TEM study of defects in AlxGa1−xN layers with different polarity

Transmission electron microscopy (TEM) studies of defects in Al_xGa_1?xN layers with various Al mole fractions (x=0.2, 0.4) and polarities were carried out. The samples were grown by ammonia molecular beam epitaxy on sapphire substrates and consisted of low-temperature AlN (LT-AlN) and high-temperature AlN (HT-AlN) buffer layers, a complex AlN/AlGaN superlattice (SL) and an Al_xGa_1?xN layer (x=0.2, 0.4). It was observed that at the first growth stages a very high density of dislocations is introduced in both Al-polar and N-polar structures. Then, at the interface of the LT-AlN and HT-AlN layers half-loops are formed and the dislocation density considerably decreases in Al-polar structures, whereas in the N-polar structures such a behavior was not observed.The AlN/AlGaN superlattice efficiently promotes the bend and annihilation of threading dislocations and respectively the decrease of the dislocation density in the upper Al_xGa_1?xN layer with both polarities.The lattice relaxation of metal-polar Al_0.2Ga_0.8N was observed, while N-polar Al_0.2Ga_0.8N did not relax. The dislocation densities in the N-polar Al_0.2Ga_0.8N and Al_0.4Ga_0.6N layers were 5.5×10⁹ cm^?2 and 9×10⁹ cm^?2, respectively, and in metal-polar Al_0.2Ga_0.8N and Al_0.4Ga_0.6N layers these were 1×10¹⁰ cm^?2 and 6×10⁹ cm^?2, respectively.Moreover, from TEM images the presence of inversion domains (IDs) in N-polar structures has been observed. The widths of IDs varied from 10 to 30 nm. Some of the IDs widen during the growth of the AlN buffer layers. The IDs formed hills on the surface of the N-polar structures.     

Electrical Properties of Praseodymium-doped GaAs and Al0.3Ga0.7As Epilayers

Praseodymium-doped GaAs and Al_0.3Ga_0.7As epilayers grown on Semi-Insulating (SI) GaAs substrates by Liquid Phase Epitaxy (LPE) were first studied in this present work. Measurement techniques, such as microscopic observation, X-ray diffraction, Secondary Ion Mass Spectroscopy (SIMS), and Hall measurement were employed. Layers doped with Pr resulted in a mirror-like surface, except several high Pr-doped layers having droplet surfaces. Hall measurements reveal that the grown layers contained p-type layers, carrier concentrations from 6.3 × 10¹⁵ to 1.2 × 10¹⁶ cm⁻³, and from 6.3 × 10¹⁵ to 3.5 × 10¹⁶ cm⁻³ for Pr-doped GaAs and Al_0.3Ga_0.7As epilayers, respectively. Although p-type conduction exists, in the light of electrical features, doping of Pr into the GaAs and Al_0.3Ga_0.7As growth melts, is still considered to exhibit gettering properties rather than to become a new acceptor itself. Additional photoluminescence examinations were taken. Their results also indicate that Pr-doped layers produce no new emission lines and support the electrical observations.     

High-resolution imaging of 1:1 [0 0 0 1] ordered a-plane Al0.3Ga0.7N

We report the observation of ordering in Al_0.3Ga_0.7N as part of an epitaxial lateral overgrowth (ELO) of GaN carried out using (1 1 2¯ 2) GaN templates grown by metal-organic chemical vapor deposition on m-plane sapphire. Transmission electron microscopy showed that the crystalline quality of the ELO GaN was greatly improved when the ELO SiO₂ mask was patterned along the [1 1 2¯ 0]_sapphire direction. The ELO GaN wings had an inclined columnar shape with smooth (0 0 0 1) and (1 1 2¯ 0) facets. Layers of 1:1 [0 0 0 1] ordered a-plane Al_0.3Ga_0.7N were observed on the a-plane GaN facets by high-resolution transmission electron microscopy and high-angle annular-dark-field scanning transmission electron microscopy. However, no ordering was observed for c-plane Al_0.3Ga_0.7N layers grown at the same time on the c-plane GaN facets.     

