Pre-growth Treatment Research Articles

Optimizing single crystal diamond mosaic growth: A study on seed thickness variation and pre-growth treatment

The limitations on the size of single-crystal diamond applications still need to be addressed for practical use. This study systematically investigates the impact of variations in seed crystal thickness and pre-growth treatments on step bonding, defect formation, and stress distribution in the mosaic growth of single-crystal diamond films. The findings suggest that variations in seed crystal thickness within 50 μm result in a smooth mosaic junction and high crystal quality, while a 100 μm variation results in noticeable defects and stress concentrations, hindering adequate bonding. Pre-growth treatment improves crystal quality by reducing polycrystalline formations and stress concentrations. Optical microscopy and Raman mapping confirm that pre-growth treated mosaic regions exhibit smooth steps and seamless bonding, with FWHM values between 3 and 5 cm−1, consistent with the quality of non-mosaic growth regions.

Impact of surface preparation on the epitaxial growth of SrTiO3 on ScAlN/GaN heterostructures

Heterogeneous integration of functional oxides with ultra-wide bandgap (UWBG) semiconductors is desired for the realization of novel hybrid systems applicable to a wide array of commercial electronics and defense applications. In this work, we demonstrate the growth of crystalline SrTiO3 (STO) thin films on high-electron-mobility transistor (HEMT) heterostructures based on an emergent UWBG semiconductor ScAlN, used as the barrier layer on a GaN channel, and determine the effects of the pre-growth chemical treatments of the ScAlN surface on resultant heterostructure properties. We investigate wet chemical cleans of ScAlN with solvents, piranha solution, UV ozone and hydrofluoric acid, and a sulfuric-phosphoric acid mix prior to STO growth, and show that the commonly used piranha solution degrades the ScAlN surface, thereby reducing the crystal quality of the deposited STO layers and lowering the channel mobility. We determine that among the treatments studied, the solvent and sulfuric-phosphoric acid cleans were the least disruptive to the electrical properties of the GaN channel as evidenced from Hall effect measurements, but the sulfuric-phosphoric acid clean results in best oxide crystallinity, as determined from structural characterizations. We perform transmission electron microscopy imaging on the piranha-treated and sulfuric-phosphoric-treated samples to compare the microstructure and find that while intermixing occurs at the oxide-nitride interfaces for both samples, the interface roughness is lower and the STO grain size is larger in the sample with sulfuric-phosphoric acid treatment. This work demonstrates the first epitaxial growth of STO on an UWBG semiconductor and motivates STO/ScAlN/GaN as material platforms for high-frequency, high-power-density HEMTs.

Over 80% power-added-efficiency GaN high-electron-mobility transistors on free-standing GaN substrates

This paper demonstrates highly efficient GaN high-electron-mobility transistors (HEMTs) on GaN substrates with reduced interface contamination. By applying a hydrofluoric acid-based pre-growth treatment to a GaN substrate, the Si impurity concentration at the interface between the GaN substrate and the epitaxial layer can successfully be reduced. RF performance was enhanced by pre-growth treatment owing to the suppression of Si-induced parasitic loss. As a result, GaN HEMTs on GaN substrates exhibited an excellent power-added efficiency of 82.8% at a 2.45 GHz. To the best of our knowledge, this exceeds that of the previously reported discrete GaN HEMTs at around this frequency range.

Photoluminescence of pre-growth treatment induced below-bandgap states in GaAs

Below-bandgap states induced by pre-epitaxial growth treatments in semi-insulating (s.i.) GaAs has previously been investigated by photoluminescence (PL). Various electronic states giving rise to near-infrared PL peaks are generated through processes such as annealing or buffer growth that are standard for epitaxial growth of InAlGaAs structures on (001) GaAs substrates. These states often overlap with those from InGaAs structures, not only complicating the interpretation but also altering the electronic properties of the devices. The present study extends the investigation to the case of doped n- and p-type (001) GaAs substrates. These substrates are widely utilized in various optoelectronic applications, and therefore information on below bandgap states are important to distinguish the desired electronic states from the pre-growth treatment-induced defect states. It was found that the behavior of n- and p-type substrates subjected to pre-growth treatment are different from that of s.i. GaAs.

