Low-temperature Buffer Layer Research Articles

Optimized InGaAs growth on GaP/Si(1 0 0) templates with different low-temperature layers

In this paper, the effect of low-temperature buffer layer on the quality of InGaAs grown on GaP/Si templates is investigated. Three different materials, InP, GaAs/InAs and InAs/InP, were grown on GaP/Si templates by molecular beam epitaxy (MBE). The interfacial states between the above materials and the GaP layer were calculated using first principles methods. It was found that the interfacial structure formed with the InAs/InP material was the most robust due to the shorter bond lengths and stronger interaction forces between the interfaces. Theoretical calculations are in agreement with the experimental results that the InGaAs grown using the InAs/InP digital alloy structure as a low temperature buffer layer has the best material quality.

Growth of high quality CuO thin film and investigation of its abnormal luminescence behavior

Given the widespread application of Copper oxide in fields such as energy and environment, it is crucial to conduct in-depth research on its growth and basic physical properties. In this investigation, Copper oxide films were fabricated using spin-coating method, and the impact of heat-treatment temperature on the structure and optical characteristics of the samples was revealed. The experimental results indicate that the use of a low-temperature homogeneous buffer layer can improve the densification of CuO thin films. Polishing up the quality of metal-oxide films through buffer layers is a commonly used method. Raman spectra show that all samples have a monoclinic structure, and their crystalline quality gradually improves with the increase of annealing temperature. The photoluminescence spectra show that all samples exhibit similar emission spectral profiles, including three emission bands with central wavelengths at 404, 478, and 569 nm, respectively. Based on the results of EDX, XPS, and other researchers’ reports, we suggest that the violet emission located at 404 nm comes from the interband transition of electrons; The blue emission with a wavelength of 478 nm is ascribed to the transition of electrons from the conduction band (CB) to the copper vacancy level; The yellow-green emission located at 569 nm maybe originates from electronic transitions associated with surface oxygen vacancy defects.

Improving crystal quality of β-phase MgGaO thin films by using low-temperature homo-buffer layer

Ultra-wide bandgap (UWBG) semiconductors are promising for many applications, such as power electronics and deep-ultraviolet photonics. In this research, UWBG β-phase magnesium gallium oxide (MgGaO) thin films with a bandgap of 5.1 eV were grown using low-temperature homo-buffer layers in a plasma-assisted molecular beam epitaxy system. The role of the growth temperature and thickness of low-temperature buffer layer on the quality of the active layer was studied using x-ray diffraction and transmission electron microscopy and by analyzing the properties of metal–semiconductor–metal photodetector devices based on these films. It is found that lower buffer growth temperature at 300 °C leads to higher crystal quality of active layer. For the same low buffer growth temperature, different crystal quality in the active layer is attained with different buffer layer thickness. A buffer layer thickness at 40 nm has the best active layer quality with the highest photo current under 265 nm illumination and long decay time as a result of reduced recombination of photo-generated carriers through fewer defects in the active layer.

Improved crystallinity and surface morphology of a-plane AlN grown on high temperature annealed AlN/sapphire template by pulsed-flow mode metal-organic vapor phase epitaxy

Preparing high quality non-polar aluminum nitride (AlN) templates is the key to improving the performance of non-polar deep-ultraviolet light-emitting diodes. In this study, we investigated the effect of buffer layer on the crystallinity and surface morphology of a-plane AlN films regrown by pulsed-flow mode (PM) metal-organic vapor phase epitaxy (MOVPE). Three buffer layers were compared including low-temperature AlN buffer layer grown by MOVPE (MO-buffer), sputtered AlN buffer layer (SP-buffer), and high-temperature annealed sputtered AlN buffer layer (HTA-buffer). It is found that the (11-20) plane x-ray rocking curve-full width at half maximum (XRC-FWHM) values of a-plane AlN films are significantly reduced after the regrowth process. Thanks to the high crystalline quality of HTA-buffer, AlN regrown on HTA-buffer exhibits the smallest (11-20) plane XRC-FWHM values of 1260/1440 arcsec along the [0001]/[1-100] direction. The mosaic tilt and basal plane stacking fault density are estimated to be 0.41° and 1.76 106 cm−1, respectively. The surface shows a uniform stripe-like pattern with the lowest root mean square value of 0.82 nm. Furthermore, the a-plane AlN epilayer regrown on HTA-buffer displays a weak in-plane stress anisotropy and high optical transmittance in the ultraviolet-visible region. Our work suggests that combining the PM regrowth with HTA-AlN buffer layer is a promising route to prepare high-quality a-plane AlN templates for efficient non-polar deep-ultraviolet light emitters.

