Effect of high-temperature buffer thickness on quality of AlN epilayer grown on sapphire substrate by metalorganic chemical vapor deposition

The effect of an initially grown high-temperature AlN buffer (HT-AlN) layer's thickness on the quality of an AlN epilayer grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD) in a two-step growth process is investigated. The characteristics of AlN epilayers are analyzed by using triple-axis crystal X-ray diffraction (XRD) and atomic force microscopy (AFM). It is shown that the crystal quality of the AlN epilayer is closely related to its correlation length. The correlation length is determined by the thickness of the initially grown HT-AlN buffer layer. We find that the optimal HT-AlN buffer thickness for obtaining a high-quality AlN epilayer grown on sapphire substrate is about 20 nm.     

Effect of high-temperature annealing on AIN thin film grown by metalorganic chemical vapor deposition

The effect of high-temperature annealing on A1N thin film grown by metalorganic chemical vapor deposition was investigated using atomic force microscopy, Raman spectroscopy, and deep ultra-violet photoluminescence （PL） with the excitation wavelength as short as ~ 177 nm. Annealing experiments were carded out in either N2 or vacuum atmosphere with the annealing temperature ranging from 1200 ℃ to 1600 ℃. It is found that surface roughness reduced and compres- sive strain increased with the annealing temperature increasing in both annealing atmospheres. As to optical properties, a band-edge emission peak at 6.036 eV and a very broad emission band peaking at about 4.7 eV were observed in the photoluminescence spectrum of the as-grown sample. After annealing, the intensity of the band-edge emission peak varied with the annealing temperature and atmosphere. It is also found that a much stronger emission band ranging from 2.5 eV to 4.2 eV is superimposed on the original spectra by annealing in either N2 or vacuum atmosphere. We attribute these deep-level emission peaks to the VAL--ON complex in the A1N material.     

Effect of high-temperature annealing on AlN thin film grown by metalorganic chemical vapor deposition

The effect of high-temperature annealing on AlN thin film grown by metalorganic chemical vapor deposition was investigated using atomic force microscopy, Raman spectroscopy, and deep ultra-violet photoluminescence(PL) with the excitation wavelength as short as ～ 177 nm. Annealing experiments were carried out in either N2 or vacuum atmosphere with the annealing temperature ranging from 1200℃ to 1600℃. It is found that surface roughness reduced and compressive strain increased with the annealing temperature increasing in both annealing atmospheres. As to optical properties,a band-edge emission peak at 6.036 eV and a very broad emission band peaking at about 4.7 eV were observed in the photoluminescence spectrum of the as-grown sample. After annealing, the intensity of the band-edge emission peak varied with the annealing temperature and atmosphere. It is also found that a much stronger emission band ranging from 2.5 eV to 4.2 eV is superimposed on the original spectra by annealing in either N2 or vacuum atmosphere. We attribute these deep-level emission peaks to the VAL–ONcomplex in the AlN material.     

Improved surface morphology of flow-modulated MOVPE grown AIN on sapphire using thin medium-temperature AIN buffer layer

High-temperature (HT) AIN films were grown on (0 0 0 1) sapphire by low-pressure flow-modulated (FM) metal organic vapor phase epitaxy (MOVPE) with and without inserting a thin medium-temperature (MT) AIN layer. To suppress parasitic reactions between the sources of trimethylaluminum (TMA) and ammonia (NH₃), TMA and NH₃ was introduced to the reactor of MOVPE by alternating supply way. Surface morphology and crystalline quality were characterized by a scanning electronic microscopy (SEM), atomic force microscopy (AFM) and X-ray rocking curve (XRC) measurements of (0 0 0 2) and (10-12) diffractions. The AFM and SEM measurements indicated that the thin MT-AIN layer had a strong influence on the surface morphology of the HT-AIN films. The surface morphology became quite smooth by inserting the thin MT-AIN layer and surface RMS roughness values were 0.84 nm and 13.4 nm for the HT-AIN films with and without inserting the thin MT-AIN buffer layer, respectively. By etching the samples in aqueous KOH solution, it was found that the polarity of AIN films was different, the HT-AIN film with the thin MT-AIN layer could not be etched, indicating that the film had an Al-polar surface; however, the film without the MT-AIN layer was etched, which was explained that that film had a N- or mixed-polar surface. The mechanism for the origin of the different polarity of HT-AIN with and without the thin MT-AIN layer was proposed and discussed in detail.     