Superconducting Diamond on Silicon Nitride for Device Applications

Chemical vapour deposition (CVD) grown nanocrystalline diamond is an attractive material for the fabrication of devices. For some device architectures, optimisation of its growth on silicon nitride is essential. Here, the effects of three pre-growth surface treatments, often employed as cleaning methods, were investigated. Such treatments provide control over the surface charge of the silicon nitride substrate through modification of the surface functionality, allowing for the optimisation of electrostatic diamond seeding densities. Zeta potential measurements and X-ray photoelectron spectroscopy (XPS) were used to analyse the silicon nitride surface following each treatment. Exposing silicon nitride to an oxygen plasma offered optimal surface conditions for the electrostatic self-assembly of a hydrogen-terminated diamond nanoparticle monolayer. The subsequent growth of boron-doped nanocrystalline diamond thin films on modified silicon nitride, under CVD conditions, produced coalesced films for oxygen plasma and solvent treatments, whilst pin-holing of the diamond film was observed following RCA-1 treatment. The sharpest superconducting transition was observed for diamond grown on oxygen plasma treated silicon nitride, demonstrating it to be of the least structural disorder. Modifications to the substrate surface optimise the seeding and growth processes for the fabrication of diamond on silicon nitride devices.

A simple route to synchronized nucleation of self-catalyzed GaAs nanowires on silicon for sub-Poissonian length distributions

We demonstrate a simple route to grow ensembles of self-catalyzed GaAs nanowires with a remarkably narrow statistical distribution of lengths on natively oxidized Si(111) substrates. The fitting of the nanowire length distribution (LD) with a theoretical model reveals that the key requirements for narrow LDs are the synchronized nucleation of all nanowires on the substrate and the absence of beam shadowing from adjacent nanowires. Both requirements are fulfilled by controlling the size and number density of the openings in SiOx, where the nanowires nucleate. This is achieved by using a pre-growth treatment of the substrate with Ga droplets and two annealing cycles. The narrowest nanowire LDs are markedly sub-Poissonian, which validates the theoretical predictions about temporally anti-correlated nucleation events in individual nanowires, the so-called nucleation antibunching. Finally, the reproducibility of sub-Poissonian LDs attests the reliability of our growth method.

Reduced twinning and surface roughness of Bi2Se3 and Bi2Te3 layers grown by molecular beam epitaxy on sapphire substrates

The authors investigate the structural properties of Bi2Se3 and Bi2Te3 topological insulator layers grown on sapphire (0001) substrates by molecular beam epitaxy, using various pregrowth optimization methods. Samples of Bi2Se3 grown on sapphire with a particular combination of pregrowth surface treatment steps showed evidence of greatly reduced twinning and significantly reduced surface roughness. Evidence of twinning, and its suppression by the appropriate choice of pregrowth steps, is obtained from x-ray diffraction Φ-scan measurements as well as atomic force microscopy (AFM) images. Improved surface roughness is also evident from the AFM images. Growth of Bi2Te3 on sapphire exhibited similar reduced twinning by the application of the pregrowth surface preparation steps, although the surface roughness was significantly greater than that of Bi2Se3. Additional improvements in surface roughness of the Bi2Te3 were achieved by growing the Bi2Te3 on a Bi2Se3 buffer layer.

Heteroepitaxy of orientation-patterned nonlinear optical materials

We report some recent results on thick heteroepitaxial growth of GaP on GaAs substrates and on orientation-patterned (OP) GaAs templates conducted in a hot-wall horizontal quartz reactor for Hydride Vapor Phase Epitaxy. The growths on the plain substrates resulted in up to 500 µm thick GaP with smooth surface morphology (RMS < 1–2 nm) and high crystalline quality (FWHM = 100–150 arcsec), comparable to the quality of the related homoepitaxial growths of GaP on GaP. Up to 300 µm thick OPGaP quasi-phase matching structures with excellent domain fidelity were also heteroepitaxially grown with high reproducibility on OPGaAs templates in support of frequency conversion laser source development for the mid and longwave infrared. We studied the GaAsxP1−x ternary transition layer that forms between the growing film and the substrate. We also undertook steps to determine some important characteristics of heteroepitaxy such as thickness of the pseudomorphous growth and periodicity of the expected misfit dislocations. The formation of these and some other defects and their distribution within the layer thickness was also investigated. Samples were characterized by Nomarski optical microscopy, transmission optical measurements, transmission electron microscopy, scanning electron microscopy, atomic force microscopy, X-ray diffraction and energy dispersive X-ray spectroscopy. The focus was predominantly on the interface and, more precisely, on what influence the pre-growth surface treatment of the substrate has on the initial and the following stages of growth, as well on the mechanisms of the strain relaxation from the lattice and thermal mismatch between layer and substrate. The efforts to accommodate the growing film to the foreign substrate by engineering an intermediate buffer layer were extended to thick growths of GaAsxP1−x ternary with the idea to combine in one material the best of the nonlinear properties of GaP and GaAs that are strictly relevant to the pursued applications.

