xN Buffer Layers Research Articles

Effect of AlxGa1−xN buffer layer on the structural and electrical properties of AlGaN/GaN/AlxGa1−xN double heterojunction high electron mobility transistor structures

In this paper, the authors report the effect of the AlxGa1−xN buffer layer on the structural and electrical properties of an AlGaN/GaN/AlxGa1−xN double heterojunction high electron mobility transistor (HEMT). As the Al composition of the buffer layer increased, the two-dimensional electron gas (2DEG) confinement of the channel was shown to improve, which was confirmed by the simulation. The AlGaN buffer HEMT showed improved structural characteristics, such as the surface morphology, crystal quality, and interface roughness compared with the conventional HEMT with a C-doped GaN buffer. A slight decrease in 2DEG characteristics owing to the negative polarization charge was observed. However, in the breakdown voltage characteristics, comparable results were obtained as 652 V for the HEMT with C-doped GaN, 624 V for the HEMT with an Al0.044Ga0.956N buffer, and 642 V for the HEMT with an Al0.088Ga0.912N buffer, although the AlGaN buffers were not doped for semi-insulating.

Selectively Enhanced UV-A Photoresponsivity of a GaN MSM UV Photodetector with a Step-Graded AlxGa1-xN Buffer Layer.

The UV-to-visible rejection ratio is one of the important figure of merits of GaN-based UV photodetectors. For cost-effectiveness and large-scale fabrication of GaN devices, we tried to grow a GaN epitaxial layer on silicon substrate with complicated buffer layers for a stress-release. It is known that the structure of the buffer layers affects the performance of devices fabricated on the GaN epitaxial layers. In this study, we show that the design of a buffer layer structure can make effect on the UV-to-visible rejection ratio of GaN UV photodetectors. The GaN photodetector fabricated on GaN-on-silicon substrate with a step-graded AlxGa−xN buffer layer has a highly-selective photoresponse at 365-nm wavelength. The UV-to-visible rejection ratio of the GaN UV photodetector with the step-graded AlxGa1−xN buffer layer was an order-of-magnitude higher than that of a photodetector with a conventional GaN/AlN multi buffer layer. The maximum photoresponsivity was as high as 5 × 10−2 A/W. This result implies that the design of buffer layer is important for photoresponse characteristics of GaN UV photodetectors as well as the crystal quality of the GaN epitaxial layers.

Overcoming the poor crystal quality and DC characteristics of AlGaN/GaN/AlGaN double‐heterostructure high electron mobility transistors

We report Al0.30Ga0.70N/GaN/Al0.07Ga0.93N double‐heterostructure high electron mobility transistors (DH HEMTs) with admirable DC characteristics fully surpassing those of the Al0.30Ga0.70N/GaN single‐heterostructure (SH) HEMTs. The DH HEMTs feature a 1400‐nm graded AlxGa1–xN (x = 0–0.06) buffer layer, a 300‐nm Al0.07Ga0.93N back barrier layer, and a 70‐nm thick GaN channel layer. Due to the improved cystal quality and enhanced confinement of the carriers, the DH HEMTs presented have shown improved performance with respect to the conventional SH HEMTs, including electron mobility promoted from 1701 to 1744 cm2 V−1 s−1, surface roughness in terms of root mean square values (RMS) reduced from 0.19 to 0.16 nm, (10–12) full widths at half‐maximum (FWHMs) reduced from 726 to 540 arcsec, subthreshold swing (SS) reduced from 113 to 78 mV dec−1, Ion/Ioff ratio increased from 105.3 to 106.2, drain‐induced barrier lowering (DIBL) reduced from 24 and 14 mV V−1, and breakdown voltage promoted from 59 to 109 V.

Off-state electrical breakdown of AlGaN/GaN/Ga(Al)N HEMT heterostructure grown on Si(111)

Electrical breakdown characteristics of AlxGa1−xN buffer layers grown on Si(111) are investigated by varying the carbon concentration ([C]: from ∼1016 to 1019 cm−3), Al-composition (x = 0 and 7%), and buffer thickness (from 1.6 to 3.1 μm). A quantitative relationship between the growth conditions and carbon concentration ([C]) is established, which can guide to grow the Ga(Al)N layer with a given [C]. It is found that the carbon incorporation is sensitive to the growth temperature (T) (exponential relationship between [C] and 1/T) and the improvement of breakdown voltage by increasing [C] is observed to be limited when [C] exceeding 1019 cm−3, which is likely due to carbon self-compensation. By increasing the highly resistive (HR) Al0.07Ga0.93N buffer thickness from 1.6 to 3.1 μm, the leakage current is greatly reduced down to 1 μA/mm at a bias voltage of 1000 V.

