Growth optimization and characterization of lattice-matched Al0.82In0.18N optical confinement layer for edge emitting nitride laser diodes

We present the growth optimization and the doping by the metal organic chemical vapor deposition of lattice-matched Al_0.82In_0.18N bottom optical confinement layers for edge emitting laser diodes. Due to the increasing size and density of V-shaped defects in Al_1?xIn_xN with increasing thickness, we have designed an Al_1?xIn_xN/GaN multilayer structure by optimizing the growth and thickness of the GaN interlayer. The Al_1?xIn_xN and GaN interlayers in the multilayer structure were both doped using the same SiH₄ flow, while the Si levels in both layers were found to be significantly different by SIMS. The optimized 8×(Al_0.82In_0.18N/GaN=54/6 nm) multilayer structures grown on free-standing GaN substrates were characterized by high resolution X-ray diffraction, atomic force microscopy and transmission electron microscopy, along with the in-situ measurements of stress evolution during growth. Finally, lasing was obtained from the UV (394 nm) to blue (436 nm) wavelengths, in electrically injected, edge-emitting, cleaved-facet laser diodes with 480 nm thick Si-doped Al_1?xIn_xN/GaN multilayers as bottom waveguide claddings.     

TEM study of defects in AlxGa1−xN layers with different polarity

Transmission electron microscopy (TEM) studies of defects in Al_xGa_1?xN layers with various Al mole fractions (x=0.2, 0.4) and polarities were carried out. The samples were grown by ammonia molecular beam epitaxy on sapphire substrates and consisted of low-temperature AlN (LT-AlN) and high-temperature AlN (HT-AlN) buffer layers, a complex AlN/AlGaN superlattice (SL) and an Al_xGa_1?xN layer (x=0.2, 0.4). It was observed that at the first growth stages a very high density of dislocations is introduced in both Al-polar and N-polar structures. Then, at the interface of the LT-AlN and HT-AlN layers half-loops are formed and the dislocation density considerably decreases in Al-polar structures, whereas in the N-polar structures such a behavior was not observed.The AlN/AlGaN superlattice efficiently promotes the bend and annihilation of threading dislocations and respectively the decrease of the dislocation density in the upper Al_xGa_1?xN layer with both polarities.The lattice relaxation of metal-polar Al_0.2Ga_0.8N was observed, while N-polar Al_0.2Ga_0.8N did not relax. The dislocation densities in the N-polar Al_0.2Ga_0.8N and Al_0.4Ga_0.6N layers were 5.5×10⁹ cm^?2 and 9×10⁹ cm^?2, respectively, and in metal-polar Al_0.2Ga_0.8N and Al_0.4Ga_0.6N layers these were 1×10¹⁰ cm^?2 and 6×10⁹ cm^?2, respectively.Moreover, from TEM images the presence of inversion domains (IDs) in N-polar structures has been observed. The widths of IDs varied from 10 to 30 nm. Some of the IDs widen during the growth of the AlN buffer layers. The IDs formed hills on the surface of the N-polar structures.     

N-type doping of GaN/Si(1 1 1) using Al0.2Ga0.8N/ALN composite buffer layer and Al0.2Ga0.8N/GaN superlattice

The characteristics of Si-doped and undoped GaN/Si(1 1 1) heteroepitaxy with composite buffer layer (CBL) and superlattice are compared and discussed. While as-grown Si-doped GaN/Si(1 1 1) heteroepitaxy shows lower quality compared to undoped GaN, crack-free n-type and undoped GaN with the thickness of 1200 nm were obtained by metalorganic chemical vapor deposition (MOCVD). In order to achieve the crack-free GaN on Si(1 1 1), we have introduced the scheme of multiple buffer layers; composite buffer layer of Al_0.2Ga_0.8N/AlN and superlattice of Al_0.2Ga_0.8N/GaN on 2-in. Si(1 1 1) substrate, simultaneously. The FWHM values of the double-crystal X-ray diffractometry (DCXRD) rocking curves were 823 arcsec and 745 arcsec for n-GaN and undoped GaN/Si(1 1 1) heteroepitaxy, respectively. The average dislocation density on GaN surface was measured as 3.85×10⁹ and 1.32×10⁹ cm⁻² for n-GaN and undoped GaN epitaxy by 2-D images of atomic force microscopy (AFM). Point analysis of photoluminescence (PL) spectra was performed for evaluating the optical properties of the GaN epitaxy. We also implemented PL mapping, which showed the distribution of edge emission peaks onto the 2 inch whole Si(1 1 1) wafers. The average FWHMs of the band edge emission peak was 367.1 and 367.0 nm related with 3.377 and 3.378 eV, respectively, using 325 nm He-Cd laser as an excitation source under room temperature.     