Role of Cu foil in-situ annealing in controlling the size and thickness of CVD graphene domains

Producing graphene with minimal crystalline defects and a controllable number of layers is highly desirable for its integration in advanced technological applications. Here we show how the chemical vapor deposition (CVD) protocol can be adjusted in order to control the size, the shape and the thickness of graphene domains. More particularly, this work focuses on the correlation between the conditions employed during the Cu foil in-situ thermal treatment and the subsequent graphene growth. The influence of Cu pre-growth treatments has been systematically investigated by considering two different pressure regimes (1 and 800 mbar) and various gas compositions (pure Ar, Ar/H2 and Ar/O2 mixtures) while using identical graphene growth conditions. We show that exposing the Cu foil to oxygen, either present as residuals in the Ar feedstock or precisely dosed using the Ar/O2 gas line, effectively contributes to reduce the catalyst carbon content prior to graphene growth and hence significantly decreases the nucleation site density. It is also found that annealing the Cu catalyst in a hydrogen-free environment favors the formation of defect-free multi-layer graphene branches that extend underneath the millimeter-size single-crystalline graphene top layer. Our results provide clear guidelines to control the formation of graphene nuclei and additional layers.

(Invited) Material Challenges in Low Offcut Substrates for High Power SiC Devices

Silicon carbide is a material of interest for high-voltage and high-power switching device applications. Basal plane dislocations (BPDs) are a major concern for SiC bipolar devices as they source Shockley-type stacking faults in the presence of an electron-hole plasma and reduce minority carrier lifetimes [1, 2]. Many researchers have investigated methods to reduce the BPD density by experimenting with pre-growth treatments [3-5], substrate orientation [6], growth parameters [6, 7] and growth interrupts [8]. This work investigates extended defects, morphology and lifetime in 4H-SiC epilayers grown on substrates offcut 2° toward the [11-20].Epilayers were synthesized on 2° offcut substrates in a horizontal hot-wall reactor using the standard chemistry of silane (2% in H2) and propane. Epilayers were grown at various growth rates, C/Si ratios, and growth temperatures. The pressure was maintained at 100 mbar for all growths. Some samples were grown with a 5 µm highly doped n+ buffer layer using ultra high purity nitrogen prior to the low-doped epilayers. Ultraviolet photoluminescence (UVPL) imaging was used to identify BPDs in the low doped epilayers. Time resolved photoluminescence measurements were performed to determine the minority carrier lifetime of the layers and analysis of Raman spectroscopic maps revealed the location of polytype inclusions. Electron trap concentrations were determined using deep level transient spectroscopy (DLTS). Surface roughness was measured by atomic force microscopy and the morphology was also characterized using Nomarski microscopy and white light interferometry. When a 15 µm epilayer was grown without a buffer layer, step bunching was observed and the surface roughness was 6.0 nm RMS. For comparison, a standard 4° offcut sample typically has 3.0 nm RMS for a 20 µm epilayer. Using UVPL, it was found that after 4 µm of epi, 90% of the BPDs had converted in the low doped layer as compared to 70% in a 4° offcut sample, indicating the conversion is faster in the lower offcut material. The conversion results were from an older substrate and vendor A. For newer substrates, vendor B, the density of BPDs at the epilayer/substrate interface was ≤ 0.2 cm-2. 3C-SiC inclusions were present in the epilayers as verified using Raman spectroscopy for both unintentionally doped (UID) and N+ epilayers. These inclusions were eliminated by increasing the growth temperature and lowering the C/Si ratio for N+ epilayers, but by increasing C/Si ratios for UID films. Changing these growth parameters resulted in specular film morphology and resulted in minority carrier lifetimes of approximately 1 µs. [1] J.P. Bergman, et al. Mater. Sci. Forum Vol. 353-356, 299 (2001). [2] R.E. Stahlbush, et al., J. Electron. Mater. 31, 370 (2002). [3] Z. Zhang, et al., Appl. Phys. Lett. 89, 081910 (2006). [4] J.J. Sumakeris, et al., Mater. Sci. Forum 527-529, 529 (2006). [5] H. Tsuchida, et al., Mater. Sci. Forum 483-485, 97 (2005). [6] W. Chen and M.A. Capano J. Appl. Phys. 98, 114907 (2005). [7] T. Ohno, et al., J. Cryst. Growth 271, 1 (2004). [8] R. E. Stahlbush, et al., Jr., Appl. Phys. WeLett. 94, 041916 (2009).

