Layer Growth Temperature Research Articles

Two-step growth of β-Ga2O3 on c-plane sapphire using MOCVD for solar-blind photodetector

In this work, a two-step metal organic chemical vapor deposition (MOCVD) method was applied for growing β-Ga2O3 film on c-plane sapphire. Optimized buffer layer growth temperature (T B) was found at 700 °C and the β-Ga2O3 film with full width at half maximum (FWHM) of 0.66° was achieved. A metal−semiconductor−metal (MSM) solar-blind photodetector (PD) was fabricated based on the β-Ga2O3 film. Ultrahigh responsivity of 1422 A/W @ 254 nm and photo-to-dark current ratio (PDCR) of 106 at 10 V bias were obtained. The detectivity of 2.5 × 1015 Jones proved that the photodetector has outstanding performance in detecting weak signals. Moreover, the photodetector exhibited superior wavelength selectivity with rejection ratio (R 250 nm/R 400 nm) of 105. These results indicate that the two-step method is a promising approach for preparation of high-quality β-Ga2O3 films for high-performance solar-blind photodetectors.

Metamorphic growth of 0.1 eV InAsSb on InAs/GaAs virtual substrate for LWIR applications

Epitaxial growth of bulk InAs1-xSbx layer on GaAs substrate could open an opportunity for cost-competitive long-wavelength infrared sensors. Achieving this requires a high-quality metamorphic InAsSb layer with a uniform strain relief buffer design. We report a comprehensive analysis of metamorphic InAsSb layers grown on InAs/GaAs virtual substrate for 0.1 eV low bandgap material with an optimized growth condition and group-V flux control. We find that InAsSb surface roughens significantly with increasing Sb composition up to 58%. Lowering the growth temperature of InAsSb layers from 450 ℃ to 425 ℃ mitigated surface roughening while allowing greater than 90% strain relaxation. Moreover, we also present a dramatically increasing threading dislocation density by more than 200 times as a consequence of high Sb composition. Finally, we demonstrate a narrow energy bandgap of 0.13 eV at 10 K in the InAs0.42Sb0.58 layer, which is close to 0.1 eV at room temperature. This InAsSb film grown on InAs/GaAs template paves the way for long-wavelength infrared optoelectronics applications.

Controlling Transport Properties of 2D Electron System Hosted by SrTiO3‐Based Bilayers

Compared with the conventional 2D electron system (2DES) located between the LaAlO3 layer and SrTiO3 substrate, the LaAlO3/SrTiO3 bilayers are also capable of hosting 2DES with additional functional tunabilities. Herein, the LaAlO3 and SrTiO3 layers are epitaxially grown on the (La,Sr)(Al,Ta)O3 substrate to build a bilayer 2DES. The transport properties can be subtly tuned by the SrTiO3 layer thickness (tSTO), growth temperature (TG), and oxygen pressure (PO2), leading to various controllable electronic states. For the metallic bilayer 2DES, the low‐temperature carrier density (≈1014 cm−2) is one‐order‐of‐magnitude higher than that of the conventional 2DES (1012–1013 cm−2). Meanwhile, the bilayer 2DES exhibits the enhanced carrier mobility on increasing the carrier density, which contradicts the experimental results obtained from the conventional 2DES and suggests an important role of charge screening in the high‐carrier‐density bilayer 2DES. Moreover, the semiconducting bilayer 2DES, featured by the lower carrier density and thus less charge screening, shows the low‐field negative minimum and enhanced fourfold anisotropy in magnetoresistances. Those results infer that the majority of scattering centers shall be interfacial localized Ti3+ ions, which are screened by the high carrier density in the metallic bilayer 2DES.

Effect of buffer layer and substrate growth temperature on the microstructural evolution of hexagonal boron nitride thin films