Nonpolar a-plane GaN grown on r-plane sapphire using multilayer AlN buffer by metalorganic chemical vapor deposition

Mirror-like and pit-free non-polar a-plane (1 1 −2 0) GaN films are grown on r-plane (1 −1 0 2) sapphire substrates using metalorganic chemical vapor deposition (MOCVD) with multilayer high-low-high temperature AlN buffer layers. The buffer layer structure and film quality are essential to the growth of a flat, crack-free and pit-free a-plane GaN film. The multilayer AlN buffer structure includes a thin low-temperature-deposited AlN (LT-AlN) layer inserted into the high-temperature-deposited AlN (HT-AlN) layer. The results demonstrate that the multilayer AlN buffer structure can improve the surface morphology of the upper a-plane GaN film. The grown multilayer AlN buffer structure reduced the tensile stress on the AlN buffer layers and increased the compressive stress on the a-plane GaN film. The multilayer AlN buffer structure markedly improves the surface morphology of the a-plane GaN film, as revealed by scanning electron microscopy. The effects of various growth V/III ratios was investigated to obtain a-plane GaN films with better surface morphology. The mean roughness of the surface was 1.02 nm, as revealed by atomic force microscopy. Accordingly, the multilayer AlN buffer structure improves the surface morphology and facilitates the complete coalescence of the a-plane GaN layer.     

Study of transient photoconductivity of GaN epilayer grown by metalorganic chemical vapor deposition

Received: 24 March 1998/Accepted: 10 July 1998     

Observation of inversion domain boundaries in Mg-doped AlGaN layers grown by metalorganic chemical vapor deposition

We have investigated the formation of inversion domain boundaries in Al_0.13Ga_0.87N layers grown on sapphire substrates by metalorganic chemical vapor deposition using transmission electron microscopy (TEM). We found that the shape of inversion domain boundaries strongly depends on Mg source flow rate in the Mg-doped Al_0.13Ga_0.87N layers. By increasing the Mg source flow rate, the shape of inversion domain boundaries is changed from the vertical shape to the horizontal shape. In addition, the change of polarity by the horizontal shape inversion domain boundary (IDB) resulted in the inverted rotation of pyramidal shape IDB within the Mg-doped Al_0.13Ga_0.87N layers.     

Characteristics of ZnO epilayer on the post-annealed buffer layer on GaN/sapphire substrate by pulsed laser deposition

We employed epi-GaN substrates for ZnO film growth, and studied the deposition and post-annealing effects. ZnO films were grown by pulsed laser deposition (PLD) method. The as-grown films were annealed for one hour under atmospheric pressure air. ZnO morphologies after annealing were investigated and the post-annealed ZnO films grown at T _g =700^oC have very smooth surfaces and the rms with roughness is about 0.5 nm. Finally, ZnO post-annealed buffer layer was inserted between ZnO epilayer and GaN/sapphire substrates. It is confirmed by AFM that growth temperature of 700^oC helps the films grow in step-flow growth mode. It is observed by cathode luminescence spectrum that the ZnO film grown at 700^oC has very low visible luminescence, indicating the decrease of the deep level defects. It is also revealed by Hall measurements that carrier concentration is decreased by increasing the growth temperatures. It is suggested that low temperature buffer layer growth and post-annealing technique can be used to fabricate ZnO hetero-epitaxy.     

Influence of AlN buffer layer thickness on structural properties of GaN epilayer grown on Si (111) substrate with AlGaN interlayer

We present the growth of GaN epilayer on Si (111) substrate with a single AlGaN interlayer sandwiched between the GaN epilayer and AlN buffer layer by using the metalorganic chemical vapour deposition. The influence of the AlN buffer layer thickness on structural properties of the GaN epilayer has been investigated by scanning electron microscopy, atomic force microscopy, optical microscopy and high-resolution x-ray diffraction. It is found that an AlN buffer layer with the appropriate thickness plays an important role in increasing compressive strain and improving crystal quality during the growth of AlGaN interlayer, which can introduce a more compressive strain into the subsequent grown GaN layer, and reduce the crack density and threading dislocation density in GaN film.     

Improved crystal quality of GaN film with the in-plane lattice-matched Ino.17Alo.s3N interlayer grown on sapphire substrate using pulsed metal-organic chemical vapor deposition

We report on an improvement in the crystal quality of GaN film with an Ino.17Alo.83N interlayer grown by pulsed metal-organic chemical vapor deposition, which is in-plane lattice-matched to GaN films. The indium composition of about 17% and the reductions of both screw and edge threading dislocations （TDs） in GaN film with the InA1N interlayer are estimated by high resolution X-ray diffraction. Transmission electron microscopy （TEM） measurements are employed to understand the mechanism of reduction in TD density. Raman and photoluminescence measurements indicate that the InA1N interlayer can improve the crystal quality of GaN film, and verify that there is no additional residual stress induced into the GaN film with InA1N interlayer. Atomic force microscopy measurement shows that the InA1N interlayer brings in a smooth surface morphology of GaN film. All the results show that the insertion of the InA1N interlayer is a convenient method to achieve excellent crystal quality in GaN epitaxy.     