Cathodoluminescence study of the excitons localization in AlGaN/GaN and InGaN/GaN quantum wells grown on sapphire

Al_0.1Ga_0.9N(5 nm)/GaN(2 nm) and In_0.2Ga_0.8N/GaN quantum wells (QWs) grown on GaN/sapphire have been studied by cathodoluminescence (CL) spectroscopy and imaged using an experimental setup especially developed for scanning near-field CL microscopy, which combines a scanning force microscope and a scanning electron microscope. The CL spectra show the characteristic band edge emission peak of GaN at λ= 364 nm and the emission peaks related to the presence of QWs, at λ= 353 and 430 nm for the AlGaN/GaN and the InGaN/GaN samples, respectively. Monochromatic CL images reveal that the emission of the AlGaN/GaN and InGaN/GaN QWs is localized at the level of the grains observed by SFM. A cross sectional analysis of the InGaN/GaN sample gives insight into its growth and an estimation of the exciton diffusion length of about L=180 nm.     

Organometallic vapor phase epitaxy growth of upright metamorphic multijunction solar cells

We demonstrate an integrated metamorphic AlGaInP/AlGaInAs/GaInAs/Ge 4 J solar cell on Ge substrate using organometallic vapor phase epitaxy (OMVPE). A step graded GaInAs buffer was grown right after the Ge subcell was formed to change the lattice constant from that of Ge to that of Ga_0.8In_0.2As lattice constant followed by a 1.14 eV Ga_0.8In_0.2As subcell, a 1.5 eV (AlGa)_0.8In_0.2As subcell, and a 1.85 eV Al_xGa_0.32?xIn_0.68P subcell. Transmission electron microscope (TEM) study shows the threading dislocation density (TDD) is about 6×10⁶ cm^?2. The X-ray diffraction reciprocal space map (RSM) shows that the structure is 100% relaxed. Bandgap dependent (Al_xGa_1?x)_0.32In_0.68P subcell performance is systematically investigated. As the Al_xGa_0.32?xIn_0.68P cell bandgap goes up to 1.9 eV, the external quantum efficiency (EQE) goes down significantly. Theoretical simulation shows that the decrease of diffusion length causes the lower EQE, which indicates the material quality degrades with the increasing Al content. Integrated 4 J solar cells are fabricated and characterized with spectral response and tested under the AM1.5D terrestrial spectrum at both 1 sun and 2000 suns.     

Low temperature II–VI nanowire growth using Au–Sn catalysts

Epitaxial lateral overgrowth is reported for semi-polar (Al,Ga)N(1 1 .2) layers. The mask pattern consisted of periodic stripes of SiO₂ oriented parallel to either the GaN[1 1 .0] or the GaN[1 1 .1] direction. Lateral growth occurred either along GaN[1 1 .1] or along GaN[1 1 .0]. For growth along the [1 1 .0] direction, coalescence was achieved for layer thicknesses >4 μm. However, planarization was not observed yielding extremely corrugated surfaces. For growth in [1 1 .1] direction, coalescence was delayed by a diminishing lateral growth rate. Growth of AlGaN during ELOG resulted in coalescence. Improvement in crystal quality of such buffer layers for the growth of InGaN/GaN quantum wells was confirmed by X-ray diffraction and photoluminescence spectroscopy.     

Growth of thick,continuous GaN layers on 4-in. Si substrates by metalorganic chemical vapor deposition

We report on the growth of thick GaN epilayers on 4-in. Si(1 1 1) substrates by metalorganic chemical vapor deposition. Using intercalated AlN layers that contribute to counterbalance the tensile strain induced by the thermal mismatch between gallium nitride and the silicon substrate, up to 6.7 μm thick crack-free group III-nitride layers have been grown. Root mean-squares surface roughness of 0.5 nm, threading dislocation densities of 1.1×10⁹ cm^?2, as well as X-ray diffraction (XRD) full widths at half-maximum (FWHM) of 406 arcsec for the GaN(0 0 2) and of 1148 arcsec for the GaN(3 0 2) reflection have been measured. The donor bound exciton has a low-temperature photoluminescence line width of 12 meV. The correlation between the threading dislocation density and XRD FWHM, as well as the correlation between the wafer curvature and the GaN in-plane stress is discussed. An increase of the tensile stress is observed upon n-type doping of GaN by silicon.     