X-ray diffraction study and quantitative measurements of high dislocation densities in superlattices and single crystal layers of heteroepitaxial systems with pronounced lattice mismatch

Plastic deformation in a single-crystal layer of the In_0.12Ga_0.88As/(111)InP solid solution is identified by the methods of X-ray diffractometry (XRD) and the double-crystal pseudorocking curves (DCPRC). X-ray topographs showed the generation of three intersecting systems of straight dislocations in the layer. In a one-layer ZnSe/GaAs structure and multilayer ZnSe/ZnSe_{1 − x} S_x/ZnSe/GaAs structures, the elastic and plastic strains were detected by the combined XRD-DCPRC method. The major components of the thermoelastic and plastic-deformation tensors were determined as ε_xx = ε_yy = 3.5 × 10⁻³ and ε_zz = 2.35 × 10⁻³. Using these data, the dislocation densities were determined as N _d ∼ 2.5 × 10⁸ cm⁻² and N _d ∼ 3 × 10¹⁰ cm⁻² for the 7 μm-thick ZnSe and 1 μm-thick InAs layers, respectively. In a superlattice of the Al_xGa_{1 − x} As/GaAs/⋯/GaAs-type with a large lattice parameter, the plastic deformation was detected. X-ray topography confirmed that the dislocation density in this superlattice equals ∼10⁵ cm⁻². __________ Translated from Kristallografiya, Vol. 45, No. 2, 2000, pp. 326–331. Original Russian Text Copyright ? 2000 by Kuznetsov.     

N-type doping of GaN/Si(1 1 1) using Al0.2Ga0.8N/ALN composite buffer layer and Al0.2Ga0.8N/GaN superlattice

The characteristics of Si-doped and undoped GaN/Si(1 1 1) heteroepitaxy with composite buffer layer (CBL) and superlattice are compared and discussed. While as-grown Si-doped GaN/Si(1 1 1) heteroepitaxy shows lower quality compared to undoped GaN, crack-free n-type and undoped GaN with the thickness of 1200 nm were obtained by metalorganic chemical vapor deposition (MOCVD). In order to achieve the crack-free GaN on Si(1 1 1), we have introduced the scheme of multiple buffer layers; composite buffer layer of Al_0.2Ga_0.8N/AlN and superlattice of Al_0.2Ga_0.8N/GaN on 2-in. Si(1 1 1) substrate, simultaneously. The FWHM values of the double-crystal X-ray diffractometry (DCXRD) rocking curves were 823 arcsec and 745 arcsec for n-GaN and undoped GaN/Si(1 1 1) heteroepitaxy, respectively. The average dislocation density on GaN surface was measured as 3.85×10⁹ and 1.32×10⁹ cm⁻² for n-GaN and undoped GaN epitaxy by 2-D images of atomic force microscopy (AFM). Point analysis of photoluminescence (PL) spectra was performed for evaluating the optical properties of the GaN epitaxy. We also implemented PL mapping, which showed the distribution of edge emission peaks onto the 2 inch whole Si(1 1 1) wafers. The average FWHMs of the band edge emission peak was 367.1 and 367.0 nm related with 3.377 and 3.378 eV, respectively, using 325 nm He-Cd laser as an excitation source under room temperature.     

Growth optimization and characterization of lattice-matched Al0.82In0.18N optical confinement layer for edge emitting nitride laser diodes

We present the growth optimization and the doping by the metal organic chemical vapor deposition of lattice-matched Al_0.82In_0.18N bottom optical confinement layers for edge emitting laser diodes. Due to the increasing size and density of V-shaped defects in Al_1?xIn_xN with increasing thickness, we have designed an Al_1?xIn_xN/GaN multilayer structure by optimizing the growth and thickness of the GaN interlayer. The Al_1?xIn_xN and GaN interlayers in the multilayer structure were both doped using the same SiH₄ flow, while the Si levels in both layers were found to be significantly different by SIMS. The optimized 8×(Al_0.82In_0.18N/GaN=54/6 nm) multilayer structures grown on free-standing GaN substrates were characterized by high resolution X-ray diffraction, atomic force microscopy and transmission electron microscopy, along with the in-situ measurements of stress evolution during growth. Finally, lasing was obtained from the UV (394 nm) to blue (436 nm) wavelengths, in electrically injected, edge-emitting, cleaved-facet laser diodes with 480 nm thick Si-doped Al_1?xIn_xN/GaN multilayers as bottom waveguide claddings.     

