 0 1)-, (1 1 0)- and (1 1 1)-oriented slices. Such high-quality crystal can be grown reproducibly with starting materials of 4N grade quality.     

The effect of the AlxGa1−xN/AIN buffer layer on the properties of GaN/Si(1 1 1) film grown by NH3-MBE

We describe the growth of GaN on Si(1 1 1) substrates with Al_xGa_1−xN/AlN buffer layer by ammonia gas source molecular beam epitaxy (NH₃-GSMBE). The influence of the AlN and Al_xGa_1−xN buffer layer thickness and the Al composition on the crack density of GaN has been investigated. It is found that the optimum thickness is 120 and 250 nm for AlN and Al_xGa_1−xN layers, respectively. The optimum Al composition is between 0.3<x<0.6.     

A simple method to synthesize single-crystalline β-wollastonite nanowires

The single-crystalline β-wollastonite (β-CaSiO₃) nanowires were prepared via a simple hydrothermal method, in the absence of any template or surfactant using cheap and simple inorganic salts as raw materials. Xonotlite [Ca₆(Si₆O₁₇)(OH)₂] nanowires were first obtained after hydrothermal treatment at a lower temperature of 200 °C for 24 h, and after being calcinated at 800 °C for 2 h, xonotlite nanowires completely transformed into β-wollastonite nanowires and the wire-like structure was preserved. The synthesized β-wollastonite nanowires had a diameter of 10–30 nm, and a length up to tens of micrometers, and the single-crystalline monoclinic parawollastonite structured β-wollastonite was identified by XRD with the space group of P2₁/a and cell constants of a=15.42 Å, b=7.325 Å, c=7.069 Å and β=95.38°. A possible growth mechanism of β-wollastonite nanowires was also proposed. The advantages of this method for the nanowire synthesis lie in the high yield, low temperature and mild reaction conditions, which will allow large-scale production at low cost.     

Correlation between interfacial structure and c-axis-orientation of LiNbO3 films grown on Si and SiO2 by electron cyclotron resonance plasma sputtering

Thin films of crystalline lithium niobate (LN) grown on Si(1 0 0) and SiO₂ substrates by electron cyclotron resonance plasma sputtering exhibit distinct interfacial structures that strongly affect the orientation of respective films. Growth at 460–600 °C on the Si(1 0 0) surface produced columnar domains of LiNbO₃ with well-oriented c-axes, i.e., normal to the surface. When the SiO₂ substrate was similarly exposed to plasma at temperatures above 500 °C, however, increased diffusion of Li and Nb atoms into the SiO₂ film was seen and this led to an LN–SiO₂ alloy interface in which crystal-axis orientations were randomized. This problem was solved by solid-phase crystallization of the deposited film of amorphous LN; the degree of c-axis orientation was then immune to the choice of substrate material.     

Luminescence studies of defects and piezoelectric fields in InGaN/GaN single quantum wells

Transmission electron microscopy (TEM), cathodoluminescence in the scanning electron microscope (SEM-CL) and photoluminescence (PL) studies were performed on a 30 nm GaN/2 nm In_0.28Ga _0.72N/2 μm GaN/(0 0 0 1) sapphire single quantum well (SQW) sample. SEM-CL was performed at low temperatures ≈8 K, and at an optimum accelerating voltage, around 4–6 kV to maximise the quantum well (QW) luminescence. The CL in the vicinity of characteristic “V-shaped” pits was investigated. The near band edge (BE) luminescence maps from the GaN showed bright rings inside the boundaries of the pits while the QW luminescence maps showed pits to be regions of low intensity. These observations are consistent with TEM observations showing the absence of QW material in the pits. Variations in both the BE and QW maps in the regions between the pits are ascribed to threading edge dislocations. The CL and PL QW luminescence was observed to blue-shift and broaden with increasing excitation intensity. This was accompanied by decreasing spatial resolution in the CL QW maps implying an increasing carrier diffusion length in the InGaN layer. The reasons for this behaviour are discussed. It is argued that screening of the piezoelectric field in the material may account for these observations.     

Study on the Ce substitution effects of BiFeO3 films on ITO/glass substrates

Ce substituted Bi_1−xCe_xFeO₃ (BCFO) films with x=0–0.15 were deposited on indium tin oxide (ITO)/glass substrates by sol–gel process annealed at 500 °C. Rhombohedral phase was confirmed by XRD study and no impure phases were observed till x=0.15. Substantially enhanced ferroelectricity was observed at room temperature due to the substitution of Ce. In the films with x=0.05 and 0.10, the double remnant polarization are 75.5 and 57.7 μC/cm² at an applied field 860 kV/cm. Moreover, the breakdown field was enhanced in the films with Ce substitution.     

MBE growth of cubic AlyGa1−yN/GaN heterostructures—structural, vibrational and optical properties

Cubic Al_yGa_1−yN/GaN heterostructures on GaAs(0 0 1) substrates were grown by radio-frequency plasma-assisted molecular beam epitaxy. High resolution X-ray diffraction, micro-Raman, spectroscopic ellipsometry, and cathodoluminescence measurements were used to characterize the structural, optical and vibrational properties of the Al_yGa_1−yN epilayers. The AlN mole fraction y of the alloy was varied between 0.07<y<0.20. X-ray diffraction reciprocal space maps demonstrate the good crystal quality of the cubic Al_yGa_1−yN films. The measured Raman shift of the phonon modes of the Al_yGa_1−yN alloy was in excellent agreement with theoretical calculations. Both SE and CL of the Al_yGa_1−yN epilayer showed a linear increase of the band gap with increasing Al-content.     

Impedance spectroscopy analysis of AlGaN/GaN HFET structures

For HFET application a series of samples with 30 nm Al_xGa_1−xN (x=0.02–0.4) layers deposited at 1040°C onto optimised 2 μm thick undoped GaN buffers were fabricated. The Al_xGa_1−xN/GaN heterostructures were grown on c-plane sapphire in an atmospheric pressure, single wafer, vertical flow MOVPE system. Electrical properties of the Al_xGa_1−xN/GaN heterostructures and thick undoped GaN layers were evaluated by impedance spectroscopy method performed in the range of 80 Hz–10 MHz with an HP 4192A impedance meter using a mercury probe. The carrier concentration distribution through the layer thickness and the sheet carrier concentration were evaluated. A non-destructive, characterisation technique for verification of device heterostucture quality from the measured C–V and G–V versus frequency characteristics of the heterostructure is proposed.     

Nucleation of AlN on SiC substrates by seeded sublimation growth

The nucleation of aluminum nitride (AlN) on silicon carbide (SiC) seed by sublimation growth was investigated. Silicon-face, 8 off-axis 4H-SiC (0 0 0 1) and on-axis 6H-SiC (0 0 0 1) were employed as seeds. Initial growth for 15 min and extended growth for 2 h suggested that 1850 °C was the optimum temperature of AlN crystal growth: on an 8 off-axis substrate, AlN grew laterally forming a continuous layer with regular “step” features; on the on-axis substrate, AlN grew vertically as well as laterally, generating an epilayer with hexagonal sub-grains of different sizes. The layer's c-lattice constant was larger than pure AlN, which was caused by the compression of the AlN film and impurities (Si, C) incorporation. Polarity sensitive and defect selective etchings were performed to examine the surface polarity and dislocation density. All the samples had an Al-polar surface and no N-polar inversion domains were observed. Threading dislocations were present regardless of the substrate misorientation. Basal plane dislocations (BPDs) were revealed only on the AlN films on the 8 off-axis substrates. The total dislocation density was in the order of when the film was 20– thick.