A study of the vacancy-defect distribution in a GaAs/AlxGa1−xAs multi-layer structure grown at low temperature

We have grown a multilayer structure of GaAs and Al_xGa_1−xAs (x=0.25) by molecular beam epitaxy at low substrate temperature (250°C) in order to investigate the effects of annealing on arsenic precipitation and vacancy-defect formation. The as-grown heterostructure contained ∼5×10¹⁷ cm⁻³ gallium vacancies, but information about the individual layers was lost because the layer width (∼45 nm) was smaller than the average positron diffusion length (∼70 nm). Annealing at 500 and 600°C showed increases in the S-parameter (above the bulk value) of ∼2.5% and ∼1.5%, respectively, smaller than for a single LT-GaAs layer (∼3–4%). We explain this phenomenon by suggesting that the excess arsenic which migrates from the material with the higher precipitate/matrix energy (LT-Al_xGa_1−xAs) reduces the gallium vacancy concentration in the LT-GaAs and hence the S-parameter. This hypothesis is supported by SIMS data which shows a build-up of As in the LT-GaAs layers, and TEM images which indicate that arsenic precipitation occurs to a greater extent in the LT-GaAs than in the LT-Al_xGa_1−xAs.     

The effect of annealing on the surface morphology of strained and unstrained InxAl1−xN thin films

In_xAl_1−xN is a particularly useful group-III nitride alloy because by adjusting its composition it can be lattice matched to GaN. Such lattice-matched layers may find application in distributed Bragg reflectors (DBRs) and high electron mobility transistors (HEMTs). However, compared with other semiconducting nitride alloys, In_xAl_1-xN has not been researched extensively. In this study, thin In_xAl_1−xN epilayers were grown by metal-organic vapour phase epitaxy (MOVPE) on GaN and Al_yGa_1−yN layers. Samples were subjected to annealing at their growth temperature of 790 °C for varying lengths of time, or alternatively to a temperature ramp to 1000 °C. Their subsequent surface morphologies were analysed by atomic force microscopy (AFM). For both unstrained In_xAl_1−xN epilayers grown on GaN and compressively strained epilayers grown on Al_yGa_1−yN, surface features and fissures were seen to develop as a consequence of thermal treatment, resulting in surface roughening. It is possible that these features are caused by the loss of In-rich material formed on spinodal decomposition. Additionally, trends seen in the strained In_xAl_1−xN layers may suggest that the presence of biaxial strain stabilises the alloy by suppressing the spinode and shifting it to higher indium compositions.     

Pseudomorphic growth of thick n-type AlxGa1−xN layers on low-defect-density bulk AlN substrates for UV LED applications

Recently it has been discovered that when growing Al_xGa_1−xN on low-defect-density bulk AlN substrates pseudomorphic layers can be achieved with a thickness far exceeding the critical thickness as given by the Matthews and Blakeslee model. For instance, the critical thickness of an Al_xGa_1−xN layer (with x=0.6) is about 40 nm thick. However we have been able to grow layers with this composition that are pseudomorphic with a thickness exceeding the critical thickness by more than an order of magnitude. This work defines the limits of pseudomorphic growth on low defect density, bulk AlN substrates to obtain low defect density, high-power UV LEDs.     

Strain relaxation in multilayer III–N structures on Si(111) substrates

X-ray diffractometry and X-ray scattering reciprocal space maps have been used to study strain relaxation in a complex buffer composed of seven intermediate layers of Al _x Ga_{1 ? x} N composition with different values of x, decreasing with an increase in the distance from the substrate. The layers have been grown by hydride metalorganic vapor phase epitaxy on silicon and sapphire substrates. Differences in the structural quality of the first four layers of a multilayer buffer grown on different substrates have been revealed. A gradual smoothing out of these differences in the next three layers with an increase in the layer serial number has been shown. The last grown intermediate Al _x Ga_{1 ? x} N layer and the GaN layer grown on it have identical thicknesses and degrees of mosaicity, regardless of the substrate type. Device structures grown on a complex buffer demonstrate emission in approximately the same wavelength range.     

