Carbon Impurity Incorporation Research Articles

Yellow luminescence and carrier distribution due to polarity-dependent incorporation of carbon impurities in bulk GaN by Na flux

We have grown a high growth rate, and low dislocation density (DD) Ga-polar GaN (DD: 4 × 105/cm2) and N-polar GaN (DD: 8 × 105/cm2) bulk single crystal using Na Flux. Variable temperature PL shows that the yellow luminescence (YL) peak energy of Ga-polar GaN (Ga-GaN) and N-polar GaN (N-GaN) are 2.31–2.30 eV and 2.28–2.26 eV, respectively. It can be found that N-GaN has a more vigorous YL intensity and a lower peak energy of YL than Ga-GaN. The variable-temperature and variable-power PL results suggest that the YL origin may be a donor-acceptor pair defect. Dislocations and defects associated with gallium vacancies that could cause YL behavior were ruled out by dislocation density determination and Raman spectroscopy, respectively. The different concentrations of carbon and oxygen in the Ga-GaN and N-GaN that were associated with the adsorption energy were discussed and proved by SIMS. The Raman results show that the carrier concentrations of Ga-polar and N-polar GaN are comparable, which may be because the higher content of carbon impurities in N-polar GaN can act as a deep acceptor to compensate for its higher content of shallow donor impurity oxygen. Therefore, it is suggested that carbon-related energy level transitions are the primary source of YL in both Ga-polar and N-polar GaN, and the compensating effect of carbon impurities leads to similar carrier concentrations in both polar planes.

Yellow-Green Luminescence Due to Polarity-Dependent Incorporation of Carbon Impurities in Self-Assembled GaN Microdisk.

Yellow-green luminescence (YGL) competes with near-bandgap emission (NBE) for carrier recombination channels, thereby reducing device efficiency; yet uncovering the origin of YGL remains a major challenge. In this paper, nearly stress-free and low dislocation density self-assembled GaN microdisks were synthesized by Na-flux method. The YGL of GaN microdisks highly depend on their polar facets. Variable accelerating voltage/power CL, variable temperature PL, and Raman spectroscopy are further performed to clarify the origin of polarity dependence of GaN microdisk YGL behavior, which indicates its independence of dislocations, surface effects, stress, crystalline quality, and gallium vacancies. It was found that the incorporation ability of carbon impurities in the polar (0001) facet is greater than that in the semipolar (101̅1) facets, producing higher content of CN or CNON defects, resulting in a more pronounced YGL in the polar (0001) facet of GaN.

Designing Multifunctional Cobalt Oxide Layers for Efficient and Stable Electrochemical Oxygen Evolution

AbstractDisordered and porous metal oxides are promising earth‐abundant and cost‐effective alternatives to noble‐metal electrocatalysts. Herein, nonsaturated oxidation in plasma‐enhanced atomic layer deposition is leveraged to tune the structural, mechanical, and optical properties of biphasic cobalt hydroxide films, thereby tailoring their catalytic activities and chemical stabilities. Short oxygen plasma exposure times and low plasma powers incompletely oxidize the cobaltocene precursor to Co(OH)2 and result in carbon impurity incorporation. These Co(OH)2 films are highly porous and catalytically active, but their electrochemical stability is impacted by poor substrate adhesion. In contrast, long exposure times and high powers completely oxidize the precursor to Co3O4, reduce the carbon incorporation, and improve the crystallinity. While the Co3O4 films have high electrochemical stability, they are characterized by low oxygen evolution reaction activity. To overcome these competing properties, the established relation between deposition parameters and functional film properties is applied to design bilayer films exhibiting simultaneously improved electrochemical performance and chemical stability. The bilayer films combine a highly active Co(OH)2 surface with a stable Co3O4 interface layer. These coatings exhibit minimal light absorption, thus making them suitable as protective catalytic layers on semiconductor light absorbers for application in photoelectrochemical devices.