SURFACTANT EFFECT OF In ON THE MOVPE GROWTH OF Al- AND N-POLAR AlN

Al- and N-polar AlN have been grown by metalorganic vapor phase epitaxy (MOVPE) with the assistance of In dopant and characterized by in situ interferometry, ellipsometry, scanning electron microscopy, atomic force microscopy, and X-ray diffractometry. The growth of Al-polar AlN is faster with smoother surfaces than the N-polar ones, which is explained by theoretical calculations. The surfactant effect of In is confirmed by improving the growth rate and surface flatness without getting into the epilayer. Additionally, In is also favorable for reducing the density of dislocations and improving the crystalline quality, especially that of Al-polar AlN. The results suggest that using In surfactant to grow the Al-polar AlN epilayer leads to a better crystal quality under proper pre-growth treatments, low- and high-temperature AlN growth conditions.

Increased monolayer domain size and patterned growth of tungsten disulfide through controlling surface energy of substrates

We report a surface energy-controlled low-pressure chemical vapor deposition growth of WS2 monolayers on SiO2 using pre-growth oxygen plasma treatment of substrates, facilitating increased monolayer surface coverage and patterned growth without lithography. Oxygen plasma treatment of the substrate caused an increase in the average domain size of WS2 monolayers by 78% ± 2% while having a slight reduction in nucleation density, which translates to increased monolayer surface coverage. This substrate effect on growth was exploited to grow patterned WS2 monolayers by patterned plasma treatment on patterned substrates and by patterned source material with resolutions less than 10 µm. Contact angle-based surface energy measurements revealed a dramatic increase in polar surface energy. A growth model was proposed with lowered activation energies for growth and increased surface diffusion length consistent with the range of results observed. WS2 samples grown with and without oxygen plasma were similar high quality monolayers verified through transmission electron microscopy, selected area electron diffraction, atomic force microscopy, Raman, and photoluminescence measurements. This technique enables the production of large-grain size, patterned WS2 without a post-growth lithography process, thereby providing clean surfaces for device applications.

Controlling the polarity of metalorganic vapor phase epitaxy-grown GaP on Si(111) for subsequent III-V nanowire growth

Nanowire growth on heteroepitaxial GaP/Si(111) by metalorganic vapor phase epitaxy requires the [-1-1-1] face, i.e., GaP(111) material with B-type polarity. Low-energy electron diffraction (LEED) allows us to identify the polarity of GaP grown on Si(111), since (2×2) and (1×1) surface reconstructions are associated with GaP(111)A and GaP(111)B, respectively. In dependence on the pre-growth treatment of the Si(111) substrates, we were able to control the polarity of the GaP buffers. GaP films grown on the H-terminated Si(111) surface exhibited A-type polarity, while GaP grown on Si surfaces terminated with arsenic exhibited a (1×1) LEED pattern, indicating B-type polarity. We obtained vertical GaAs nanowire growth on heteroepitaxial GaP with (1×1) surface reconstruction only, in agreement with growth experiments on homoepitaxially grown GaP(111).

Management and interpretation of heterogeneous observational data: Using insulin-like growth factor-I data from the NordiNet® International Outcome Study

ObjectiveThe NordiNet® International Outcome Study (IOS), a large-scale, non-interventional, multi-centre, real-world study of Norditropin® treatment, registers insulin-like growth factor-I (IGF-I) values, as measured by different assays. This paper considers the potential biases introduced by using a single IGF-I reference data set in analysing NordiNet® IOS data. DesignTo evaluate possible biases from different IGF-I assays used across NordiNet® IOS, a mixed-effect linear model was fitted to IGF-I data (analyses on log-transformed data). Pre-growth hormone treatment (pre-GHT) IGF-I values were assumed to depend on diagnosis, sex and age. During GHT, a treatment-effect dependent on these factors was added. Differences between assays were assumed multiplicative on the original scale. Individual measurements were scaled to a common level (Nichols Advantage) giving adjusted IGF-I standard deviation score (SDS) values. ResultsIn total, 49 495 IGF-I measurements were available from 9481 paediatric patients. Mixed-effect linear modelling showed a systematic difference between IGF-I levels measured by different assays. Differences were minimised when assessing change in IGF-I SDS from the start of GHT to 1-year follow-up. This applied to values adjusted for actual-assay used and for unadjusted delta IGF-I SDS values. Largest differences between unadjusted change in IGF-I SDS values were: for growth hormone deficiency 0.1 (girls) and 0.3 (boys); for small-for-gestational age 0.1; and for Turner syndrome 0.2. Similar magnitude differences were seen for data with unknown assay. ConclusionsAnalysis and modelling suggest the current approach to IGF-I data collection and analyses in the NordiNet® IOS is sound: in a large cohort without assay-used information, potential bias is minimised by analysing changes in IGF-I SDS.