Hexagonal boron nitride (hBN) thin films have excellent optical, electrical, and corrosion resistance properties that can enable a wide range of applications. The current study explores pulsed laser deposition technique for synthesizing hBN thin films. The microstructure, mechanical, and wetting properties of hBN thin films were analyzed as a function of substrate growth temperature (25 °C, 300 °C, 500 °C, and 800 °C) and buffer layer addition (Cu or Ti). The microstructure of hBN thin films was found to be influenced by the deposition temperature. This study revealed that the layered growth (for samples deposited at 25 °C) and layer plus island growth (for samples deposited at ≥300 °C) to be the primary growth modes of the thin films regardless of the buffer layer. The roughness of hBN thin films with the Cu buffer layer, in general, increased with the increase in deposition temperature, whereas no significant differences in roughness were observed for films deposited on Si and Ti buffer layer. Grazing Incidence X-ray diffraction results showed the presence of hexagonal close packed crystal structure for all the deposited thin films. These results were corroborated with Raman spectroscopy. The hBN thin films exhibited the hardness and modulus values of ~10–14 GPa and ~120–180 GPa, respectively. No considerable differences in mechanical properties were observed for hBN thin films deposited at various temperatures except for the films deposited with the Cu buffer layer, which showed lower mechanical properties. The wettability of hBN thin films was analyzed using contact angle measurements. The wettability was found to be directly influenced by the surface roughness of hBN thin films. Characterization of hBN thin films indicates that the deposition temperature and buffer layer addition had an influence on the growth and microstructural evolution of hBN thin films.

Statistical investigation of dislocation induced leakage current paths in AlGaN/GaN HEMT structures on Si and the impact of growth conditions

Threading dislocations in the AlGaN-barrier of four pairwise differently grown AlGaN/GaN high electron mobility transistor structures on Si were investigated with respect to their structural and electrical properties in direct comparison simultaneously ensuring statistical significance of the results. Portions of pure screw and mixed type dislocations were observed to serve as leakage current paths and to be clearly dependent on growth conditions like the AlN nucleation layer growth temperature. The role of impurity segregation at dislocation cores due to growth-dependent locally characteristic strain fields as for example induced by specific dislocation reactions at the AlGaN/GaN interface is discussed as the origin.

Emissions of the InGaN/GaN MQW LEDs with the InGaN well layer grown at different temperatures

Two light-emitting diodes (LEDs) enabled by InGaN/GaN multiple quantum wells (MQWs) with different well layer growth temperatures (WLGTs) were prepared. The dependences of electroluminescence (EL) spectra of these two structures on temperature at various fixed injection currents indicate that, a decreasing WLGT can result in a conversion of the well layer structure from a one-zone structure with better homogeneity in the localization depth into a two-zone structure with different average In contents and different localization depths, due to the increased In content-induced enhanced component fluctuation. The former is inferred from an “inverted-V-shaped” (increasing-decreasing) temperature-dependent behavior of peak energy at all fixed currents; the latter is mainly inferred from an “M-shaped” (increasing-decreasing-increasing-decreasing) temperature-dependent behavior of peak energy at intermediate fixed currents. These explanations also match those given for temperature-dependent behaviors in terms of external quantum efficiency (EQE) of these two LEDs, including “M-shaped” temperature-dependent behaviors of the EQE of LED B at the intermediate fixed currents. • EL properties of two different InGaN/GaN MQW-based LEDs A and B were studied. • LED B has a lower well layer growth temperature (WLGT) than LED A. • “M-shaped” temperature behavior of EL efficiency was observed only for LED B. • This is due to two-zone structural feature of well layer induced by its lower WLGT.

Growth of highly relaxed InGaN pseudo-substrates over full 2-in. wafers

A highly relaxed InGaN buffer layer was demonstrated over a full two-inch c-plane sapphire substrate by metalorganic chemical vapor deposition. The InGaN buffer layer was grown on a 100 nm GaN decomposition stop layer with a 3 nm thick high indium composition InGaN underlayer. After thermal decomposition of the underlayer at 1000 °C, a 200 nm thick In0.04Ga0.96N buffer showed 85% biaxial relaxation measured by a high resolution x-ray diffraction reciprocal space map. When used as a pseudo-substrate for the regrowth of InGaN/InGaN multi-quantum wells, the sample showed a 75 nm red-shift in room temperature photoluminescence when compared to a co-loaded GaN template reference. The longer emission wavelength is associated with higher indium incorporation in the InGaN layers from the lessening of the compositional pulling effect caused by compressive strain. Using this technique, a simple red light emitting diode was demonstrated with an active layer growth temperature of 825 °C and a peak wavelength of 622 nm at a current density of 20 A cm−2. This work represents a unique method to relax a III-nitride based layer over a full substrate.