The growth of high-quality and self-separation GaN thick-films by hydride vapor phase epitaxy

About 1.2 mm thick GaN bulk crystals were obtained by combining a pulsed NH₃-flow modulation (PFM) method and a self-separation method of short-shutting NH₃ flow when using hydride vapor phase epitaxy (HVPE). High crystal quality of bulk GaN was evaluated by X-ray rocking curves (XRC) and the full width at half maximum (FWHM) values were 110, 72 and 83 arcsec for (002), (102) and (100) reflection planes, respectively. The PFM method is proved to be effective in reducing cracks and keeping the surface smooth. And the method of short-shutting NH₃ flow can lead to GaN thick layer separate from sapphire substrate when cooling from the high growth temperature. Growth and separation mechanisms were investigated. Two states were found in PFM method. With PFM method modulating between high quality state and low stress state, 300 μm thick GaN layers without cracks were obtained. Study of spontaneous separation mechanism revealed that the separation attributed to formation of voids inside the GaN layer.     

Digitally alloyed modulated precursor flow epitaxial growth of AlxGa1−xN layers with AlN and AlyGa1−yN monolayers

We propose a new growth scheme of digitally alloyed modulated precursor flow epitaxial growth (DA-MPEG) using metalorganic and hydride precursors for the growth of Al_xGa_1−xN layers with high-Al content at relatively low temperatures. The growth of high-quality, high-Al content Al_xGa_1−xN layers (x_Al>50%) that are composed of AlN and Al_yGa_1−yN monolayers on AlN/sapphire template/substrates by DA-MPEG was demonstrated. The overall composition of the ternary Al_xGa_1−xN material by DA-MPEG can be controlled continuously by adjusting the Column III mole fraction of the atomic Al_yGa_1−yN sub-layer. X-ray diffraction and optical transmittance results show that the AlGaN materials have good crystalline quality. The surface morphology of DA-MPEG AlGaN samples measured by atomic force microscopy are comparable to high-temperature-grown AlGaN and are free from surface features such as nano-pits.     

High quality MOCVD GaN film grown on sapphire substrates using HT-AlN buffer layer

In this work we report on the growth and characterization of high quality MOCVD GaN film grown on Al₂O₃ substrates by using a HT (>1150 °C)-AlN buffer layer. We have investigated the most favorable growth conditions in terms of temperature, thickness and growth rate of AlN buffer layer in order to optimize the high temperature GaN layer. The improved morphological and structural properties of GaN layer were verified by AFM and XRD measurements. The optimized GaN layer presents a smooth surface with a rms value of 1.4 Å. The full width at half maximum (FWHM) for 800 nm thick GaN films is 144″. Furthermore PL measurements and C–V analysis confirm that in GaN layer grown on HT-AlN buffer layer defect density is drastically reduced.     

Ga and In incorporation rates in Ga1−xInxAs growth by chemical beam epitaxy

GaAs, InAs and Ga_1?xIn_xAs layers were grown by chemical beam epitaxy (CBE) using triethylgallium, trimethylindium and tertiarybutylarsine as precursors for Ga, In and As, respectively. The growth rate during the homoepitaxial growth of GaAs and InAs, deduced from the frequency of reflection high-energy electron diffraction intensity oscillations, was used to calibrate the incorporation rates for the III elements. The In content of the Ga_1?xIn_xAs layers was measured by Rutherford backscattering spectrometry and compared with the value predicted from the above calibration data; while the measured In mole fraction is close to the predicted value for the samples grown for low In to Ga flux ratios (x<0.2), the In incorporation is enhanced for larger values of this ratio. The results obtained on layers grown at different substrate temperatures show that In mole fraction is almost constant at growth temperatures in the range 400–500 °C, but a strong dependence on the substrate temperature has been found outside this range. The above results, not observed for samples grown by solid source molecular beam epitaxy, indicate that some interaction between Ga and In precursors at the sample surface could take place during the growth by CBE.     