Innovative Solutions to Materials Challenges for High-Voltage SiC Power Devices

Silicon carbide (SiC) is a material of interest for high-temperature, high-voltage and high-power switching device applications. Key materials challenges inhibiting such devices include elimination of basal plane dislocations (BPDs) and enhancement of minority carrier lifetimes in the drift or blocking regions of the device. Recent progress in addressing both of these issues is presented. BPDs are a major concern for the SiC bipolar devices required for high-voltage applications as they source Shockley-type stacking faults in the presence of an electron-hole plasma and reduce minority carrier lifetimes. Many methods have been investigated to reduce the BPD density including pre-growth treatments, substrate orientation, growth parameters and growth interrupts. It has been shown that the conversion of BPDs to threading edge dislocations (TEDs) continues throughout the epitaxial growth process in 4° off-axis SiC material and that a minimum thickness of ~16 µm is required to convert all BPDs to TEDs. Here we show that optimizing a hydrogen etch of the substrate prior to epitaxial growth significantly enhances conversion efficiency in a thin highly doped n+ buffer layer (BL). IN this work, epitaxial layers were grown on 4° off-axis substrates in an Aixtron/Epigress VP508 horizontal hot-wall reactor using the standard chemistry of silane (2% in H2) and propane. In addition, using various growth approaches, low-doped epitaxial layers of only 20 µm in thickness on a 5 µm highly doped buffer layer have demonstrated minority carrier lifetimes up to 4 µs, as measured by time resolved photoluminescence, without any pre- or post- processing treatment. Interface recombination likely dominates these measurements. We extend this approach by presenting new data investigating the non-uniformity of lifetime found on as-grown material under various conditions.This work is supported by the Office of Naval Research

Monte Carlo Study of the Early Growth Stages of 3C-SiC on Misoriented and 6H-Sic Substrates

In this paper we used three dimensional kinetic Monte Carlo simulations on super-lattices to study the hetero-polytypical growth of cubic silicon carbide polytype (3C-SiC) on hexagonal 6H-SiC substrates with miscuts towards the &lt;11-20&gt; and &lt;1-100&gt; directions. We analyze the grown film for different miscut angles (in the range 2° to 12° degrees) and different growth rates, finding that substrates with miscut of 3-4° degrees towards the &lt;1-100&gt; direction should be the best choice for the growth of high quality cubic epitaxial films, being able to promote, given a suitable pre-growth treatment to induce step bunching, the nucleation of single domain 3C-SiC films.

高温预生长对图形化蓝宝石衬底GaN薄膜质量的提高

高温预生长对图形化蓝宝石衬底GaN薄膜质量的提高

Native-oxide-based selective area growth of InP nanowires via metal–organic molecular beam epitaxy mediated by surface diffusion

The growth of InP nanowires on an InP(111) B substrate is reported. The substrate native oxide was not removed from the surface prior to growth. Nanowires were grown at 400 °C from gold catalysts in a selective area manner, without bulk growth. Unlike SiO2-based metal–organic molecular beam epitaxy selective area growth, the growth reported here is mediated by surface diffusion with a characteristic diffusion length of 4 μm, about an order of magnitude larger than values for diffusion on bare substrates. A pre-growth heating treatment at 450 °C was found to increase the yield of nanowire nucleation from the gold catalysts.

Monte Carlo study of the hetero-polytypical growth of cubic on hexagonal silicon carbide polytypes

In this article we use three dimensional kinetic Monte Carlo simulations on super-lattices to study the hetero-polytypical growth of cubic silicon carbide polytype (3C-SiC) on misoriented hexagonal (4H and 6H) substrates. We analyze the quality of the 3C-SiC film varying the polytype, the miscut angle and the initial surface morphology of the substrate. We find that the use of 6H misoriented (4°–10° off) substrates, with step bunched surfaces, can strongly improve the quality of the cubic epitaxial film whereas the 3C/4H growth is affected by the generation of dislocations, due to the incommensurable periodicity of the 3C (3) and the 4H (4) polytypes. For these reasons, a proper pre-growth treatment of 6H misoriented substrates can be the key for the growth of high quality, twin free, 3C-SiC films.

Effect of High-Temperature Pregrowth Treatment on the Surface Morphology of GaInP Epilayers on Ge Grown by Metal–Organic Vapor-Phase Epitaxy

Effect of High-Temperature Pregrowth Treatment on the Surface Morphology of GaInP Epilayers on Ge Grown by Metal–Organic Vapor-Phase Epitaxy