AlGaAs/InGaP interfaces in structures prepared by MOVPE

Al_0.3Ga_0.7As/In_1−xGa_xP structures were prepared by low-pressure MOVPE. Lattice matched and strained ones with top In_1−xGa_xP layers as well as reverse ones with top Al_0,3Ga_0,7As layers were examined. The structures were studied by photoluminescence, X-ray and atomic force microscope (AFM) methods. An additional photoluminescence peak from the Al_0.3Ga_0.7As/In_1−xGa_xP interface was observed in our samples and it was attributed to a type-II band offset. A conduction band offset of 0.121 eV was measured in the Al_0.3Ga_0.7As/In_0.485Ga_0.515P lattice-matched structure and a linear dependence of the conduction band offset on In_1−xGa_xP composition, with a zero offset in the Al_0.3Ga_0.7As/In_0.315Ga_0.685P structure, was determined. The valence band discontinuity had a nearly constant value of 0.152 eV.     

Study of the surface state density and potential in MIS diode Schottky using the surface photovoltage method

ABSTRACT

The effects of surface preparation and illumination on electric parameters of Au/GaN/GaAs Schottky diode were investigated. The thin GaN film is realized by nitridation of GaAs substrates with different thicknesses of GaN layers (0.7 – 2.2 nm). In order to study the electrical characteristics under illumination, we use an He-Ne laser of 632 nm wavelength. The I(V) current- voltage, the surface photovltage SPV measurement were plotted and analysed taking into consideration the influence of charge exchange between a continuum of the surface states and the semiconductor. The barrier height Ф_Bn, the serial resistance R_s and the ideality factor n are respectively equal to 0.66 eV, 1980 Ω, 2.75 under dark and to 0.65 eV, 1160 Ω, 2.74 under illumination for simple 1 (GaN theckness of 0.7 nm). The interface states density N_ss in the gap and the excess of concentration δn are determined by fitting the experimental curves of the surface photovltage SPV with the theoretical ones and are equal to 4.5×10¹² eV^?1 cm^?2, 5×10⁷ cm^?3, respectively, for sample 1 and 3.5×10¹² eV^?1 cm^?2, 7×10⁸ cm^?3 for sample 2 (GaN theckness of 2 nm). The results confirm that the surface photovoltage is an efficient method for optical and electrical characterizations.     

Strain relaxation in multilayer III–N structures on Si(111) substrates

X-ray diffractometry and X-ray scattering reciprocal space maps have been used to study strain relaxation in a complex buffer composed of seven intermediate layers of Al _x Ga_{1 ? x} N composition with different values of x, decreasing with an increase in the distance from the substrate. The layers have been grown by hydride metalorganic vapor phase epitaxy on silicon and sapphire substrates. Differences in the structural quality of the first four layers of a multilayer buffer grown on different substrates have been revealed. A gradual smoothing out of these differences in the next three layers with an increase in the layer serial number has been shown. The last grown intermediate Al _x Ga_{1 ? x} N layer and the GaN layer grown on it have identical thicknesses and degrees of mosaicity, regardless of the substrate type. Device structures grown on a complex buffer demonstrate emission in approximately the same wavelength range.     

Growth kinetics and boron doping of very high Ge content SiGe for source/drain engineering

We have studied the in-situ boron doping of high Ge content Si_1?xGe_x layers (x=0.3, 0.4 and 0.5). These layers have been grown at low pressure (20 Torr) and low temperature (600–650 °C) with a heavily chlorinated chemistry on blanket Si(0 0 1) substrates. Such a chemistry yields a full selectivity versus SiO₂ (isolation) and Si₃N₄ (sidewall spacers) on patterned wafers with gate stacks. We have quantified the impact of the diborane flow on the SiGe layer crystalline quality, its resistivity, the SiGe:B growth rate and the apparent Ge concentration. Resistivity values lower than 1 mΩ cm are easily achieved, all the more so for high Ge content layers. The SiGe growth rate increases and the apparent Ge concentration (from X-ray diffraction) decreases as the diborane flow increases. B atoms (much smaller than Si or Ge) indeed partially compensate the compressive strain in the SiGe:B layers. We have also probed the in-situ boron and phosphorus doping of Si at 750 °C, 20 Torr with a heavily chlorinated chemistry. The B ions concentration increases linearly with the diborane flow, then saturates at a value close to 4×10¹⁹ cm^?3. By contrast, the P ions concentration increases sub-linearly with the phosphine flow, with a maximum value close to 9×10¹⁸ cm^?3. Adding diborane (phosphine) to the gaseous mixture leads to a sharp increase (decrease) of the Si:B (the Si:P) growth rates, which has to be taken into account in device layers. All the know-how acquired will be most handy for the formation of in-situ doped recessed or raised sources and drains in metal-oxide semiconductor devices.     