Formation of ZnO nanodot arrays along the step edges on R-face sapphire by metalorganic chemical vapor deposition

Dot array formation of zinc oxide (ZnO) along the linear single steps was demonstrated on sapphire substrates under near atmosphere pressure by metalorganic chemical vapor deposition (MOCVD). Sapphire substrates of , (0 0 0 1) and planes (A-plane, C-plane and R-plane, respectively) were employed as templates for manufacturing the nanostructures. For highly controlling dot array formation the substrates were prepared by annealing at 1000 °C for 3 h in air after chemical treatment. The step arrays were easily prepared on C-plane and R-plane sapphire. The linearly aligned ZnO nanodot arrays were formed on R-plane sapphire along the step edges over several ten micrometers. The result can be attributed to the smaller number of dangling bonds on R-plane than on A-plane and C-plane, enhancing the surface diffusion length. Sapphire can be a good template for manipulating II–VI semiconductor on it to form nanostructures even at near atmosphere pressure by a conventional MOCVD.     

Raman-spectroscopy study of PbTiO3 thin films grown on Si substrates by metalorganic chemical vapor deposition

Raman spectra have been investigated in PbTiO₃ thin films grown on Si by metalorganic chemical vapor deposition. A large grazing-angle scattering technique was taken to measure the temperature dependence of Raman spectra below room temperature. All Raman modes in the thin films are assigned and compared with those in the bulk single crystal, a newA ₁(TO) soft mode at 104 cm^–1 was recorded which satisfies the Curie-Weiss relation ² =A(T _c –T). Intensities of theA ₁(1TO) andE(1TO) modes were anomalously strengthened with increasing temperature. Raman modes for the thin films exhibit remarkable frequency downshift and upshift which is related to the effect of internal stress.     

Effect of Al incorporation on the AlGaN growth by metalorganic chemical vapor deposition

The effect of Al incorporation on the AlGaN growth by metalorganic chemical vapor deposition is investigated. With the increase of trimethylalluminum (TMAl) flux, the crystal quality becomes worse, and the epilayer surface becomes rougher. An interesting phenomenon is that the growth rate of AlGaN decrease with increasing TMAl flux, which is opposite to the AlN growth rate dependence on the TMAl flux. All these effects are attributed to the different properties of Al atoms due to the higher bond strength of Al-N compared with Ga-N, which lead to lower surface mobility and stronger competitive ability of Al atoms during the growth. The enhancement of the surface mobility of Al is especially important for improving the quality of AlGaN.     

Improved crystal quality of GaN film with the in-plane lattice-matched In_(0.17)Al_(0.83)N interlayer grown on sapphire substrate using pulsed metal organic chemical vapor deposition

We report on an improvement in the crystal quality of GaN film with an In0.17Al0.83N interlayer grown by pulsed metal–organic chemical vapor deposition, which is in-plane lattice-matched to GaN films. The indium composition of about 17% and the reductions of both screw and edge threading dislocations(TDs) in GaN film with the InAlN interlayer are estimated by high resolution X-ray diffraction. Transmission electron microscopy(TEM) measurements are employed to understand the mechanism of reduction in TD density. Raman and photoluminescence measurements indicate that the InAlN interlayer can improve the crystal quality of GaN film, and verify that there is no additional residual stress induced into the GaN film with InAlN interlayer. Atomic force microscopy measurement shows that the InAlN interlayer brings in a smooth surface morphology of GaN film. All the results show that the insertion of the InAlN interlayer is a convenient method to achieve excellent crystal quality in GaN epitaxy.     

Semipolar(1122) and polar(0001) InGaN grown on sapphire substrate by using pulsed metal organic chemical vapor deposition

High indium semipolar(1122) and polar(0001) In Ga N layers each with a thickness of about 100 nm are realized simultaneously on sapphire substrates by pulsed metal organic chemical vapor deposition(MOCVD). The morphology evolution, structural and optical characteristics are also studied. The indium content in the layer of the surface(1122)is larger than that of the surface(0001), which is confirmed by reciprocal space map, photoluminescence spectrum and secondary ion mass spectrometer. Additionally, the(0001) surface with island-like morphology shows inhomogeneous indium incorporation, while the(1122) surface with a spiral-like morphology shows a better homogeneous In composition.This feature is also demonstrated by the monochromatic cathodoluminescence map.     

