The influence of the Al pre-deposition on the properties of AlN buffer layer and GaN layer grown on Si (1 1 1) substrate

The influence of Al pre-deposition on the properties of AlN buffer layer and GaN layer grown on Si (1 1 1) substrate by metalorganic chemical vapor deposition (MOCVD) has been systematically studied. Compared with the sample without Al pre-deposition, optimum Al pre-deposition time could improve the AlN buffer layer crystal quality and reduce the root mean square (RMS) roughness. Whereas, overlong Al-deposition time deteriorated the AlN crystal quality and Al-deposition patterns could be found. Cracks and melt-back etching patterns appeared in the GaN layer grown without Al pre-deposition. With suitable Al-deposition time, crack-free 2.0 μm GaN was obtained and the full-width at half-maximum (FWHM) of (0 0 2) plane measured by double crystal X-ray diffraction (DCXRD) was as low as 482 arcsec. However, overlong Al-deposition time would result in a great deal of cracks, and the crystal quality of GaN layer deteriorated. The surface of GaN layer became rough in the region where the Al-deposition patterns were formed due to overlong Al-deposition time.     

Reduction of tensile stress in GaN grown on Si(1 1 1) by inserting a low-temperature AlN interlayer

We studied the selective growth behaviors of InP through narrow openings (<2 μm) by metal-organic chemical vapor deposition. The lateral overgrowth was observed to be significantly affected by both the opening width and orientation. It was found that the lateral overgrowth length reached the maximum at 60° off [0 1 1] direction. The lateral overgrowth also showed a ‘diffraction-like’ behavior, with the overgrowth length increasing with decreasing opening width. Based on these results, a novel InP/InGaAs heterojunction bipolar transistor (HBT) structure with extrinsic base laterally overgrown on SiO₂ is proposed. The device behaviors of the laterally regrown-base HBT prototypes are demonstrated.     

Correlations between photoluminescence and Hall mobility in GaN/sapphire grown by metalorganic chemical vapor deposition

We have investigated photoluminescence (PL) and electron Hall mobility for unintentionally doped GaN epitaxial layers grown by low-pressure metalorganic chemical vapor deposition on c-plane Al₂O₃ substrates. Four GaN films having identical dislocation density but remarkably different electron Hall mobility were exploited. At low temperature (12 K), a PL line associated with a bound exciton was observed and strong correlations were found between the Hall mobility and the PL intensity of the exciton transition. That is, relative PL intensity of the bound exciton to a donor-bound exciton monotonously increased with decreasing the electron mobility of the GaN films. This correlation was interpreted in terms of electrical compensation. Efforts to find the chemical origin of the PL line led to the conclusion that the BE line originated neither from threading dislocations nor from extrinsic point defects. Intrinsic acceptors such as Ga vacancy and GaN anti-site were suspected as plausible origin.     

Influence of AlGaN/GaN/InGaN superlattice on the characteristics of LEDs grown by metalorganic chemical vapor deposition

This study examined the influence of strain-compensated triple AlGaN/GaN/InGaN superlattice structures (SLs) in n-GaN on the structural, electrical and optical characteristics of LEDs by analyzing the etch pits density (EPD), stress measurement, high-resolution X-ray diffraction (HRXRD), sheet resistance, photoluminescence (PL) and light–current–voltage (L–I–V). EPD, stress measurement and HRXRD studies showed that the insertion of AlGaN/GaN/InGaN SLs during the growth of n-GaN effectively distributed and compensated for the strong compressive stress, and decreased the dislocation density in n-GaN. The operating voltage at 20 mA for the LEDs grown with SLs decreased to 3.18 V from 3.4 V for the LEDs grown without SLs. In addition, a decrease in the spectral blue shift compared to the LEDs grown without SLs was observed in the LEDs grown with the SLs.     

GaN/GaAs(1 0 0) superlattices grown by metalorganic vapor phase epitaxy using dimethylhydrazine precursor

Superlattices of cubic gallium nitride (GaN) and gallium arsenide (GaAs) were grown on GaAs(1 0 0) substrates using metalorganic vapor phase epitaxy (MOVPE) with dimethylhydrazine (DMHy) as nitrogen source. Structures grown at low temperatures with varying layer thicknesses were characterized using high resolution X-ray diffraction and atomic force microscopy. Several growth modes of GaAs on GaN were observed: step-edge, layer-by-layer 2D, and 3D island growth. A two-temperature growth process was found to yield good crystal quality and atomically flat surfaces. The results suggest that MOVPE-grown thin GaN layers may be applicable to novel GaAs heterostructure devices.     