The Role of Defects on the Performance of Quantum Dot Intermediate Band Solar Cells

Electrically active defects present in three InAs/GaAs quantum dots (QDs) intermediate band solar cells grown by metalorganic vapor phase epitaxy have been investigated. The devices’ structures are almost identical, differing only in the growth temperature and thickness of the GaAs layers that cover each InAs QD layer. These differences induce significant changes in the solar energy conversion efficiency of the photovoltaic cells, as previously reported. In this work, a systematic investigation was carried out using deep level transient spectroscopy (DLTS) and Laplace DLTS measurements on control samples and solar cell devices, which have clearly shown that electrically active traps play an important role in the device figures of merit, such as open circuit voltage, short circuit current, and shunt resistance. In particular, it was found that the well-known EL2 defect negatively affects both the open circuit voltage and shunt resistance, more in structures containing QDs, as a consequence of the temperature cycle required to deposit them. Other unidentified defects, that are absent in samples in which the QDs were annealed at 700 °C, contribute to a reduction of the short circuit current, as they increase the Shockley-Read-Hall recombination. Photoluminescence results further support the DLTS-based assignments.

Effect of nitrogen gas flow and growth temperature on extension of GaN layer on Si* *Project supported by the National Key Research and Development Program of China (Grant No. 2017YFB0402900) and the National Natural Sciences Foundation of China (Grant No. 62074144).

The effect of nitrogen flow and growth temperature on extension of GaN on Si substrate has been studied. By increasing the nitrogen flow whose outlet is located in the center of the MOCVD (metal–organic chemical vapor deposition) gas/particle screening flange and by increasing the growth temperature of HT-AlN and AlGaN buffer layers near the primary flat of the wafer, the GaN layer has extended more adequately on Si substrate. In the meantime, the surface morphology has been greatly improved. Both the AlN and GaN crystal quality uniformity has been improved. X-ray diffraction results showed that the GaN (0002) XRD FWHMs (full width at half maximum) decreased from 579 arcsec∼ 1655 arcsec to around 420 arcsec.

High current density tunnel diodes for multi-junction photovoltaic devices on InP substrates

InAlGaAs tunnel diodes, lattice-matched to InP and grown by molecular beam epitaxy, are demonstrated with peak tunneling current densities exceeding 1200 A/cm2. This was achieved by a 20 °C reduction in growth temperature for the p-type tunnel diode layers, resulting in up to two orders of magnitude improvement in the peak tunneling current density. Secondary ion mass spectrometry measurements reveal that the lower growth temperature reduces unwanted segregation of p-type Be dopants, improving dopant incorporation within the active tunnel diode layers. The diodes are transparent to wavelengths above 1000 nm and are compatible with the bottom junctions of InP-based multi-junction solar cells and with InP-based photonic power converters operating in the telecommunication O- and C-bands. When incorporated into a dual-junction photonic power converter test structure, measurements under 1319-nm laser illumination demonstrate integrated tunnel diode operation, enabling a halving of the short-circuit current and doubling of the open-circuit voltage as compared to a single junction reference device.

Diminishing the Induced Strain and Oxygen Incorporation on Aluminium Nitride Films Deposited Using Pulsed Atomic-Layer Epitaxy Techniques at Standard Pressure MOCVD

A pulsed atomic-layer epitaxy growth technique has been introduced to substantially diminish the induced strain and oxygen incorporation on aluminium nitride films grown at standard pressure by metal organic chemical vapour deposition. The qualities of the as-deposited aluminium nitride films were studied by varying the aluminium nitride nucleation layer growth temperature at 700°C, 800°C, 900°C, 1000°C and 1100°C, respectively. The compressive strain inside the as-deposited aluminium nitride films, induced by the hetero-epitaxial growth on sapphire, was investigated through Raman spectroscopy by focusing on the evolution of E2 (high) peak frequency, where almost stress-free aluminium nitride films were attained at nucleation layer growth temperature of 1100°C. Then, the correlation between luminescence defect and level of foreign impurities respective to the varied nucleation layer growth temperatures were also systematically analysed through photoluminescence spectroscopy and x-ray photoelectron spectroscopy, respectively.

Optimized InAlAs graded buffer and tensile-strained dislocation filter layer for high quality InAs photodetector grown on Si

We demonstrate a low threading dislocation density (TDD) and smooth surface InAs layer epitaxially grown on Si by suppressing phase separation of InxAl1−xAs (x = 0 to 1) graded buffer and by inserting a tensile-strained In0.95Al0.05As dislocation filter layer. While keeping the total III–V layer below 2.7 μm to avoid thermal cracks, we have achieved a sixfold reduction of TDD in InAs on Si compared to the unoptimized structure. We found a strong correlation between the metamorphic InAs surface roughness and TDD as a function of InxAl1−xAs buffer thickness. An optimal thickness of 175 nm was obtained where both phase separation and 3D islanding growth were suppressed. Moreover, a tensile-strained In0.95Al0.05As dislocation filter layer and high growth temperature of the InAs cap layer further assisted the dislocation reduction process, which led to a TDD to 1.37 × 108 cm−2. Finally, an InAs p-i-n photodetector grown on the optimized InAs/Si template confirmed its high quality by showing an improved responsivity from 0.16 to 0.32 A/W at a 2 μm wavelength.