High-mobility n−-GaN drift layer grown on Si substrates

We have investigated the interaction between carbon impurities and threading dislocations and their impact on the transport properties of GaN grown on Si substrates. The incorporation of carbon impurity was found to be associated with dislocation density, with a linear dependence. It indicates that the carbon may accumulate around the dislocations. The temperature-dependent Hall-effect measurement further confirmed that those carbon-decorated dislocations can act as acceptor-like traps, existing at every c-lattice spacing along a threading dislocation. The acceptor-like traps are important scattering centers and, thus, cannot be neglected. By reducing the density of the carbon-decorated dislocation via introducing a thick dislocation filtering layer to reduce the dislocation-related acceptor-like trap scattering, a record room-temperature electron mobility of 1090 cm2/V s with a carrier concentration of ∼2 × 1016 cm−3 for n--GaN on Si was achieved. Our results provide an effective approach to obtain high-quality n−-GaN on Si for vertical GaN based devices.

Effect of growth temperature on surface morphology and optical properties of undoped Al0.32Ga0.68As

Undoped Al0.32Ga0.68As material which was grown by metal-organic chemical vapor deposition at different growth temperature. The evolution mechanism of surface morphology of Al0.32Ga0.68As was explained by the variation of step growth direction and growth rate, which were affected by growth temperature. Electrochemical capacitance-voltage results showed the dependence of conductivity type on carbon incorporation. Al0.32Ga0.68As changed from p-type to n-type conductivity and carrier concentration decreased as growth temperature increased from 640 °C to 740 °C. The disorder of Al0.32Ga0.68As ternary alloy originated from its structural defects, which were attributed to aluminum (Al) compositional fluctuations by the incorporation of carbon impurity. The crystal quality of undoped Al0.32Ga0.68As was characterized by the intensity of Fermi-edge singularity peak. The change rule of the intensity of Al0.32Ga0.68As and GaAs peaks was discussed in terms of carrier transport and recombination properties.

Continuous-wave electrically injected GaN-on-Si microdisk laser diodes.

Silicon photonics has been calling for an electrically pumped on-chip light source at room temperature for decades. A GaN-based microdisk laser diode with whispering gallery modes grown on Si is a promising candidate for compact on-chip light source. By suppressing the unintentional incorporation of carbon impurity in the p-type AlGaN cladding layer of the laser, we have significantly reduced the operation voltage and threshold current of the GaN-on-Si microdisk laser. Meanwhile the radius of the microdisk laser was shrunk to 8 µm to lower the thermal power. The overall junction temperature of the microdisk laser was effectively reduced. As a result, the first continuous-wave electrically pumped InGaN-based microdisk laser grown on Si was achieved at room temperature.

Influence of substrate misorientation on carbon impurity incorporation and electrical properties of p-GaN grown by metalorganic chemical vapor deposition

The influence of substrate misorientation angle on carbon impurity incorporation and electrical properties of p-GaN grown at a low temperature of 900 °C has been explored. Secondary ion mass spectrometry results reveal that the concentration of unintentionally incorporated carbon impurity decreases remarkably (from 2 × 1017 cm−3 to 7 × 1016 cm−3) with the increasing misorientation angle. The step motion model is introduced to explain the reason for decreasing carbon concentration with increasing misorientation angle. It has also been found the hole concentration of p-GaN increases and the resistivity of p-GaN decreases with the increasing misorientation angle since carbon acts as compensating donor in p-GaN.

Resistivity reduction of low temperature grown p-Al0.09Ga0.91N by suppressing the incorporation of carbon impurity

Reducing the resistivity of low temperature grown p-Al0.09Ga0.91N layers is crucial to improving the performance of GaN-based laser diodes. In this study, growth conditions of low temperature grown p-Al0.09Ga0.91N layers are monitored and the role of C impurity is investigated systematically. On the basis of the dependence of resistivity on C concentration and the photoluminescence analysis, it is found that C impurities act as donors in p-Al0.09Ga0.91N layer, and reducing the C concentration can reduce its compensation effect on Mg acceptor. Finally, a low resistivity of 4.2 Ω·cm is achieved for the low temperature grown p-Al0.09Ga0.91N.

