Thermal stability of thin InGaN films on GaN

The thermal stability of ∼200-nm-thick InGaN thin films on GaN was investigated using isothermal and isochronal post-growth anneals. The In_xGa_1−xN films (x=0.08–0.18) were annealed in N₂ at 600–1000 °C for 15–60 min, and the resulting film degradation was monitored using X-ray diffraction (XRD) and photoluminescence (PL) measurements. As expected, films with higher indium concentration showed more evidence for decomposition than the samples with lower indium concentration. Also for each alloy composition, decreases in the PL intensity were observed starting at much lower temperatures compared to decreases in the XRD intensity. This difference in sensitivity of the PL and XRD techniques to the InGaN decomposition suggest that defects that quench luminescence are generated prior to the onset of structural decomposition. For the higher indium concentration films, the bulk decomposition proceeds by forming metallic indium and gallium regions as observed by XRD. For the 18% indium concentration film, measurement of the temperature-dependent InGaN decomposition yields an activation energy, E_A, of 0.87±0.07 eV, which is similar to the E_A for bulk InN decomposition. The InGaN integrated XRD signal of the 18% film displays an exponential decrease vs. time, implying InGaN decomposition proceeds via a first-order reaction mechanism.     

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.     

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.     

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.     

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.     

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.     

Selective etching of dislocations in GaN grown by low-pressure solution growth

This work presents an experimental study on the identification and quantification of different types of dislocations in GaN grown by low-pressure solution growth. A reliable defect selective etching procedure in a NaOH-KOH melt is developed and validated using transmission electron microscopy that permits to define groups of etch pits that belong each to dislocations with a specific Burgers vector. This way a comparably fast method is provided for determining the total, the specific dislocation densities and the type of dislocation in a statistically representative way. The results for the solution grown samples are compared to those obtained for MOCVD GaN.     

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.     

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.     

Room-temperature luminescence study on the effect of Mg activation annealing on p-GaN layers grown by MOCVD

An Mg-doped p-GaN layer was grown by the metalorganic chemical vapor deposition method. The dissociation extent of hydrogen-passivated Mg acceptors in the p-GaN layer through Mg activation annealing was estimated by using room-temperature cathodoluminescence (CL) spectroscopy. The CL measurement revealed that the CL spectra intensities tend to increase with increasing the activation annealing temperature. The sample annealed at 925 °C showed the most intense emission and the narrowest width among the emission peaks. Consequently, it was the most excellent dissociation extent of Mg–H complexes caused by the Mg activation annealing. The hole concentration under this optimum condition was 1.3×10¹⁷ cm⁻³ at room temperature. The photoluminescence (PL) measurement showed a 2.8 eV band having characteristically a broad peak in heavily Mg-doped GaN at room temperature. By analyzing the PL results, we learned that this band was associated with the deep donor–acceptor pair (DAP) emission rather than with the emission caused by the transition from the conduction band to deep acceptor level. The four emission peaks in the resolved 2.8 eV band were emitted by transiting from deep donor levels of 0.14, 0.26, 0.40, and 0.62 eV below the conduction band to the shallow Mg acceptor level of 0.22 eV above the valence band.     

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.     

Properties of Ge-doped,high-quality bulk GaN crystals fabricated by hydride vapor phase epitaxy

We investigated the properties of Ge-doped, high-quality bulk GaN crystals with Ge concentrations up to 2.4×10¹⁹ cm⁻³. The Ge-doped crystals were fabricated by hydride vapor phase epitaxy with GeCl₄ as the dopant source. Cathodoluminescence imaging revealed no increase in the dislocation density at even the highest Ge concentration, with values as low as 3.4×10⁶ cm⁻². The carrier concentration, as determined by Hall measurement, was almost identical to the combined concentration of Ge and unintentionally incorporated Si. The electron mobilities were 260 and 146 cm² V⁻¹ s⁻¹ for n=3.3×10¹⁸ and 3.35×10¹⁹ cm⁻³, respectively; these values are markedly larger than those reported in the past for Ge-doped GaN thin films. The optical absorption coefficient was quite small below the band gap energy; it slightly increased with increase in Ge concentration. Thermal conductivity, estimated by the laser-flash method, was virtually independent of Ge concentration, maintaining an excellent value around 2.0 W cm⁻¹ K⁻¹. Thermal expansion coefficients along the a- and m-axes were approximately constant at 5.0×10⁻⁶ K⁻¹ in the measured doping concentration range.     

Growth of height-controlled InGaN quantum dots on GaN

InGaN height-controlled quantum dots (HCQDs) were grown by alternately depositing In_0.4Ga_0.6N QD and In_0.1Ga_0.9N spacer layers on a seed In_0.4Ga_0.6N QD layer. Structural and optical studies showed that the height of the InGaN QDs was controlled by the deposition cycle of In_0.4Ga_0.6N/In_0.1Ga_0.9N layers. Photoluminescence studies showed that the In_0.4Ga_0.6N HCQDs provided deep potential wells and the piezoelectric field-induced quantum-confined Stark effect was negligibly small. These phenomena are attributed to variation in quantum confinement energy in the electronically coupled InGaN HCQDs providing deep potential wells.     

Polar dependence of impurity incorporation and yellow luminescence in GaN films grown by metal-organic chemical vapor deposition

We have investigated the unintentional impurities, oxygen and carbon, in GaN films grown on c-plane, r-plane as well as m-plane sapphire by metal-organic chemical vapor deposition. The GaN layer was analyzed by secondary ion mass spectroscopy. The different trend of the incorporation of oxygen and carbon has been explained in the polar (0 0 0 1), nonpolar (1 1 2¯ 0) and semipolar (1 1 2¯ 2) GaN by a combination of the atom bonding structure and the origin direction of the impurities. Furthermore, it has been found that there is a stronger yellow luminescence (YL) in GaN with higher concentration of carbon, suggesting that C-involved defects are originally responsible for the YL.     

Revealing extended defects in HVPE-grown GaN

In this communication we will summarize the results of a complementary study of structural and chemical non-homogeneities that are present in thick HVPE-grown GaN layers. It will be shown that complex extended defects are formed during HVPE growth, and are clearly visible after photo-etching on both Ga-polar surface and on any non-polar cleavage or section planes. Large chemical (electrically active) defects, such as growth striations, overgrown or empty pits (pinholes) and clustered irregular inclusions, are accompanied by non-uniform distribution of crystallographic defects (dislocations). Possible reasons of formation of these complex structures are discussed. The nature of defects revealed by selective etching was subsequently confirmed using TEM, orthodox etching and compared with the CL method. The non-homogeneities were studied in GaN crystals grown in different laboratories showing markedly different morphological characteristics.     

A study of the degree of relaxation of AlGaN epilayers on GaN template

The strain state of 570 nm Al_xGa_1−xN layers grown on 600 nm GaN template by metal organic chemical vapor deposition was studied using Rutherford backscattering (RBS)/channeling and triple-axis X-ray diffraction measurements. The results showed that the degree of relaxation (R) of Al_xGa_1−xN layers increased almost linearly when x0.42 and reached to 70% when x=0.42. Above 0.42, the value of R varied slowly and Al_xGa_1−xN layers almost full relaxed when x=1 (AlN). In this work the underlying GaN layer was in compressive strain, which resulted in the reduction of lattice misfit between GaN and Al_xGa_1−xN, and a 570 nm Al_xGa_1−xN layer with the composition of about 0.16 might be grown on GaN coherently from the extrapolation. The different shape of (0 0 0 4) diffraction peak was discussed to be related to the relaxation.