Efficiency droop behavior improvement through barrier thickness modification for GaN-on-silicon light-emitting diodes

Crack-free GaN-based light-emitting diodes (LEDs) were grown on 150-mm-diameter Si substrates by using low-pressure metal-organic chemical vapor deposition. The relationship between the LED devices and the thickness of quantum barriers (QBs) was investigated. The crystal quality and surface cracking of GaN-on-Si were greatly improved by an AlxGa1−xN buffer layer composed of graded Al. The threading dislocation density of the GaN-on-Si LEDs was reduced to <7×108 cm−2, yielding LEDs with high internal quantum efficiency. Simulation results indicated that reducing the QB thickness improved the carrier injection rate and distribution, thereby improving the droop behavior of the LEDs. LEDs featuring 6-nm-thick QBs exhibited the lowest droop behavior. However, the experimental results showed an unanticipated phenomenon, namely that the peak external quantum efficiency (EQE) and light output power (LOP) gradually decreased with a decreasing QB thickness. In other words, the GaN-on-Si LEDs with 8-nm-thick QBs exhibited low droop behavior and yielded a good peak EQE and LOP, achieving a 22.9% efficiency droop and 54.6% EQE.

Improved performance of long-wavelength light-emitting diodes grown with wide quantum wells on stress-relaxed InxGa1−xN buffer layers

Long-wavelength (>540 nm) single-quantum-well light-emitting diodes were grown pseudomorphically on stress-relaxed InxGa1−xN buffer layers on the orientation. Basal plane dislocation glide led to the formation of misfit dislocations at the bottom of the InxGa1−xN buffer layer. This provided one-dimensional plastic relaxation in the film interior, including the active region. For wider quantum wells (>3.5 nm), the efficiency of devices grown on relaxed buffer layers showed significant improvement compared to reference devices grown with no relaxation, indicating that degradation of thicker active regions can be prevented with partial relaxation via basal plane slip.

Compositionally graded relaxed AlGaN buffers on semipolar GaN for mid-ultraviolet emission

In this Letter, we report on the growth and properties of relaxed, compositionally graded AlxGa1 − xN buffer layers on freestanding semipolar (202¯1) GaN substrates. Continuous and step compositional grades with Al concentrations up to x = 0.61 have been achieved, with emission wavelengths in the mid-ultraviolet region as low as 265 nm. Coherency stresses were relaxed progressively throughout the grades by misfit dislocation generation via primary (basal) slip and secondary (non-basal) slip systems. Threading dislocation densities in the final layers of the grades were less than 106/cm2 as confirmed by plan-view transmission electron microscopy and cathodoluminescence studies.

Transmission-mode GaN photocathode based on graded AlxGa1 xN buffer layer

Transmission-mode GaN photocathode based on graded AlxGa1 xN buffer layer

Effects of AlxGa1−xN interlayer for GaN epilayer grown on Si substrate by metal-organic chemical-vapor deposition

GaN film grown on Si substrate using multilayer AlN/AlxGa1−xN buffer is studied by the low-pressure metal-organic chemical-vapor deposition method. The AlxGa1−xN films with Al composition varying from 1 to 0.66 were used to accommodate the stress induced between GaN and the Si substrate during GaN growth. The correlation of the Al composition in the AlxGa1−xN films with respect to the stress induced in the GaN film grown was studied using high-resolution x-ray diffraction, including symmetrical and asymmetrical ω/2θ scans and reciprocal space maps. It is found that with proper design of the Al composition in the AlxGa1−xN buffer layer, crack-free GaN film can be successfully grown on 6 in. Si (111) substrates using multilayer AlN and AlxGa1−xN buffer layers.