High crystallinity N-polar InGaN layers grown on cleaved ScAlMgO4 substrates

We have grown high-crystallinity InGaN layers on ScAlMgO4 (SAM) substrates using metalorganic vapor-phase epitaxy. We have prepared atomically flat SAM substrates by cleaving them along the c-plane and have utilized direct InGaN growth without any low-temperature buffer layer. The resulting InGaN layer has a distinct hexagonal hillock morphology and remarkable crystalline quality. The x-ray rocking curve measurements showed that (0002̄) and (10–1–2) peaks full widths at half-maximum are as good as 384 and 481 arcsec, respectively. The calculated threading dislocations densities are as low as 2.9 × 108 and 1.6 × 109 cm−2 in the case of screw-type and edge-type dislocations, respectively.

Interface formation mechanism of GaN on Al-pretreated ScAlMgO4 (0001) substrates

GaN layers are grown on Al-pretreated ScAlMgO4 (0001) substrates by metal-organic vapor-phase epitaxy (MOVPE) without using low-temperature (LT) buffer layers. The Al pretreatment is performed under a H2 and N2 gas mixture and results in the nucleation of AlN on the ScAlMgO4. High-resolution transmission electron microscopy reveals that the ScAlMgO4 (0001) surface is terminated with the rocksalt phase ScO (111). Because of the chemical nature of the ScO layer, the AlN nuclei are N-polar. However, subsequent GaN MOVPE inverts the polarity to metal-polar in the vicinity of the boundary between the interfacial AlN nucleation layer and GaN layer, which is attributed to the GaN surface during MOVPE being more stable when it is metal-polar. The grown GaN layers exhibit better crystalline quality compared with GaN grown on ScAlMgO4 or sapphire (0001) substrates using conventional LT-buffer technology, thereby demonstrating the promise of the method to produce high-quality GaN epilayers on ScAlMgO4.

Enhancing the electron mobility in Si-doped (010) β-Ga2O3 films with low-temperature buffer layers

We demonstrate a new substrate cleaning and buffer growth scheme in β-Ga2O3 epitaxial thin films using metal–organic vapor phase epitaxy (MOVPE). For the channel structure, a low-temperature (LT, 600 °C) un-doped Ga2O3 buffer was grown, followed by a transition layer to a high-temperature (HT, 810 °C) Si-doped Ga2O3 channel layers without growth interruption. The (010) Ga2O3 Fe-doped substrate cleaning uses solvent cleaning, followed by additional hydrofluoric acid (49% in water) treatment for 30 min before the epilayer growth. This step is shown to compensate the parasitic Si channel at the epilayer–substrate interface that originates from the substrate polishing process or contamination from the ambient. From secondary ion mass spectroscopy (SIMS) analysis, the Si peak atomic density at the substrate interface is found to be several times lower than the Fe atomic density in the substrate—indicating full compensation. The elimination of the parasitic electron channel at the epi–substrate interface was also verified by electrical (capacitance–voltage profiling) measurements. In the LT-grown (600 °C) buffer layers, it is seen that the Fe forward decay tail from the substrate is very sharp, with a decay rate of ∼9 nm/dec. X-ray off-axis rocking curve ω-scans show very narrow full width at half maximum (FWHM) values, similar to the as-received substrates. These channels show record high electron mobility in the range of 196–85 cm2/V⋅s in unintentionally doped and Si-doped films in the doping range of 2 × 1016–1 × 1020 cm−3. Si delta-doped channels were also grown utilizing this substrate cleaning and the hybrid LT buffers. Record high electron Hall mobility of 110 cm2/V⋅s was measured for sheet charge density of 9.2 × 1012 cm−2. This substrate cleaning, combined with the LT buffer scheme, shows the potential of designing Si-doped β-Ga2O3 channels with exceptional transport properties for high-performance Ga2O3-based electron devices.

Heteroepitaxial Growth of GaP Photocathode by Hydride Vapor Phase Epitaxy for Water Splitting and CO2 Reduction

Heteroepitaxial Zn-doped p-GaP was grown on (001) GaAs, (001) Si and (111) Si substrates by hydride vapor phase epitaxy for solar-driven photoelectrochemical applications of hydrogen generation by water splitting and CO2 reduction. Growth of GaP on Si was realized through the implementation of a low-temperature buffer layer, and the morphology and crystalline quality were enhanced by optimizing the precursor flows and pre-heating ambient substrate. The p-GaP/GaAs and p-GaP/Si samples were processed to photoelectrodes with an amorphous TiO2 coating for CO2 reduction and a combination of TiO2 layer and mesoporous tungsten phosphide catalyst for water splitting. P-GaP/GaAs with suitable Zn-doping concentration exhibited photoelectrochemical performance comparable to homoepitaxial p-GaP/GaP for water splitting and CO2 reduction. Degradation of photocurrent in p-GaP/Si photoelectrodes is observed in PEC water splitting due to the high density of defects arising from heteroepitaxial growth.