The effect of annealing on the surface morphology of strained and unstrained InxAl1−xN thin films

In_xAl_1−xN is a particularly useful group-III nitride alloy because by adjusting its composition it can be lattice matched to GaN. Such lattice-matched layers may find application in distributed Bragg reflectors (DBRs) and high electron mobility transistors (HEMTs). However, compared with other semiconducting nitride alloys, In_xAl_1-xN has not been researched extensively. In this study, thin In_xAl_1−xN epilayers were grown by metal-organic vapour phase epitaxy (MOVPE) on GaN and Al_yGa_1−yN layers. Samples were subjected to annealing at their growth temperature of 790 °C for varying lengths of time, or alternatively to a temperature ramp to 1000 °C. Their subsequent surface morphologies were analysed by atomic force microscopy (AFM). For both unstrained In_xAl_1−xN epilayers grown on GaN and compressively strained epilayers grown on Al_yGa_1−yN, surface features and fissures were seen to develop as a consequence of thermal treatment, resulting in surface roughening. It is possible that these features are caused by the loss of In-rich material formed on spinodal decomposition. Additionally, trends seen in the strained In_xAl_1−xN layers may suggest that the presence of biaxial strain stabilises the alloy by suppressing the spinode and shifting it to higher indium compositions.     

Structural and electrical characteristics of InGaAsN layers grown by LPE

Crystallographic and transport properties of nominally undoped and Sn-doped InGaAsN layers grown by low-temperature LPE have been studied and related to the growth conditions.In the case of lattice matching, flat and uniform mirror-like layers of 8–10 μm in thickness are obtained. The compositions of the layers under study have been determined by combination of X-ray microanalysis and X-ray diffraction methods to be In_0.035Ga_0.065As_0.086N_0.014. The lattice mismatch between layer and substrate Δa_l/a_s calculated from X-ray diffraction curves is less than?7×10^?4 for all samples. The layers grown at lower epitaxy temperatures exhibit the highest crystalline quality, better lattice match and better homogeneity. This is in good agreement with the results of morphological study by atomic force microscopy which show root mean-square surface roughness of 0.18 nm for the best layers.CV and Hall measurements reveal that intentionally undoped InGaAsN layers are n-type with free carrier concentration about one order of magnitude higher in comparison to layers not containing nitrogen and high electron mobility values over 2000 cm²/Vs. A dramatic reduction in the free carrier concentration and slightly increase in mobility are observed for Sn-doped InGaAsN layers.     

Molecular beam epitaxy as a growth technique for achieving free-standing zinc-blende GaN and wurtzite AlxGa1-xN

Currently there is a high level of interest in the development of ultraviolet (UV) light sources for solid-state lighting, optical sensors, surface decontamination and water purification. III-V semiconductor UV LEDs are now successfully manufactured using the AlGaN material system; however, their efficiency is still low. The majority of UV LEDs require Al_xGa_1-xN layers with compositions in the mid-range between AlN and GaN. Because there is a significant difference in the lattice parameters of GaN and AlN, Al_xGa_1-xN substrates would be preferable to those of either GaN or AlN for many ultraviolet device applications. However, the growth of Al_xGa_1-xN bulk crystals by any standard bulk growth techniques has not been developed so far.There are very strong electric polarization fields inside the wurtzite (hexagonal) group III-nitride structures. The charge separation within quantum wells leads to a significant reduction in the efficiency of optoelectronic device structures. Therefore, the growth of non-polar and semi-polar group III-nitride structures has been the subject of considerable interest recently. A direct way to eliminate polarization effects is to use non-polar (001) zinc-blende (cubic) III-nitride layers. However, attempts to grow zinc-blende GaN bulk crystals by any standard bulk growth techniques were not successful.Molecular beam epitaxy (MBE) is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. In this study we have used plasma-assisted molecular beam epitaxy (PA-MBE) and have produced for the first time free-standing layers of zinc-blende GaN up to 100 μm in thickness and up to 3-inch in diameter. We have shown that our newly developed PA-MBE process for the growth of zinc-blende GaN layers can also be used to achieve free-standing wurtzite Al_xGa_1-xN wafers. Zinc-blende and wurtzite Al_xGa_1-xN polytypes can be grown on different orientations of GaAs substrates - (001) and (111)B respectively. We have subsequently removed the GaAs using a chemical etch in order to produce free-standing GaN and Al_xGa_1-xN wafers. At a thickness of ～30 µm, free-standing GaN and Al_xGa_1-xN wafers can easily be handled without cracking. Therefore, free-standing GaN and Al_xGa_1-xN wafers with thicknesses in the 30–100 μm range may be used as substrates for further growth of GaN and Al_xGa_1-xN-based structures and devices.We have compared different RF nitrogen plasma sources for the growth of thick nitride Al_xGa_1-xN films including a standard HD25 source from Oxford Applied Research and a novel high efficiency source from Riber. We have investigated a wide range of the growth rates from 0.2 to 3 µm/h. The use of highly efficient nitrogen RF plasma sources makes PA-MBE a potentially viable commercial process, since free-standing films can be achieved in a single day.Our results have demonstrated that MBE may be competitive with the other group III-nitrides bulk growth techniques in several important areas including production of free-standing zinc-blende (cubic) (Al)GaN and of free-standing wurtzite (hexagonal) AlGaN.     

