AlGaN Epitaxial Layers Research Articles

Erratic dislocations within funnel defects in AlN templates for AlGaN epitaxial layer growth

We report our transmission electron microscopy observations of erratic dislocation behavior within funnel-like defects on top of AlN templates filled with AlGaN from an overlying epitaxial layer. This dislocation behavior is observed in material where phase separation is also observed. Several bare AlN templates were examined to determine the formation mechanism of the funnels. Our results suggest that they are formed prior to epitaxial layer deposition due to the presence of impurities during template regrowth. We discuss the erratic dislocation behavior in relation to the presence of the phase-separated material and the possible effects of these defects on the optoelectronic properties.

Investigation of Composition-dependent Optical Phonon Modes in AlxGa1-xN Epitaxial Layers Grown on Sapphire Substrates

AbstractWe reported the systematical study of optical properties of hexagonal AlxGa1-xN epitaxial films grown on c-sapphire substrate using metal-organic chemical vapor deposition. By performing Fourier transform infrared spectroscopy measurements, the high-frequency dielectric constants and phonon frequencies can be obtained by theoretically fitting the experimental infrared reflectance spectra using a four-phase layered model. The high-frequency dielectric constant of AlxGa1-xN varies between 4.98 and 4.52 for ε∝,│(polarization perpendicular to the optical axis) and between 4.95 and 4.50 for ε∝,//(polarization parallel to the optical axis) respectively when the aluminum composition changes from 0.15 to 0.24. Furthermore, from experimental infrared reflectance spectra of AlxGa1-xN films, a specific absorption dip at 785 cm-1 was observed when the aluminum composition is larger than 0.24. The dip intensity increases and the dip frequency shifts from 785 to 812 cm-1 as aluminum composition increases from 0.24 to 0.58. According to the reciprocal space map of x-ray diffraction measurements, the emergence of this dip could be resulted from the effects of strain relaxation in AlGaN epitaxial layers due to the large lattice mismatch between GaN and AlGaN epitaxial film.

High‐resolution Rutherford backscattering spectrometry for InAlGaN/AlGaN single quantum well structure

AbstractInAlGaN/AlGaN single quantum well (SQW) structure has been studied by means of high‐resolution Rutherford backscattering spectrometry (HRBS). The HRBS spectrum of a SQW structure was clearly observed. The composition of each layer was estimated to be In0.02Al0.17Ga0.81N/Al0.28Ga0.72N from the HRBS measurement. The channeling measurement shows the high degree of the crystalline perfection that can be achieved in such epitaxial layers from the minimum yield of 3.2% in the direction of the <11–23> axis. The channeling measurement also shows atomic disorder differences between InAlGaN epitaxial layer and AlGaN epitaxial layer. Furthermore Al, Ga and In minimum yields in the InAlGaN layer were estimated to be χmin(Ga) = 4.9%, χmin(Al) = 9.4% and χmin(In) = 6.6%, respectively. Larger normalized yield of Al than those of Ga and In in the InAlGaN layer indicates that Al atomic disorder becomes larger to compensate the compressive stress of Ga and In. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improvement of crystal quality of AlN and AlGaN epitaxial layers by controlling the strain with the (AlN/GaN) multi-buffer layer

AlN and AlGaN epitaxial layers with Al composition of 0.6–0.8 were grown by low-pressure metal organic vapor phase epitaxy on a (0 0 0 1) 6H–SiC substrate using the (AlN/GaN) multi-buffer layer structure (MBLS). Strain of the grown layer could be controlled by structure of the inserted (AlN/GaN) MBLS. It was found that the crystal quality of the grown layer could be improved with increase of the tensile strain in a-axis (compressive strain in c-axis): full-width at half-maximum (FWHM) of X-ray rocking curves (XRC) of both (0 0 0 2) plane ( ω scan) and (1 0 1¯ 2) plane ( ϕ scan) were decreased from 1530 arcsec to 79 arcsec and 3962 arcsec to 853 arcsec for the AlN template, respectively. FWHM of the ϕ scan XRC was several times larger than that of the ω scan, though the former was roughly proportional to the latter.

Growth and Investigation of n-AlGaN/p-6H-SiC/n-6H-SiC Heterostructures

The investigated AlGaN epitaxial layers were grown by hydride vapor phase epitaxy (HVPE) on a commercial P+ SiC substrate or on an N+ SiC Lely substrate with a p+ SiC layer previously grown by sublimation epitaxy. To investigate the electrical characteristics of the n-p heterojunction, mesa structures of 100, 200 and 1500 microns in diameter were fabricated by reactive ion etching. Investigation of electrical characteristics shows good quality of grown n- AlGaN/p-SiC heterojunctions. This shows applicability of this technological combination for producing n-AlGaN/p-SiC bipolar or FET transistors.