Impact of Defects for AlN Single Crystal Thin Film by Metal Nitride Vapor Phase Epitaxy.

Herein, the defect-related properties of an AlN sample prepared based on the optimal process parameters by metal nitride vapor phase epitaxy (MNVPE) were investigated. The FWHM values of the (0002)/(101̅2) planes of the sample by MNVPE are 397/422 arcsec; the advantages of similar FWHM values of (0002) and (101̅2) planes will have a huge advantage over other preparation methods such as MOCVD. From the cross-sectional TEM images of the AlN sample, it is found that the fusion of a large number of a + c type dislocations occur at the interface of the low temperature buffer layer and the epitaxial layer, which affects the growth mode of the epitaxial layer. The lower FHWM value of the E 2(high) peak of the Raman spectrum, the lower the point defect concentration, which made the sample gain higher energy defect emission bands in the PL spectra and higher transmittance in the UV-vis transmission spectrum.

Structural and optical properties of cubic GaN on U-grooved Si (100)

Cubic GaN epitaxy on large-area U-grooved silicon (100) dies is demonstrated by metalorganic chemical vapor deposition, and its structural and optical properties are reported. Scanning electron, atomic force, and transmission electron microscopy studies reveal that cubic GaN shows no discernible threading dislocations and a low stacking fault density of 3.27 ± 0.18 × 104 cm−1. Temperature-dependent photoluminescence studies reveal as-grown cubic GaN band edge emission internal quantum efficiency as 25.6% ± 0.9%. Selective etching of the low-temperature AlN buffer layer, SiO2 sidewalls, and hexagonal-phase GaN is demonstrated, which increases the cubic GaN band edge emission internal quantum efficiency to 31.6% ± 0.8%. This increase is attributed to the decrease in the radiative recombination lifetime via the removal of defective hexagonal-phase GaN. Overall, cubic GaN on U-grooved silicon with high structural and optical quality is reported, promising its suitability for next-generation devices.

Structural characterization of ELO-GaN film on mask-stripe patterned GaAs (0 0 1) substrate grown by metalorganic vapor phase epitaxy

GaN films were epitaxial laterally overgrown on the [110]-stripe-patterned GaAs (001) substrate by MOVPE. AlGaN intermediate layers and low-temperature GaN buffer layer were inserted in order to prevent structural defects. The epitaxial lateral overgrowth of GaN results in a trapezoidal shape with flat facets. Structural phases and defects distribution were studied by TEM. Selective-area diffraction pattern indicates the mixed phase of GaN and a high density of stacking faults at nucleation. However, with the deposition of dielectric SiNx masks, a lateral overgrowth resulted in nearly free of defects, and the cubic phase transformed into hexagonal despite a cubic template. Moreover, dark-field TEM images were used to demonstrate the structural phase transformation.

Microstructural analysis of N-polar InGaN directly grown on a ScAlMgO4(0001) substrate

We report the characterization of a N-polar InGaN layer deposited by metalorganic vapor-phase epitaxy on a ScAlMgO4(0001) (SAM) substrate without a low-temperature buffer layer. The InGaN layer was tensile-strained, and its stoichiometry corresponded to In0.13Ga0.87N. We also present the microstructural observation of the InGaN/SAM interface via integrated differential phase contrast-scanning transmission electron microscopy. The results show that the interface between N-polar InGaN and SAM occurs between the O atoms of the O–Sc SAM surface and the (Ga,In) atoms of InGaN.