Improvement of Ohmic contact to p-GaN by controlling the residual carbon concentration in p++-GaN layer

Growth conditions are used to control the residual carbon impurity incorporation in p++-GaN layers. Specific contact resistance (ρc) with various residual carbon concentrations has been investigated through the circular transmission line model (CTLM) method and secondary ion mass spectroscopy (SIMS) analysis. A correlation between residual carbon and ρc indicates that incorporation of proper carbon impurity can be an advantage for Ohmic contact, although carbon can also act as a compensating donor to worsen the Ohmic contact at a very high concentration. Finally, ρc is improved to 6.80×10−5Ω×cm2 with a carbon concentration of 8.3×1017cm−3 in p++-GaN layer, when the growth temperature, pressure and flow rate of CP2Mg and TMGa are 940°C, 100Torr, 3μmol/min and 28μmol/min, respectively.

Atomic Layer Deposition of In2O3:H from InCp and H2O/O2: Microstructure and Isotope Labeling Studies.

The atomic layer deposition (ALD) process of hydrogen-doped indium oxide (In2O3:H) using indium cyclopentadienyl (InCp) and both O2 and H2O as precursors is highly promising for the preparation of transparent conductive oxides. It yields a high growth per cycle (>0.1 nm), is viable at temperatures as low as 100 °C, and provides a record optoelectronic quality after postdeposition crystallization of the films ( ACS Appl. Mat. Interfaces , 2015 , 7 , 16723 - 16729 , DOI: 10.1021/acsami.5b04420 ) . Since both the dopant incorporation and the film microstructure play a key role in determining the optoelectronic properties, both the crystal growth and the incorporation of the hydrogen dopant during this ALD process are studied in this work. This has been done using transmission electron microscopy (TEM) and atom probe tomography (APT) in combination with deuterium isotope labeling. TEM studies show that an amorphous-to-crystalline phase transition occurs in the low-temperature regime (100-150 °C), which is accompanied by a strong decrease in carrier density and an increase in carrier mobility. At higher deposition temperatures (>200 °C), enhanced nucleation of crystals and the incorporation of carbon impurities lead to a reduced grain size and even an amorphous phase, respectively, resulting in a strong reduction in carrier mobility. APT studies on films grown with deuterated water show that the incorporated hydrogen mainly originates from the coreactant and not from the InCp precursor. In addition, it was established that the incorporation of hydrogen decreased from ∼4 atom % for amorphous growth to ∼2 atom % after the transition to crystalline film growth.

Investigation of NH3 input partial pressure for N-polarity InGaN growth on GaN substrates by tri-halide vapor phase epitaxy

The influence of NH3 input partial pressure on N-polarity InGaN grown by tri-halide vapor phase epitaxy was investigated. It was found that surface morphology, solid composition and optical properties were affected by NH3 input partial pressure. As shown in thermodynamic analyses, the indium content increased due to an increase in the driving force for InN deposition caused by increased NH3 input partial pressure. In addition, the deep level emission around 2.1 eV in photoluminescence measurements drastically decreased at higher NH3 input partial pressures. Ab initio calculations and subsequent secondary ion mass spectrometry measurements suggested the reduction of metal-vacancies and/or carbon impurity incorporation in the InGaN layers.

Conductivity enhancement in AlGaN:Mg by suppressing the incorporation of carbon impurity

Growth conditions were explored to suppress the carbon impurity incorporation in AlGaN:Mg grown at low temperatures. Electrical properties of Al0.07Ga0.93N:Mg samples with various carbon concentrations were investigated by Hall measurements. A clear correlation between carbon concentration and electrical conductivity has been found. By reducing the carbon concentration from 2 × 1018 to 5 × 1016 cm−3, the resistivity of p-Al0.07Ga0.93N decreases from 7.4 to 2.2 Ω·cm. From the results of the analysis of the charge neutrality equation, we found that the carbon concentration is close to the compensating donor concentration in the AlGaN:Mg samples, which suggests that carbon acts as the main compensating donor in AlGaN:Mg.