AlGaN epitaxial layers grown by HVPE on sapphire substrates

AbstractGrowth of AlxGa1–xN (x ∼ 0.0‐1.0) alloy layers by hydride vapor pahse epitaxy (HVPE) is reported. Crack‐free undoped AlGaN layers from 0.1 to 2 μm thick were grown on 2″ SiC and 2″and 4″ sapphire substrates. For AlxGa1–xN (x ∼ 0.7‐0.8) layers grown on sapphire, high‐resolution symmetric/asymmetric X‐ray diffraction (XRD) measurements resulted in ω‐scan rocking curve widths ranging from 250 to 650 arcsec and from 1400 to 1900 arcsec, for the (00.2) and (10.2) reflections, respectively. Minimum XRD‐estimated screw and edge dislocation densities in AlGaN layers grown on sapphire were <6 x 108 cm–2 and <2 x 109 cm–2, respectively. Raman studies revealed that the composition dependences of the phonon modes of HVPE‐grown AlxGa1–xN layers are in a good agreement with the one, which have been observed for MOCVD‐grown AlxGa1–xN alloys earlier. Grown layers had n‐type electrical conductivity for the composition range up to x = 0.4. The layer became highly resistive for higher AlN contents. First results on 40 µm thick crack free AlGaN growth on SiC substrates are reported. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of an in situ-deposited [formula omitted] intermediate layer inside GaN and AlGaN layers on SiC substrates

In this work we present a study on the influence of an in situ grown SiN x intermediate layer inside (Al)GaN epitaxial layers grown on SiC substrates on dislocation densities and material strain of the epitaxial films. A defect density of 6 × 10 8 cm - 2 was achieved by reducing the number of pure edge dislocations in the order of one magnitude. It was found that a reduction of the dislocation density is only successful by inserting the SiN x layer inside GaN films and no defect reduction takes place with SiN x inside AlGaN films. The presence of stacking faults at the close vicinity of the SiN x layer plays an important role for the mechanism of dislocation termination at the SiN x interface. To determine dislocation densities an etch pit density method was carried out combining transmission electron microscopy (TEM) investigations of plan-view samples with atomic force microscopy (AFM) scans. Structural properties were investigated by cross-sectional TEM samples. Composition of AlGaN at the SiN x interface was determined by energy dispersive X-ray analysis (EDX) and secondary ion mass spectroscopy (SIMS). Material strain was investigated by micro-photoluminescence measurements ( μ -PL).

Lateral tunneling injection and peripheral dynamic charging in nanometer-scale Schottky gates on AlGaN/GaN hetrosturucture transistors

The gate leakage and gate control characteristics of AlGaN/GaN heterostructure field effect transistors (HFETs) were systematically investigated in an attempt to clarify possible effects of surface states. The experiments were compared to rigorous computer simulations. We observed large amounts of leakage currents in the Schottky diodes fabricated on the AlGaN epitaxial layers. By the calculation based on a thin surface barrier model in which the effects of surface defect donor were taken into account, this large leakage was well explained by enhancement of tunneling transport processes due to the barrier thinning associated with ionization of surface-defect donor. On the other hand, the analysis on the current-voltage characteristics for the nanometer-scale Schottky contacts on AlGaN/GaN HFETs, indicated additional lateral leakage components. The comparison of the gate control characteristics between experiment and calculation clearly showed that the effective lateral expansion of gate length significantly impeded the gm enhancement by the reduction of geometrical gate length. This can be explained by the lateral electron tunneling process at the AlGaN surface stimulated by the pronounced gate leakage currents. Due to frequent tunneling transfer at the gate periphery, surface state occupancy near the gate becomes governed by the metal Fermi level, causing the dynamic surface state charging effects. This resulted in effective widening of the gate length, leading to degradation of gate control performance in AlGaN/GaN HFETs.

Composition measurement in strained AlGaN epitaxial layers using x-ray diffraction

An x-ray diffraction technique is described which, by careful choice of the x-ray reflection used, minimizes errors in composition measurements resulting from strain and uncertainties in the elastic constants of a material. The method is applied to the AlGaN system, which shows a wide range of values for Poisson’s ratio in the literature and significant variation in strain state due to the high dislocation content and large thermal expansion mismatch with the substrate. It is demonstrated that accurate composition measurements of partially relaxed AlxGa1−xN layers (x<0.3) with thickness >20nm can be made from a single measurement.