SiC epitaxial layers grown by chemical vapour deposition and the fabrication of Schottky barrier diodes

This paper presents the results of unintentionally doped 4H-SiC epilayers grown on n-type Si-faced 4H-SiC substrates with 8° off-axis toward the [11\overline 2 0] direction by low pressure horizontal hot-wall chemical vapour deposition. Growth temperature and pressure are 1580~°C and 10⁴~Pa, respectively. Good surface morphology of the sample is observed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). Fourier transform infrared spectroscopy (FTIR) and x-ray diffraction (XRD) are used to characterize epitaxial layer thickness and the structural quality of the films respectively. The carrier concentration in the unintentional 4H-SiC homoepitaxial layer is about 6.4×10¹⁴~cm^-3 obtained by c--V measurements. Schottky barrier diodes (SBDs) are fabricated on the epitaxial wafer in order to verify the quality of the wafer and to obtain information about the correlation between background impurity and electrical properties of the devices. Ni and Ti/4H-SiC Schottky barrier diodes with very good performances were obtained and their ideality factors are 1.10 and 1.05 respectively.     

Monolayer MoS2 of high mobility grown on SiO2 substrate by two-step chemical vapor deposition

We report a novel two-step ambient pressure chemical vapor deposition (CVD) pathway to grow high-quality MoS₂ monolayer on the SiO₂ substrate with large crystal size up to 110 μm. The large specific surface area of the pre-synthesized MoO₃ flakes on the mica substrate compared to MoO₃ powder could dramatically reduce the consumption of the Mo source. The electronic information inferred from the four-probe scanning tunneling microscope (4P-STM) image explains the threshold voltage variations and the n-type behavior observed in the two-terminal transport measurements. Furthermore, the direct van der Pauw transport also confirms its relatively high carrier mobility. Our study provides a reliable method to synthesize high-quality MoS₂ monolayer, which is confirmed by the direct 4P-STM measurement results. Such methodology is a key step toward the large-scale growth of transition metal dichalcogenides (TMDs) on the SiO₂ substrate and is essential to further development of the TMDs-related integrated devices.     

Observation of positive and small electron affinity of Si-doped AlN films grown by metalorganic chemical vapor deposition on n-type 6H–SiC

We have investigated the electron affinity of Si-doped AlN films(N_(Si)= 1.0 × 10~(18)–1.0 × 10_(19)cm~(-3)) with thicknesses of 50, 200, and 400 nm, synthesized by metalorganic chemical vapor deposition(MOCVD) under low pressure on the ntype(001)6H–SiC substrates. The positive and small electron affinity of AlN films was observed through the ultraviolet photoelectron spectroscopy(UPS) analysis, where an increase in electron affinity appears with the thickness of AlN films increasing, i.e., 0.36 eV for the 50-nm-thick one, 0.58 eV for the 200-nm-thick one, and 0.97 e V for the 400-nm-thick one.Accompanying the x-ray photoelectron spectroscopy(XPS) analysis on the surface contaminations, it suggests that the difference of electron affinity between our three samples may result from the discrepancy of surface impurity contaminations.     

Effect of post annealing on the characteristics of In2S3 buffer layer grown by chemical bath deposition on a CIGS substrate

In₂S₃ as an alternative Cd-free buffer in Cu(In,Ga)Se₂ (CIGS) solar cells was deposited on CIGS substrate by a chemical bath deposition and characterized after post annealing to optimize film properties for CIGS solar cells. A uniform and pinhole-free In₂S₃ film was deposited on a CIGS substrate by H₂O₂ treatment prior to chemical bath deposition. The In₂S₃ layer was an amorphous state due to the co-existence of In–S, In–O, and In–OH bonds. Annealing at 200 °C induced copper diffusion from CIGS into In₂S₃ layer and lowered the band gap from 3.3 to 1.9 eV, leading to phase change from amorphous state to crystalline state. The conduction band alignment at the In₂S₃/CIGS interface can be controlled by the post annealing. The shunt current through In₂S₃ film was prevented down to the thickness of 30 nm and a 1.15 eV shallow defect was eliminated by the annealing. The results indicated that post annealing in air is a critical to fabricate CIGS solar cells with a sub-30 nm CBD-In₂S₃ buffer layer.     

Structural and optical properties of Al1-xInxN epilayers on GaN template grown by metalorganic chemical vapor deposition

This paper reports that Al1 xInxN epilayers were grown on GaN template by metalorganic chemical vapor de-position with an In content of 7%-20%. X-ray diffraction results indicate that all these Al1 xInxN epilayers have a relatively low density of threading dislocations. Rutherford backscattering/channeling measurements provide the exact compositional information and show that a gradual variation in composition of the Al1 xInxN epilayer happens along the growth direction. The experimental results of optical reflection clearly show the bandgap energies of Al1 xInxN epilayers. A bowing parameter of 6.5 eV is obtained from the compositional dependence of the energy gap. The cathodoluminescence peak energy of the Al1 xInxN epilayer is much lower than its bandgap, indicating a relatively large Stokes shift in the Al1 xInxN sample.