High-quality a-plane GaN grown with flow-rate modulation epitaxy on r-plane sapphire substrate

We use the flow-rate modulation epitaxy (FME) technique to improve the crystal quality of a pit-free a-plane GaN (1 1 2¯ 0) film grown on r-plane sapphire (1 1¯ 0 2) substrate. With the FME technique, the width of the rocking curve in X-ray diffraction measurement is significantly reduced. Also, the surface roughness in either atomic-force-microscopy scanning or -step profiling is decreased. Here, the FME technique means to alternatively turn on and off the supply of Ga atoms, while N atoms are continuously supplied without changing the flow rate. Under the used growth conditions, the optimized FME parameters include the on/off periods at 10/10 s. During the period of closing the flow of trimethylgallium (TMGa), the continuous supply of nitrogen can lead to the formation of stoichiometry structure under the high-Ga growth condition, which is required for pit-free growth. Also, during this period, Ga atoms can further migrate to result in a flatter surface. Therefore, the crystal quality of the a-plane GaN sample can be improved.     

Selective area growth of GaN microstructures on patterned (1 1 1) and (0 0 1) Si substrates

Selective MOVPE growth of GaN microstructure on silicon substrates has been investigated using SiO₂ mask having circular or stripe window. In case of (0 0 1)substrate, grooves with (1 1 1) facets at the sides were made by using the etching anisotropy of a KOH solution. On the (1 1 1) facets of patterned silicon substrate (or on the as opened window region of (1 1 1) substrate), growth of wurtzite GaN was performed, of which the c-axis is oriented along the 1 1 1 axis of silicon. The photoluminescence and X-ray diffraction analysis were performed to characterize the single crystal to reveal the effect of the growth conditions of the intermediated layer and the microstructure.     

Study on optimal growth conditions of a-plane GaN grown on r-plane sapphire by metal-organic chemical vapor deposition

Non-polar a-plane GaN thin films were grown on r-plane sapphire substrates by metal-organic chemical vapor deposition. In order to obtain a-plane GaN films with better crystal quality and surface morphology, detailed comparisons between different growth conditions were investigated. The results showed that high-temperature and low-pressure conditions facilitating two-dimensional growth could lead to a fully coalesced a-plane GaN layer with a very smooth surface. The best mean roughness of the surface morphology was 10.5 Å. Various thickness values of AlN nucleation layers and the V/III ratios for growth of the a-plane GaN bulk were also studied to determine the best condition for obtaining a smooth surface morphology of the a-plane GaN layer.     

Control of the morphology of InGaN/GaN quantum wells grown by metalorganic chemical vapor deposition

Various techniques for morphological evolution of InGaN/GaN multiple quantum well (MQW) structures grown by metalorganic chemical vapor deposition have been evaluated. Atomic force microscopy, photoluminescence (PL) and X-ray diffraction measurements have been used for characterization. It is shown that inclusions, that are generated into the V-defects in the InGaN quantum wells (QW), can be removed by introducing a small amount of hydrogen during the growth of GaN barriers. This hydrogen treatment results in partial loss of indium from the QWs, but smooth surface morphology of the MQW structure and improved optical quality of InGaN wells are obtained. The density of the V-defects could be reduced by reducing the dislocation density of the underlying GaN buffer.     

Structural,electrical and optical properties of SnO2 films deposited on Y-stabilized ZrO2 (1 0 0) substrates by MOCVD

SnO₂ films have been deposited on Y-stabilized ZrO₂ (YSZ) (1 0 0) substrates at different substrate temperatures (500–800 °C) by metalorganic chemical vapor deposition (MOCVD). Structural, electrical and optical properties of the films have been investigated. The films deposited at 500 and 600 °C are epitaxial SnO₂ films with orthorhombic columbite structure, and the HRTEM analysis shows a clear epitaxial relationship of columbite SnO₂(1 0 0)||YSZ(1 0 0). The films deposited at 700 and 800 °C have mixed-phase structures of rutile and columbite SnO₂. The carrier concentration of the films is in the range from 1.15×10¹⁹ to 2.68×10¹⁹ cm⁻³, and the resistivity is from 2.48×10⁻² to 1.16×10⁻² Ω cm. The absolute average transmittance of the films in the visible range exceeds 90%. The band gap of the obtained SnO₂ films is about 3.75–3.87 eV.     