Growth of thick AlGaN by mixed-source hydride vapor phase epitaxy

Growth of a thick AlGaN layer on GaN/Al 2O 3 substrate is performed by mixed-source hydride vapor phase epitaxy (HVPE) method. The AlGaN material is compounded from chemical reaction between a NH 3 and an aluminum-gallium chloride formed by HCl which is flown over metallic Ga mixed with Al. The AlGaN epitaxial layer is analyzed by Auger-electron spectroscopy (AES) and X-ray diffraction (XRD) measurement to characterize the AlGaN. Al concentration is estimated to be in the range of 0.5–6%. We suggest that the metallic Ga mixed with Al can be used as a group III source material in the HVPE growth of the AlGaN layer.

Growth of AlGaN epitaxial layers and AlGaN/GaN superlattices by metal-organic chemical vapor deposition

Special features of metal-organic chemical vapor deposition of AlGaN epitaxial layers and AlGaN/GaN superlattices either in an Epiquip VP-50 RP research and development reactor (for a single wafer 2 in. in diameter) or in an AIX2000HT production-scale reactor (for up to six wafers 2 in. in diameter) are stud-ied. It is found that the dependence of the aluminum content in the solid phase on the trimethylaluminum (TMA) flux in a reactor levels off; this effect hinders the growth of the layers with a high aluminum content in both types of reactors and is more pronounced in the larger reactor (AIX2000HT). Presumably, this effect is a consequence of spurious reactions in the vapor phase and depends on the partial pressure of TMA in the reactor. The aluminum content in the layers can be increased not only by reducing the total pressure in the reactor but also by increasing the total gas flow through the reactor and reducing the trimethylgallium flux. The approaches described above were used to grow layers with a mole fraction of AlN as large as 20% in the AIX2000HT production-scale reactor at a pressure of 400 mbar (this fraction was as large as 40% at 200 mbar). AlGaN layers with the entire range of composition were grown in the Epiquip VP-50 RP reactor.

Nitride-based Schottky diodes and HFETs fabricated by photo-enhanced chemical wet etching

Photo-enhanced chemical (PEC) wet etching technology was used to etch GaN and AlGaN epitaxial layers. It was found that the maximum etch rates were 510, 1960, 300, and 0 nm/mm for GaN, Al 0.175Ga 0.825N, Al 0.23Ga 0.77N, and Al 0.4Ga 0.6N, respectively. It was also found that we could achieve a high Al 0.175Ga 0.825N to GaN etch rate ratio of 12.6. Nitride-based Schottky diodes and heterostructure field effect transistors (HFETs) were also fabricated by PEC wet etching. It was found that we could achieve a saturated I D larger than 850 mA/mm and a maximum g m about 163 mS/mm from PEC wet etched HFET with a 0.5 μm gate length. Compared with dry etched devices, the leakage currents observed from the PEC wet etched devices were also found to be smaller.

Room-temperature deep-ultraviolet lasing at 241.5 nm of AlGaN multiple-quantum-well laser

Room-temperature deep-ultraviolet lasing of AlxGa1−xN multiple-quantum-well lasers with an Al composition x of 0.66 was achieved at 241.5 nm under pulsed optical pumping. The threshold pumping power was approximately 1200 kW/cm2 at room temperature. The shortest lasing wavelength was 231.8 nm at 20 K. The laser structure was grown on a high-quality AlN layer, which was grown on a 4H-SiC substrate by inserting an AlN/GaN multibuffer-layer structure between the substrate and the AlN layer. Temperature dependence of lasing wavelength was also estimated to be 0.01 and 0.03 nm/K in the temperature region from 20 to 150 K and from 160 K to room temperature, respectively. The laser cavity was made of a cleaved facet of AlGaN epitaxial layers and a SiC substrate. For this purpose, it was necessary to polish the wafer to a thickness of less than 100 μm. The optimal wafer thickness for cleaving in our experiments was 60–70 μm.

ZnO/AlGaN ultraviolet light emitting diodes.

ABSTRACTGa doped n-type ZnO layers are grown using plasma assisted chemical vapor deposition on Mg doped p-type AlGaN epitaxial layers grown by hydride vapor phase epitaxy to form n-ZnO/p-AlGaN heterojunction light emitting diodes. I-V characteristics clearly show rectifying behavior with a threshold voltage of ∼3.2 V and intense ultraviolet electroluminescence with peak emission at 390 nm. The dominant emission mechanism is found to result from hole injection from the p-type AlGaN into the n-type ZnO. Significant emission up to 500 K is observed indicating possible applications in harsh environments.