Low-temperature/high-temperature AlN superlattice buffer layers for high-quality AlxGa1−xN on Si (1 1 1)

We present MOVPE-grown, high-quality Al_xGa_1−x N layers with Al content up to x=0.65 on Si (1 1 1) substrates. Crack-free layers with smooth surface and low defect density are obtained with optimized AlN-based seeding and buffer layers. High-temperature AlN seeding layers and (low temperature (LT)/high temperature (HT)) AlN-based superlattices (SLs) as buffer layers are efficient in reducing the dislocation density and in-plane residual strain. The crystalline quality of Al_xGa_1−xN was characterized by high-resolution X-ray diffraction (XRD). With optimized AlN-based seeding and SL buffer layers, best ω-FWHMs of the (0 0 0 2) reflection of 540 and 1400 arcsec for the (1 0 1¯ 0) reflection were achieved for a ∼1-μm-thick Al_0.1Ga_0.9N layer and 1010 and 1560 arcsec for the (0 0 0 2) and (1 0 1¯ 0) reflection of a ∼500-nm-thick Al_0.65Ga_0.35N layer. AFM and FE-SEM measurements were used to study the surface morphology and TEM cross-section measurements to determine the dislocation behaviour. With a high crystalline quality and good optical properties, Al_xGa_1−x N layers can be applied to grow electronic and optoelectronic device structures on silicon substrates in further investigations.     

Gallium vacancy ordering in Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript> thin layers on Si(100), Si(111), and Si(123) substrates

Thin Ga₂Se₃ layers deposited on silicon substrates with the (100), (111), and (123) orientations are studied by transmission electron microscopy and X-ray microanalysis. Some features and regularities of the stoichiometric gallium vacancy ordering at different substrate orientations are discussed. The Ga₃Se₄(100)с(2 × 2) and Ga₂Se₃(111)(√3 × √3)-R30° ordered structures are formed on the Si(100) and Si(111) surfaces, respectively.     

Digitally alloyed modulated precursor flow epitaxial growth of AlxGa1−xN layers with AlN and AlyGa1−yN monolayers

We propose a new growth scheme of digitally alloyed modulated precursor flow epitaxial growth (DA-MPEG) using metalorganic and hydride precursors for the growth of Al_xGa_1−xN layers with high-Al content at relatively low temperatures. The growth of high-quality, high-Al content Al_xGa_1−xN layers (x_Al>50%) that are composed of AlN and Al_yGa_1−yN monolayers on AlN/sapphire template/substrates by DA-MPEG was demonstrated. The overall composition of the ternary Al_xGa_1−xN material by DA-MPEG can be controlled continuously by adjusting the Column III mole fraction of the atomic Al_yGa_1−yN sub-layer. X-ray diffraction and optical transmittance results show that the AlGaN materials have good crystalline quality. The surface morphology of DA-MPEG AlGaN samples measured by atomic force microscopy are comparable to high-temperature-grown AlGaN and are free from surface features such as nano-pits.     

Cathodoluminescence study of the excitons localization in AlGaN/GaN and InGaN/GaN quantum wells grown on sapphire

Al_0.1Ga_0.9N(5 nm)/GaN(2 nm) and In_0.2Ga_0.8N/GaN quantum wells (QWs) grown on GaN/sapphire have been studied by cathodoluminescence (CL) spectroscopy and imaged using an experimental setup especially developed for scanning near-field CL microscopy, which combines a scanning force microscope and a scanning electron microscope. The CL spectra show the characteristic band edge emission peak of GaN at λ= 364 nm and the emission peaks related to the presence of QWs, at λ= 353 and 430 nm for the AlGaN/GaN and the InGaN/GaN samples, respectively. Monochromatic CL images reveal that the emission of the AlGaN/GaN and InGaN/GaN QWs is localized at the level of the grains observed by SFM. A cross sectional analysis of the InGaN/GaN sample gives insight into its growth and an estimation of the exciton diffusion length of about L=180 nm.     