Characterization of polycrystalline GaN grown on silica glass substrates

The structural, optical, and electrical properties of GaN films grown on silica glass substrate by metalorganic chemical vapor deposition were studied. X-ray diffraction showed that the films were grown in hexagonal structure with a predominant (0 0 0 2) peak. A broad and strong band-edge emission and very weak yellow luminescence in photoluminescence (PL) spectra were observed. And the temperature dependence of the PL spectra was extensively studied. The thermal quenching activation energy was found to be very close to the donor activation energy determined from the temperature dependence of the carrier concentration. Longitudinal optical phonons were found to be responsible for the PL broadening above 100 K.     

Photoluminescence study of homoepitaxial N-polar GaN grown on differently misoriented single crystal substrates

We report results of a photoluminescence (PL) study of homoepitaxial N-polar GaN films grown by metal-organic chemical vapour deposition on vicinal GaN single crystal substrates. Off-angles of 2° and 4° towards the direction as well as 4° in the direction were investigated. Along with a remarkable improvement of the epilayer morphology, a significant reduction of the unintentional/intrinsic donor concentration is achieved for all considered misorientations. As a consequence, PL spectra with narrow bound and free excitonic lines were observed. The misorientation of 4° towards the direction results in an N-polar epilayer of the best optical quality.     

GaP:Mg layers grown on GaN by MOCVD

We have investigated the growth of magnesium-doped GaP (GaP:Mg) layers on GaN by metalorganic chemical vapor deposition. The hole carrier concentration increased linearly from 0.8×10¹⁸ to 4.2×10¹⁸ cm⁻³ as the Bis(cyclopentadienyl) magnesium (Cp₂Mg) mole flow rate increased from 1.2×10⁻⁷ to 3.6×10⁻⁷ mol/min. However, the hole carrier concentration decreased when the CP₂Mg mole flow rate was further increased. The double crystal X-ray diffraction (DCXRD) rocking curves showed that the GaP:Mg layers were single crystalline at low CP₂Mg molar flow. However, the GaP:Mg layers became polycrystalline if the CP₂Mg molar flow was too high. The decrease in hole carrier concentration at high CP₂Mg molar flow was due to crystal quality deterioration of the GaP layer, which also resulted in the low hole mobility of the GaP:Mg layer.     

Structural and optical properties of quaternary AlInGaN epilayers grown by MOCVD with various TMGa flows

AlInGaN quaternary epilayers have been grown with various TMGa flows by metalorganic chemical vapor deposition to investigate the influence of growth rate on the structural and optical properties. Triple-axis X-ray diffraction measurements show AlInGaN epilayers have good crystalline quality. Photoluminescence (PL) measurements show that the emission intensity of AlInGaN epilayers is twenty times stronger than that of AlGaN epilayer with comparable Al content. V-shaped pits are observed at the surface of AlInGaN epilayers by atomic force microscopy (AFM) and transmission electron microscopy (TEM). High growth rate leads to increased density and size of V-shaped pits, but crystalline quality is not degraded.     

Transmission electron microscopy study of an AlN nucleation layer for the growth of GaN on a 7-degree off-oriented (0 0 1) Si substrate by metalorganic vapor phase epitaxy

We have investigated the morphology of the high-temperature-grown AlN nucleation layer and its role in the early stage of GaN growth, by means of transmission electron microscopy. The nitride was selectively grown on a 7-degree off-oriented (0 0 1) patterned Si substrate by metalorganic vapor phase epitaxy. AlN was deposited on the inclined unmasked (1 1 1) facet in the form of islands. The size of the islands varied along the slope, which is attributable to the diffusion of the growth species in the vapor phase. The GaN nucleation occurred at the region where rounded AlN islands formed densely. The threading dislocations were observed to generate in the GaN nucleated region.     