Optimisation of a carbon doped buffer layer for AlGaN/GaN HEMT devices

This work reports on the optimisation of carbon doping GaN buffer layer (BL) for AlGaN/GaN HEMT (high electron mobility transistor) structures, grown by low pressure metal–organic vapour phase epitaxy (LP-MOVPE) on 3in. SiC semi-insulating substrates. The incorporation of carbon impurities in GaN is studied as a function of the growth conditions, without using an external carbon source. We observed that the C incorporation can be effectively controlled over more than one order of magnitude by tuning the reactor pressure and the growth temperature, without degradation of the crystalline properties of the GaN layers.HEMT structures with a specific barrier design were grown with different carbon dopings in the GaN BL and processed into transistors to evaluate the impact of the BL doping on the device performances. A significant improvement of the HEMT drain leakage current and of the breakdown voltage was obtained by increasing the carbon incorporation in the GaN BL. The RF performances of the devices show a trade-off between leakage currents and trapping phenomena which are enhanced by the use of carbon doping, limiting the delivered output power. An output power as high as 6.5W/mm with a Power Added Efficiency of 70% has been achieved at 2GHz by the HEMT structures with the lowest carbon doping in the BL.

Influence of vicinal sapphire substrate on the properties of N-polar GaN films grown by metal-organic chemical vapor deposition

The influence of vicinal sapphire substrates on the growth of N-polar GaN films by metal-organic chemical vapor deposition is investigated. Smooth GaN films without hexagonal surface feature are obtained on vicinal substrate. Transmission electron microscope results reveal that basal-plane stacking faults are formed in GaN on vicinal substrate, leading to a reduction in threading dislocation density. Furthermore, it has been found that there is a weaker yellow luminescence in GaN on vicinal substrate than that on (0001) substrate, which might be explained by the different trends of the carbon impurity incorporation.

Ge1-xSnx Epitaxial Growth on Ge Substrate by MOCVD

Metal-organic chemical vapor deposition have many advantages such as low deposition temperature compared to conventional CVD using inorganic molecules. In this study, we succeeded in the Ge1-xSnx epitaxial growth on Ge substrate by MOCVD using tertiary-butyl-germane and tetra-ethyl-tin for the precursors for the first time. The Sn concentration in the deposited film was 2 at.% which exceeds solid solubility limit of 1 at.%. The carbon impurity incorporation was confirmed to be less than the XPS detection limit.

Carrier gas and VI/II ratio effects on carbon clusters incorporation into ZnO films grown by MOCVD

Undoped ZnO films were deposited by atmospheric metal-organic chemical vapor deposition (MOCVD) on (0001) ZnO substrate. The films were grown at various partial pressure ratios of oxygen and zinc precursors (VI/II) using either N2 or H2 as carrier gas. Micro-Raman scattering was employed to study the effects of carrier gas, VI/II ratio and annealing on carbon impurity incorporation into the ZnO films. Besides the well known phonon modes of ZnO, Raman spectra of the samples grown with N2 carrier gas show two additional broad peaks, which are ascribed to carbon sp2 clusters related modes, spreading in the frequency range 1300–1600cm−1 and dominate the Raman spectrum of the sample grown under oxygen deficiency (VI/II=0.25). In addition, a band centered at ∼520cm−1, considered as some defects related local vibrations, appears in the samples grown with N2 as carrier gas and its intensity increases when the VI/II ratio decreases. The average cluster size, estimated from the intensity ratio of D over G bands of the carbon sp2 clusters, ranges from 16.5 to 19.4Å. However, in all the samples grown with H2 as carrier gas, the bands related to carbon sp2 clusters and defects, are largely suppressed and the second-order-Raman scattering band (1050–1200cm−1) is clearly observed in addition to the bulk ZnO lattice modes. After annealing the samples at 900°C in oxygen ambient, the crystal quality has been improved for all the samples but the carbon related bands, formed in the as-deposited films grown with the N2 carrier gas, were only weakened.