Photoconductivity and Electroreflectance Study of Cubic GaN/GaAs(001) Heterostructures by Optical-Biasing Technique

Photoconductivity (PC) and electroreflectance (ER) spectrum measurements were carried out for the cubic GaN/GaAs and cubic AlGaN/GaN/GaAs heterostructures grown by LP-MOVPE. Both spectra contained significantly overlapped signals from the c-(Al)GaN epitaxial layer and the underlying GaAs layer, but the latter was successfully suppressed and distinguished from the former by the use of an additional steady-state illumination of Ar+ laser as an optical bias. The temperature dependence of the optically-biased PC spectrum showed that there remained structural disorder in the c-GaN layer indicated by the temperature-independent Urbach energy. From the temperature dependence of the ER spectrum, the E0 critical point of c-GaN observed likely corresponds not to excitonic transition but to free-carrier transition as it shifted by ∼30 meV to higher energies than the reported value, probably due to the dissociation of exciton by the relatively large modulation field applied to the c-(Al)GaN layer.

X-Ray Diffraction Analysis of GaN and AlGaN

ABSTRACTIn this paper, threading dislocation densities in GaN and AlGaN epitaxial layers have been evaluated using two different X-ray analysis techniques; a Williamson Hall (WH) plot and reciprocal space mapping (RSM). GaN and AlGaN have crystalline growth composed of columnar structures that can be estimated by coherence length and angular misorientation measured by X-ray. A WH plot can provide information about coherence length and tilt angle from a linear fit to the linewidth of the triple axis rocking curve (000l) symmetric reflections. RSM is typically used to obtain this data, but it is more involved in technique. The two dominant components of threading dislocation densities (screw and edge types) in the GaN and AlGaN epitaxial layers were found to be similar by both techniques. The treading dislocation density correlates to the size of columnar structure as determined by coherence length, tilt angle, and twist angle. The effect of Al composition in AlGaN alloys on these dislocation densities was investigated and found to depend on strongly on the type of nucleation layer, GaN or AlN.

Structural Defects in Mg-doped GaN and AlGaN grown by MOCVD

ABSTRACTWe have investigated the structural defects formed in Mg-doped GaN and AlGaN epitaxial layers grown by metal organic chemical vapor deposition. These defects have an inverse pyramidal shape and appear when the Mg concentration [Mg] is higher than ∼4×1019/cm3. The density of the defects increases as [Mg] increases, but the size of the defects becomes smaller as [Mg] increases. The density of the defects also has a strong correlation with the hydrogen concentration in the epitaxial layers. Transmission electron microscope analysis reveals that the defects have an inversion operation to the matrix and that their boundaries are Mg-rich. We also propose a model for defect formation.

Distinct Magnesium Incorporation Behavior in Laterally Grown AlGaN

ABSTRACTDifferent magnesium incorporation behavior has been observed in heavily Mg-doped AlGaN epitaxial layers. The films were grown by metal-organic vapor phase epitaxy involving a lateral overgrowth technique on patterned sapphire substrates. TEM observations show that direct growth on sapphire exhibits pyramidal defects, while lateral overgrowth is homogeneous and free of structural defects. The orientation of the growth front significantly influences the microstructure, and the {0001} growth facet appears to be essential for the formation of the pyramidal defects. In addition, cylindrical and funnel-shaped nanopipes have been observed at dislocations with an edge component. The relationship between Mg segregation and these defects is discussed, and formation mechanisms are proposed taking into consideration the orientation of the growth front.

Optical Characterisation of AlGaN Epitaxial Layers and GaN/AlGaN Quantum Wells

We report optical characterisation of AlGaN epitaxial layers and GaN/AIGaN quantum wells, grown by metalorganic vapour phase epitaxy on sapphire substrates. A combination of emission (photoluminescence) and absorption (photoluminescence excitation) spectroscopy provides information on the nature of the electronic states and on built-in electric fields resulting from piezoelectric and spontaneous polarisation effects. These fields are found to be considerably smaller than in previously reported work on similar structures.

Optical Characterisation of AlGaN Epitaxial Layers and GaN/AlGaN Quantum Wells

We report optical characterisation of AlGaN epitaxial layers and GaN/AIGaN quantum wells, grown by metalorganic vapour phase epitaxy on sapphire substrates. A combination of emission (photoluminescence) and absorption (photoluminescence excitation) spectroscopy provides information on the nature of the electronic states and on built-in electric fields resulting from piezoelectric and spontaneous polarisation effects. These fields are found to be considerably smaller than in previously reported work on similar structures.