The photoluminescence of ZnO thin films grown on Si (1 0 0) substrate by plasma-enhanced chemical vapor deposition

High-quality ZnO thin films have been grown on a Si(1 0 0) substrate by plasma-enhanced chemical vapor deposition (PECVD) using a zinc organic source (Zn(C₂H₅)₂) and carbon dioxide (CO₂) gas mixtures at a temperature of 180°C. A strong free exciton emission with a weak defect-band emission in the visible region is observed. The characteristics of photoluminescence (PL) of ZnO, as well as the exciton absorption peak in the absorption spectra, are closely related to the gas flow rate ratio of Zn(C₂H₅)₂ to CO₂. Full-widths at half-maximum of the free exciton emission as narrow as 93.4 meV have been achieved. Based on the temperature dependence of the PL spectra from 83 to 383 K, the exciton binding energy and the transition energy of free excitons at 0 K were estimated to be 59.4 meV and 3.36 eV, respectively.     

Structure and thermal stability of MOCVD ZrO2 films on Si (1 0 0)

The structure and thermal stability of ZrO₂ films grown on Si (1 0 0) substrates by metalorganic chemical vapor deposition have been studied by high-resolution transmission electron microscopy, selected area electron diffraction and X-ray energy dispersive spectroscopy. As-deposited films consist of tetragonal ZrO₂ nanocrystallites and an amorphous Zr silicate interfacial layer. After annealing at 850°C, some monoclinic phase is formed, and the grain size is increased. Annealing a 6 nm thick film at 850°C in O₂ revealed that the growth of the interfacial layer is at the expense of the ZrO₂ layer. In a 3.0 nm thick Zr silicate interfacial layer, there is a 0.9 nm Zr-free SiO₂ region right above the Si substrate. These observations suggest that oxygen reacted with the Si substrate to grow SiO₂, and SiO₂ reacted with ZrO₂ to form a Zr silicate interfacial layer during the deposition and annealing. Oxygen diffusion through the tetragonal ZrO₂ phase was found to be relatively easier than through the monoclinic phase.     

X-ray and cathodoluminescence study on the effect of intentional long time annealing of the InGaN/GaN multiple quantum wells grown by MOCVD

GaN-based InGaN/GaN multiple quantum wells (MQWs) structure having a high-quality epilayer and coherent periodicity was grown by metalorganic chemical vapor deposition. After thermal annealing of InGaN/GaN MQWs, the increase in temperature and annealing time caused the intermixing between the barrier and the wells, which in turn caused a decrease in periodicity on the high-resolution X-ray diffraction patterns. Thereby, we confirmed that the structural performance of InGaN MQWs is successively degrading with increasing thermal annealing temperature. Especially, InGaN MQWs of the sample annealed at 950 °C were profoundly damaged. The cathodoluminescence (CL) measurement indicated that MQWs emission intensity decreases with increasing thermal annealing temperature. Thus, the integrated CL intensity ratio of InGaN MQWs to GaN dramatically decreased while thermal annealing temperatures increased. This result caused the intermixing in MQWs to deteriorate the active layer performance. Furthermore, the peak position of MQWs showed a tendency of the red shift after high thermal annealing. It is suggested that the annealing-induced red shift in MQWs is attributed to the reduction of the inhomogeneity of the In content in the MQWs leading to the reduction of the quantized energies. Consequently, it indicates that the high temperature and the long-time thermal annealing would be inevitably followed by the structural destruction of InGaN MQWs.     

Charge storage and screening of the internal field in GaN/AlGaN quantum wells

The optical spectra of a set of high quality quantum wells, with well characterized structural parameters, are presented here. We propose a quantitative calculation for the measured emission energies which takes into account the spontaneous and piezoelectric polarization fields and the field-screening effect due to electron-hole photogenerated pairs; such an effect is especially effective in wide QW samples which in fact behave like mesoscopic capacitors.     

Structural and electrical characterizations of single tetragonal FeSe on Si substrate

(0 0 l)-Oriented FeSe thin films were successfully fabricated by metal organic chemical vapor deposition on Si substrate with large thermal mismatch. X-ray diffraction and X-ray photoelectron spectroscopy measurements indicate that the films are of single tetragonal phase FeSe. Hysteresis loop indicates that the FeSe structure is ferromagnetic at room temperature with coercive force of 260 Oe. The FeSe films show p-type conduction with carrier concentration of 10²¹ cm⁻³, and the anomalous Hall effect was discussed.