Bulk single crystals of β-Ga2O3 and Ga-based spinels as ultra-wide bandgap transparent semiconducting oxides

In the course of development of transparent semiconducting oxides (TSOs) we compare the growth and basic physical properties bulk single crystals of ultra-wide bandgap (UWBG) TSOs, namely β-Ga₂O₃ and Ga-based spinels MgGa₂O₄, ZnGa₂O₄, and Zn_1-xMg_xGa₂O₄. High melting points of the materials of about 1800 -1930 °C and their thermal instability, including incongruent decomposition of Ga-based spinels, require additional tools to obtain large crystal volume of high structural quality that can be used for electronic and optoelectronic devices. Bulk β-Ga₂O₃ single crystals were grown by the Czochralski method with a diameter up to 2 inch, while the Ga-based spinel single crystals either by the Czochralski, Kyropoulos-like, or vertical gradient freeze / Bridgman methods with a volume of several to over a dozen cm³. The UWBG TSOs discussed here have optical bandgaps of about 4.6 - 5 eV and great transparency in the UV / visible spectrum. The materials can be obtained as electrical insulators, n-type semiconductors, or n-type degenerate semiconductors. The free electron concentration (n_e) of bulk β-Ga₂O₃ crystals can be tuned within three orders of magnitude 10¹⁶ - 10¹⁹ cm^?3 with a maximum Hall electron mobility (μ) of 160 cm²V^?1s^?1, that gradually decreases with n_e. In the case of the bulk Ga-based spinel crystals with no intentional doping, the maximum of n_e and μ increase with decreasing the Mg content in the compound and reach values of about 10²⁰ cm^?3 and about 100 cm²V^?1s^?1 (at n_e > 10¹⁹ cm^?3), respectively, for pure ZnGa₂O₄.     

X-ray diffraction study of short-period AlN/GaN superlattices

The structure of short-period hexagonal GaN/AlN superlattices (SLs) has been investigated by X-ray diffraction. The samples have been grown by metalorganic vapor-phase epitaxy (MOVPE) in a horizontal reactor at a temperature of 1050°C on (0001)Al₂O₃ substrates using GaN and AlN buffer layers. The SL period changes from 2 to 6 nm, and the thickness of the structure varies in a range from 0.3 to 1 μm. The complex of X-ray diffraction techniques includes a measurement of θ-2θ rocking curves of symmetric Bragg reflection, the construction of intensity maps for asymmetric reflections, a measurement and analysis of peak broadenings in different diffraction geometries, a precise measurement of lattice parameters, and the determination of radii of curvature. The thickness and strain of separate SL layers are determined by measuring the θ-2θ rocking curves subsequent simulation. It is shown that most SL samples are completely relaxed as a whole. At the same time, relaxation is absent between sublayers, which is why strains in the AlN and GaN sublayers (on the order of 1.2 × 10^?2) have different signs. An analysis of diffraction peak half-widths allows us to determine the densities of individual sets of dislocations and observe their change from buffer layers to SLs.     

Characterization of deep levels in a-plane GaN epi-layers grown using various growth techniques

To study the correlation between defects and deep levels in a-plane GaN films grown on r-plane sapphire substrates, transmission electron microscopy (TEM) and deep level transient spectroscopy (DLTS) have been performed on three types of a-plane GaN samples grown using modified two-step growth (sample I), SiN_x interlayer (sample II), and patterned insulator on sapphire substrate (sample III). From the microstructure evolution in cross-sectional TEM images, it was shown that combination of growth techniques is highly efficient in the reduction of dislocation densities. Average dislocation densities of samples I, II, and III were about 2.2×10⁹ cm^?2, 1.1×10⁹ cm^?2, and 3.4×10⁸ cm^?2, respectively. The trap a_t E_c–E_t～0.13 eV (E1) was observed in only sample I, and three electron traps at 0.28–0.33 eV (E2), 0.52–0.58 eV (E3), and 0.89–0.95 eV (E4) from the conduction band edge were measured common to all the samples. The analysis of trap properties indicated that E2 and E3 trap levels are strongly associated with the partial dislocations in a-plane GaN films.