Growth kinetics and boron doping of very high Ge content SiGe for source/drain engineering

We have studied the in-situ boron doping of high Ge content Si_1?xGe_x layers (x=0.3, 0.4 and 0.5). These layers have been grown at low pressure (20 Torr) and low temperature (600–650 °C) with a heavily chlorinated chemistry on blanket Si(0 0 1) substrates. Such a chemistry yields a full selectivity versus SiO₂ (isolation) and Si₃N₄ (sidewall spacers) on patterned wafers with gate stacks. We have quantified the impact of the diborane flow on the SiGe layer crystalline quality, its resistivity, the SiGe:B growth rate and the apparent Ge concentration. Resistivity values lower than 1 mΩ cm are easily achieved, all the more so for high Ge content layers. The SiGe growth rate increases and the apparent Ge concentration (from X-ray diffraction) decreases as the diborane flow increases. B atoms (much smaller than Si or Ge) indeed partially compensate the compressive strain in the SiGe:B layers. We have also probed the in-situ boron and phosphorus doping of Si at 750 °C, 20 Torr with a heavily chlorinated chemistry. The B ions concentration increases linearly with the diborane flow, then saturates at a value close to 4×10¹⁹ cm^?3. By contrast, the P ions concentration increases sub-linearly with the phosphine flow, with a maximum value close to 9×10¹⁸ cm^?3. Adding diborane (phosphine) to the gaseous mixture leads to a sharp increase (decrease) of the Si:B (the Si:P) growth rates, which has to be taken into account in device layers. All the know-how acquired will be most handy for the formation of in-situ doped recessed or raised sources and drains in metal-oxide semiconductor devices.     

Relation between β relaxation and fragility in LaCe-based metallic glasses

The studies of the dynamical mechanical properties of a series of (Ce_xLa_1-x)₆₈Al₁₀Cu₂₀Co₂ (where 0 ≤ x ≤ 1) metallic glasses show that the behaviors of β relaxation are closely correlated with the properties of their corresponding supercooled liquids. Metallic glasses quenched from fragile liquids show pronounced β relaxation humps whereas unobvious excess wings are associated with the metallic glasses quenched from strong liquids. Our result suggests he behaviors of β relaxations in the metallic glasses are correlated with the fragility of their supercooled liquids.     

Network topological thresholds in gallium doped As–Te glasses – Electrical and thermal investigations

Electrical switching and differential scanning calorimetric studies are undertaken on bulk As₂₀Te_80−xGa_x glasses, to elucidate the network topological thresholds. It is found that these glasses exhibit a single glass transition (T_g) and two crystallization reactions (T_c1 & T_c2) upon heating. It is also found that there is only a marginal change in T_g with the addition of up to about 10% of Ga; around this composition an increase is seen in T_g which culminates in a local maximum around x = 15. The decrease exhibited in T_g beyond this composition, leads to a local minimum at x = 17.5. Further, the As₂₀Te_80−xGa_x glasses are found to exhibit memory type electrical switching. The switching voltages (V_T) increase with the increase in gallium content and a local maximum is seen in V_T around x = 15. V_T is found to decrease with x thereafter, exhibiting a local minimum around x = 17.5. The composition dependence of T_c1 is found to be very similar to that of V_T of As₂₀Te_80−xGa_x glasses. Based on the present results, it is proposed that the composition x = 15 and x = 17.5 correspond to the rigidity percolation and chemical thresholds, respectively, of As₂₀Te_80−xGa_x glasses.     

Microstructural analysis of Ga2S3–2MCl (M = K, Rb, Cs) glasses using Raman scattering

Raman scattering spectra of Ga₂S₃–2MCl (M = K, Rb, Cs) glasses have been conducted at room temperature. Based on the analysis of the local co-ordination surroundings of Cs⁺ ions, the similarities and differences of Raman spectra for the glass Ga₂S₃–2CsCl and the bridged molecular GaCl₃ were explained successfully. Through considering the effect of M⁺ ions on mixed anion units [GaS_4?xCl_x] and bridged units [Ga₂S_6?xCl_x] and the corresponding microstructural model, the Raman spectral evolution of the Ga₂S₃–2MCl (M = K, Rb, Cs) glasses was reasonably elucidated.     

Chemical lift-off of (11–22) semipolar GaN using periodic triangular cavities

Chemical lift-off of (11–22) semipolar GaN using triangular cavities was investigated. The (11–22) semipolar GaN was grown using epitaxial lateral overgrowth by metal-organic chemical vapor deposition on m-plane sapphire, in such a way as to keep N terminated surface of c-plane GaN exposed in the cavities. After regrowing 300 μm thick (11–22) semipolar GaN by hydride vapor phase epitaxy for a free-standing (11–22) semipolar GaN substrate, the triangular cavities of the templates were chemically etched in molten KOH. The (000-2) plane in the triangular cavities can be etched in the [0002] direction with the high lateral etching rate of 196 μm/min. The resulting free-standing (11–22) semipolar GaN substrate was confirmed to be strain-free by the Raman analysis.