Investigation of Band-Gap States in AlGaN/GaN Hetero-Structures with Different Growth Conditions of GaN Buffer Layers

ABSTRACTWe have investigated electronic band-gap states in AlGaN/GaN hetero-structures with different growth conditions of GaN buffer layers from a viewpoint of Carbon impurity incorporation into GaN, using photoluminescence (PL), capacitance-voltage (C-V) and steady-state photo-capacitance spectroscopy (SSPC) techniques. The Carbon incorporation was found to be enhanced with decreasing the growth temperature of the GaN buffer layer between 1120 and 1170 °C. Acting in concert, three specific deep levels located at ~2.07, ~2.70, and ~3.23 eV below the conduction band were found to become dense significantly at the low growth temperature. Therefore, these levels are probably attributable to Ga vacancies and/or Carbon acceptors produced by the Carbon impurity incorporation, and are likely in conjunction with each other.

Effect of Carbon Impurity Incorporation on Band-Gap States in AlGaN/GaN Hetero-Structures

Effect of Carbon Impurity Incorporation on Band-Gap States in AlGaN/GaN Hetero-Structures

Influences of unintentionally doped carbon on magnetic properties in Mn–N co-doped ZnO

Carbon impurities are usually unintentionally doped in N-doped ZnO when attempting to realize p-type conductivity by metal-organic chemical vapor deposition. Mn–N co-doping technique, which is developed to realize hole-mediated room temperature ferromagnetism in ZnO, may further enhance the carbon incorporation. In this work, two kinds of Mn–N co-doped samples, grown at low temperature (400 °C) and high temperature (600 °C), respectively, have been compared to study the influences of carbon impurities. In contrast to that found on N mono-doped ZnO, an enhanced incorporation of carbon impurities is observed in the high-temperature-grown Mn–N co-doped sample with the conductivity changed from p to n type. According to X-ray photoelectron spectroscopy measurement, the compensation effect from carbon impurities is applied to elucidate the origin of the conductivity transition. Correspondingly, superconducting quantum interference device measurement certainly shows a much smaller value of the saturation magnetization for the high-temperature-grown sample. A possible effect from carbon related complexes on magnetization is also proposed to explain the decrease of the magnetic moment from the view of weakened spin polarization induced by unintentionally incorporated carbon. This assumption is further supported by the first-principle calculation on the Mn–N co-doped ZnO system with carbon incorporation.

The influences of O/Zn ratio and growth temperature on carbon impurity incorporation in ZnO grown by metal-organic chemical vapor deposition

The strong correlations between the O/Zn ratio and carbon impurity incorporation have been observed on the ZnO films grown using N 2O or O 2 as oxygen source in metal-organic chemical vapor deposition (MOCVD). From in-situ mass spectrometric measurements, the O/Zn ratio in the MOCVD reactor is found to decrease to a minimum value as the growth temperature increased till a critical growth temperature T c, and then increased above T c due to different dissociation rates of the oxygen and Zn sources. The strongest D and G modes, which are ascribed to carbon clusters sp 2 related modes, have been observed in Raman scattering spectroscopy for the ZnO samples grown at T c, indicating the highest incorporation rate of carbon impurity in the samples grown at T c. Compared with O 2, N 2O has a low dissociation rate and that leads to a lower value of O/Zn ratio, resulting in much stronger D and G modes and higher incorporation rate of carbon impurities in the samples grown at T c. It is interesting to note that the lowest specific resistances from Hall effect measurements were also obtained on the samples grown at T c, indicating possible electrical contributions from the formation of carbon clusters, which should be highly conductive regions in ZnO. Furthermore, ionization or addition of H 2 in the case of N 2O can significantly enhance the dissociation of N 2O, with film quality improved significantly. This study shows that a high O/Zn ratio is critical to suppress carbon impurity incorporation and to grow high quality ZnO by MOCVD, especially at low growth temperature.