Growth of high-quality nitrogen-polar GaN film by two-step high-temperature method

Normally, nitrogen-polar (N-polar) GaN films with better crystalline quality tend to have poor surface morphology, whereas those with good surface morphology suffer from degraded crystalline quality. In this work, we use a two-step high-temperature (HT) growth method to grow N-polar GaN films on vicinal c-plane sapphire substrates by metalorganic chemical vapor deposition, which can effectively solve the irreconcilability between surface morphology and crystalline quality. The GaN film consists of two GaN layers (HT-GaN 1 and HT-GaN 2), which were grown at different growth parameters, respectively. The 300-nm-thick HT-GaN 1 layer was directly grown on the low-temperature GaN buffer layer under the higher V/III ratio and pressure at 1040 °C to reduce dislocation density. The 1.7-μm-thick HT-GaN 2 layer was grown under the lower V/III ratio and pressure to improve surface morphology. Besides, the growth temperature of HT-GaN 2 layer was ramped up to 1070 °C to suppress the yellow-band luminescence. Finally, high-quality N-polar GaN films with smooth surface morphology and high crystalline quality were obtained, for which the full-width at half-maximum values of (0002) and (101¯2) planes X-ray diffraction rocking curves are 131 and 390 arcsec, respectively, the root mean square roughness over an area of 20 × 20 μm2 is 1.88 nm, and the residual compressive stress is as low as 0.025 GPa. It is reasonably believed that this work provides an effective approach to preparing high-quality N-polar GaN films for the application of N-polar nitride devices.

Macrodefects investigation in a-GaN films

Investigation of the effect of crystal defects on nonpolar a-GaN films grown by metalorganic chemical vapor deposition on r-sapphire substrates is presented. Samples were grown at different temperatures and the same V/III ratio. High-temperature nucleation in combination with low-temperature GaN buffer layer growth was used to achieve a smooth morphology and “mirror” surface. The macrostructures of the defects were investigated using a scanning electron microscope. Defect concentration via dislocation type correlation was analyzed by diffractometry investigation. Structural quality was investigated via x-ray diffraction. The correlation between the densities of mixed threading dislocations and V-defects was determined. It was detected that the origin of V-defects on the a-plane and c-plane was the same despite having completely different geometries and structures. We also investigated the dependence of V-defect density on a-GaN film growth temperature at a constant hydrogen flow through a triethylgallium source, and we also investigated how the V/III ratio influenced the V-defect structure. Optimum growth parameters for defect concentration reduction were determined.

Improved growth quality of epitaxial ZnTe thin films on Si (111) wafer with ZnSe buffer layer

To achieve high-quality and low-cost ZnTe epitaxial films, a low-temperature (LT, 200 °C) ZnSe buffer layer with a thickness of 5 nm was grown on Si (111) wafers prior to the epitaxy of ZnTe layers by molecular beam epitaxy. Reflection high-energy electron diffraction patterns reveal that ZnTe epilayers grown at 335 °C with a thickness of 100 nm on the LT-ZnSe buffer layer present a completely two-dimensional growth mode. Film surfaces present smooth and flat morphology with the lowest surface roughness of ∼2.2 nm at a Te/Zn beam equivalent pressure ratio of 1.6. X-ray diffraction θ-2θ scanning indicates that the out-of-plane structure of ZnTe films is highly preferred with the (111) growth orientation of ZnSe/Si. X-ray diffraction φ scanning further confirms that the in-plane structure of ZnTe epilayer is also a single-crystal orientation growth. Thus, high-quality ZnTe (111) single-crystal epilayer with the lowest full-width at half-maximum of ∼200 arc sec and the smallest dislocation density of ∼1.12 × 108 cm−2 was grown on Si (111) wafers. In addition, the ZnSe buffer layer could alleviate the residual stress effectively for the heteroepitaxy of Si wafers. The most balanced residual stress for ZnTe/ZnSe/Si with σx of ∼96.5 MPa and σy of ∼94.2 MPa was obtained, which is essential for achieving high-performance and good mechanical properties of ZnTe-based devices.

Effect of High-Temperature Nitridation and Buffer Layer on Semi-Polar (10-13) AlN Grown on Sapphire by HVPE.

We have investigated the effect of high-temperature nitridation and buffer layer on the semi-polar aluminum nitride (AlN) films grown on sapphire by hydride vapor phase epitaxy (HVPE). It is found the high-temperature nitridation and buffer layer at 1300 °C are favorable for the formation of single (10–13) AlN film. Furthermore, the compressive stress of the (10–13) single-oriented AlN film is smaller than polycrystalline samples which have the low-temperature nitridation layer and buffer layer. On the one hand, the improvement of (10–13) AlN crystalline quality is possibly due to the high-temperature nitridation that promotes the coalescence of crystal grains. On the other hand, as the temperature of nitridation and buffer layer increases, the contents of N-Al-O and Al-O bonds in the AlN film are significantly reduced, resulting in an increase in the proportion of Al-N bonds.