Direct Heteroepitaxy and Selective Area Growth of GaP and GaAs on Si by Hydride Vapor Phase Epitaxy

Direct heteroepitaxy and selective area growth (SAG) of GaP and GaAs on Si(100) and Si(111) are implemented by low‐pressure hydride vapor phase epitaxy (LP‐HVPE), which are facilitated by buffer layers grown at 410–490 °C with reactive gas mixing directly above Si substrates. High‐density islands observed on GaP buffer layers on Si result in rough morphology and defect formation in the subsequent GaP layers grown at 715 °C. The impact of growth temperature of GaAs buffer layers on the crystal quality of GaAs/Si is studied. A decreased nucleation temperature significantly improves the morphology and crystalline quality of the overall GaAs growth on Si. It is observed that Si(111) substrates are favorable for both GaP and GaAs growths in comparison with Si(100). In SAGs of GaP/Si and GaAs/Si, the high selectivity innate to HVPE is maintained in the used unconventional growth regime. The spatially resolved photoluminescence mapping reveals the material quality of GaAs/Si is enhanced by defect filtering by SAG. The outcomes of this work will pave the way of III–V/Si integration realized by cost‐effective HVPE for photonic device applications.

Synthesis and temperature Effect of ZnO nanoparticle seeding layer and nanorods

Zinc oxide nanostructured with different seeding layer annealing temperature have been synthesized by drop casting technique on FTO coated glass substrate. The optimized seeding layer have employed to synthesis ZnO nanorods were growth aqueous solutions method of zinc nitrate and HML as precursors with different growth temperatures.FESEM supported by EDS results showed significant information of ZnO topographic surface. X- Ray diffraction scan demonstrate a hexagonal wurtzite structure with c-axis orientation of the ZnO nanorods. Strong ultraviolet (UV) emission of ZnO nanorods has detected by UV visible measurement. The obtained results have analyzed optimize annealing temperature of ZnO seeding layer and suitable growth temperature of ZnO nanorods, with crystal hexagonal ZnO nanorods and homogenous distribution with 95°C growth temperature

Effect of Strains and V-Shaped Pit Structures on the Performance of GaN-Based Light-Emitting Diodes

Strains and V-shaped pits are essential factors for determining the efficiency of GaN-based light-emitting diodes (LEDs). In this study, we systematically analyzed GaN LED structures on patterned sapphire substrates (PSSs) with two types of growth temperature employed for prestrained layers and three different thickness of n-type GaN layers by using cathodoluminescence (CL), microphotoluminescence (PL), and depth-resolved confocal Raman spectroscopy. The results indicated that V-pits formation situation can be analyzed using CL. From the emission peak intensity ratio of prestrained layers and multiple quantum wells (MQWs) in the CL spectrum, information regarding strain relaxation between prestrained layers and MQWs was determined. Furthermore, micro-PL and depth-resolved confocal Raman spectroscopy were employed to validate the results obtained from CL measurements. The growth conditions of prestrained layers played a dominant role in the determination of LED performance. The benefit of the thick layer of n-GaN was the strain reduction, which was counteracted by an increase in light absorption in thick n-type doped layers. Consequently, the most satisfactory LED performance was observed in a structure with relatively lower growth temperature of prestrained layers that exhibited larger V-pits, leading to higher strain relaxation and thinner n-type GaN layers, which prevent light absorption caused by n-type GaN layers.