Temperature dependent lattice expansions of epitaxial GaN-on-Si heterostructures characterized by in- and ex-situ X-ray diffraction

In-situ X-ray diffraction (XRD), with the sample temperature stepwise increased up to 900 °C, together with ex-situ high-resolution XRD (HRXRD), has been used to study the lattice expansion along the surface normal direction of GaN-on-Si heterostructures grown by metalorganic chemical vapor deposition (MOCVD). The thermal stabilities induced by step-graded (SG) AlxGa1−xN/AlN and multiple low-temperature (MLT) AlN buffer layers have been addressed for the heterostructures grown on 700 and 1500 µm thick Si (111) wafers, respectively. It reveals that the thermal expansion of the GaN epilayer is slightly larger than that of the Si substrate and the nitride growth tends to decrease the thermal expansion of the Si (111) wafer at lower temperatures. Onset of drop in the lattice expansions has been observed when the temperature is increased to a transition point, Ttr, and the Ttr of the MLT-AlN buffered GaN on the 1500 µm thick Si is ~500 °C, which is apparently lower than that of the SG-AlxGa1−xN/AlN buffered GaN on the 700 µm thick Si. These observations have been interpreted and attributed to convolutions among the residual lattice strains, their relaxations, and the thermal expansion coefficient mismatch induced wafer curvatures (opposite to that during MOCVD growth) at elevated temperatures.

Pseudo-bulk SAW transducers fabricated in GaN epitaxial layers grown on sapphire substrate

Gallium nitride is a material commonly applied to the fabrication of various semiconductor devices, naming High Electron Mobility Transistors (HEMT) and Light Emitting Diodes (LED). Its unique feature is that it also possesses good piezoelectric properties. This enables to produce devices that integrate both active transistor operation and signal processing and filtration in one monolithic chip. This aim requires a comprehensive analysis of the acoustic wave propagation in the gan/sapphire system. Particularly attractive in the construction of modern acoustic instruments, due to higher propagation speeds, is the utilisation of pseudo-bulk mods. This paper presents the results of research on Surface Acoustic Wave (SAW) transducers using pseudo-bulk waves propagating in thin (2-6 μm) layers of GaN on sapphire. The layers were made by the MOVPE process using a low-temperature buffer layer. Transducers with a two-finger pitch of 9 and 18 μm with 48 and 24 periods respectively were made using electron lithography. Low finger resistivity was achieved by using Ru/Au/Ti/Au metal alloy. The results of microwave measurements identifying particular pseudo-bulk mods and their dispersion characteristics are presented. Two pseudo-bulk mods were observed, the first one with a propagation velocity range of 8500-7800 ms−1 and the second one of 7200-7300 ms−1. Additionally, pseudo-bulk modes were also compared with classical Rayleigh modes generated in the same transducers at lower frequencies. The results are important from the point of view of possible monolithic integration of the tested transducers with High Electron Mobility Transistors.

Improving Transport Properties of GaN-Based HEMT on Si (111) by Controlling SiH4 Flow Rate of the SiNx Nano-Mask

The AlGaN/AlN/GaN high electron mobility transistor structures were grown on a Si (111) substrate by metalorganic chemical vapor deposition in combination with the insertion of a SiNx nano-mask into the low-temperature GaN buffer layer. Herein, the impact of SiH4 flow rate on two-dimensional electron gas (2DEG) properties was comprehensively investigated, where an increase in SiH4 flow rate resulted in a decrease in edge-type threading dislocation density during coalescence process and an improvement of 2DEG electronic properties. The study also reveals that controlling the SiH4 flow rate of the SiNx nano-mask grown at low temperatures in a short time is an effective strategy to overcome the surface desorption issue that causes surface roughness degradation. The highest electron mobility of 1970 cm2/V·s and sheet carrier concentration of 6.42 × 1012 cm−2 can be achieved via an optimized SiH4 flow rate of 50 sccm.

Low Surface Roughness GaAs/Si Thin-Film Deposition Using Three-Step Growth Method in MBE

Epitaxial GaAs-on-Si is an ideal material system for studying the physics of mismatched heteroepitaxy of a polar semiconductor layer grown on a non-polar substrate like silicon. Understanding and optimization of the initial nucleation of GaAs on silicon is the most crucial step in the success of GaAs/Si heteroepitaxy. Molecular beam epitaxy (MBE) technique has been used to deposit hetero-epitaxial GaAs thin-film on off-angle Si (100) substrate using the three-step growth method. After optimizing the growth parameters of low temperature (LT) buffer layer and high temperature (HT) epilayer, XRD analyses were performed. Characterization results of the GaAs (004) films which were not subjected to annealing, show a full-width half maximum (FWHM) of 760.1 arc sec and a root mean square (RMS) surface roughness of lower than 1 nm for a scanning area of 10 μm × 10 μm.