Effect of the cap layer growth temperature on the Sb distribution in InAs/InSb/InAs sub-monolayer heterostructures for mid-infrared devices

Sub-monolayer (SML) deposition of InSb within InAs matrix by migration enhanced epitaxy tends to form type II SML nanostructures offering efficient light emission within the mid-infrared (MIR) range between 3 and 5 μm. In this work, we report on the Sb distribution in InSb/InAs SML nanostructures with InAs cap layers grown at temperatures lower than that associated with the under-grown InSb active layer. Analysis by transmission electron microscopy (TEM) in 002 dark field conditions shows that the reduction in the growth temperature of the InAs cap layer increases the amount of Sb deposited in the layers, in good agreement with the x-ray diffraction results. TEM micrographs also show that the layers are formed by random InSbAs agglomerates, where the lower cap temperature leads to a more continuous InSb layer. Quantitative atomic column resolved high angle annular dark field-scanning (S)TEM analyses also reveal atomic columns with larger composition of Sb for the structure with the lowest InAs cap layer temperature. The dependence of the Sb distribution on InAs cap growth temperature allows tuning the corresponding emission wavelength in the MIR range, as shown by the photoluminescence emission spectra.

Optical spectroscopy studies of atom intermixing in the core versus growth temperature of the claddings in MOCVD-grown quantum cascade lasers

Here we present optical spectroscopy studies to examine structural and optical properties of active region of quantum cascade lasers grown fully by MOCVD technique. The active part is InGaAs/AlInAs based multilayer structure nominally lattice matched to InP substrate. In this communication we study a set of structures with different growth temperature of the cladding layers and its influence on the properties of active core. The x-ray spectroscopy (XRD) allowed to determine the widths of constituent layers and compare obtained values with nominal ones. Fourier-transformed photoluminescence (FTPL) and photoreflectance (PR) measurements provided high signal to noise spectra, proving good optical and structural properties of investigated samples. A model of atoms interdiffusion processes was presented to explain observed small energy shifts of the transitions energies within the investigated multilayer structures.

Correlative investigation of Mg doping in GaN layers grown at different temperatures by atom probe tomography and off-axis electron holography

Correlation between off-axis electron holography and atom probe tomography (APT) provides morphological, chemical and electrical information about Mg doping (p-type) in gallium nitride (GaN) layers that have been grown at different temperatures at a nanometric scale. APT allows access to the three-dimensional distribution of atoms and their chemical nature. In particular, this technique allows visualisation of the Mg-rich clusters observed in p-doped GaN layers grown by metal-organic chemical vapour deposition. As the layer growth temperature increases, the cluster density decreases but their size indicted by the number of atoms increases. Moreover, APT reveals that threading dislocations are decorated with Mg atoms. Off-axis electron holography provides complementary information about the electrical activity of the Mg doping. As only a small fraction of dopant atoms are ionised at room temperature, this fraction is increased by annealing the specimen to 400 °C in situ in a transmission electron microscope (TEM). A strong reduction of the dopant electrical activity is observed for increases in the layer growth temperature. The correlation of APT with TEM-based techniques was shown to be a unique approach in order to investigate how the growth temperature affects both the chemical distribution and electrical activity of Mg dopant atoms.

Enhance the electroluminescence efficiency of InGaN/GaN multiple quantum wells by optimizing the growth temperature of GaN barriers

Three InGaN/GaN MQWs samples with different QB layers growth temperatures were grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD). It is found that the electroluminescence intensity increases first and then decreases with increasing growth temperature of GaN barrier layers when the growth temperature increases from 800 °C to 880 °C. The XRD and AFM measurements show that a better interface quality exists in InGaN/GaN MQW whose GaN barrier layers grown at a higher temperature. However, when the growth temperature of GaN barriers is too high, a quality degradation of QWs will occur for underlying InGaN well layers, resulting in a decrease of the luminous intensity of the active region. Finally, we proposed that using three temperature growth scheme could improve the electroluminescence efficiency of the active region compared to conventional dual-temperature growth method.

Stimulated emission of AlGaN layers grown on sapphire substrates using ammonia molecular beam epitaxy

By optimising the growth temperature of the AlGaN layers and using high-temperature AlN buffer layers, high-quality AlxGa1 – xN layers (x = 0.15, 0.21, 0.26, and 0.3) were obtained, in which stimulated emission in the UV spectral range 330 – 297 nm was implemented with the threshold intensity of excitation Ith ≈ 0.7 – 1.4 MW cm−2, respectively. It is found that the threshold value of the stimulated emission of AlGaN layers grown by molecular beam epitaxy is largely determined by the intensity of the process of thermal decomposition of GaN, which affects the surface morphology and, consequently, the amount of optical scattering loss. It is shown that no pronounced localisation of nonequilibrium charge carriers occurs in the AlGaN layers, which is manifested in the absence of a large Stokes shift and in the realisation of optical amplification at transitions in an electron-hole plasma, and also indicates a relatively homogeneous material composition.