Effect of Graded AlxGa1-xN Interlayer Buffer on the Strain of GaN Grown on Si (111) Using MOCVD Method

ABSTRACTGaN film grown on Si substrate with AlN/AlxGa1−xN buffer is studied by low pressure metal organic chemical vapor deposition (MOCVD) method. The AlxGa1−xN film with Al composition varying from 0∼ 0.66 was used. The correlation of the Al composition in the AlxGa1−xN film with the stress of the GaN film grown was studied using high resolution X-ray diffraction including symmetrical and asymmetrical ω/2θscans and reciprocal space maps. It is found that with proper design of the Al composition in the AlxGa1−xN buffer layer, crack-free GaN films can be successfully grown on Si (111) substrates using AlN and AlxGa1−xN buffer layers.

Low-resistance graded Al xGa 1−xN buffer layers for vertical conducting devices on n-SiC substrates

We investigated the Al composition and thickness dependence of the resistance of graded Al x Ga 1− x N buffer layers for vertical conducting devices on n-type 6H-SiC substrates. To measure current–voltage characteristics, a Ti/Au ohmic contact was formed on the backside of the SiC substrate. The Al composition of the buffer layer was varied from 2% to 10%. We found that the optimal buffer layer thickness for the lowest resistance decreases with increasing Al composition because the nucleation seeds on SiC substrate coalesce faster. Since the optimal layer thickness depends on the Al composition, the sheet Al concentration, which is calculated from the Al composition and the thickness, should be considered as a critical parameter for low buffer resistance of vertical conducting devices on n-SiC substrates.

Cracking of III-nitride layers with strain gradients

Experimental results are demonstrated for the cracking of nominally compressed AlyGa1−yN layers grown on AlxGa1−xN buffer layers with smaller lattice constants (y&amp;lt;x). The authors present a theoretical analysis showing that the inclination of pure edge threading dislocation lines effectively reduces the compressive stress, causing relaxation, and after reaching a certain thickness begins to generate the tensile stress gradient responsible for cracking. The critical layer thickness for crack nucleation in such a gradient elastic field has been found. The results of the modeling are in good agreement with experimental observations of crack onset in nominally compressed nitride layers.

Deposition of AlGaN films on (111) Si substrates and optimization of GaN growth on Si using intermediate-temperature AlGaN buffer layers

AlxGa1−xN films having various Al-contents were grown on (111) Si substrates over a temperature range of 800∼1000 °C. It was found that crack free AlxGa1−xN films were achieved when the films were grown at 800 °C. High temperature (HT) GaN films were also deposited on (111) Si substrates using 800 °C grown AlxGa1−xN buffer layers with different thickness and composition combinations. The best HT GaN film was achieved on (111) Si substrate by process optimization with an 800 °C grown 180 nm-thick Al0.58Ga0.42N buffer layer. Room temperature photoluminescence (PL) spectrum of the HT GaN film shows a strong near band edge emission having a linewidth of 100 meV and a quenched yellow luminescence. It is believed that the use of intermediate temperature AlxGa1−xN buffer layer is beneficial to accommodate the misfit strain between HT GaN film and (111) Si substrate.

Properties of LT-AlGaN films and HT-GaN films using LT-AlGaN buffer layers grown on (0001) sapphire substrates

Low-temperature (LT) Al x Ga1−xN (0.1 < x < 0.8) films, 0.4 μm in thickness, were prepared on (0001) sapphire substrates at 500,∘C by alternate supply of Ga and Al alkyls and ammonia (NH3). Al composition in the solid phase was identified based on the shift of the (0002) Bragg angle of X-ray diffraction. A series of high temperature (HT) GaN films, 1.0 μm in thickness, were also grown at 1000,∘C on the LT-Al x Ga1−xN coated (0001) sapphire substrates with buffer layer thickness ranging from 7.5 to 20 nm. It was found that the optimized LT-Al x Ga1−xN buffer layer thickness decreases linearly with the Al-content. As-grown HT-GaN films having LT-LT-Al0.43Ga0.57N buffer layers show smooth surface based on optical microscopic (OM) observations. Transmission electron microscopy (TEM) confirms the mono-crystalline nature of the HT-GaN films. The quenched near band-edge photoluminescence (PL) emissions and an apparent yellow luminescence of the HT-GaN films are attributed to the LT-Al x Ga1−xN buffer layer induced mosaic microstructure and bonding defects in the films.

Properties of LT-AlGaN films and HT-GaN films using LT-AlGaN buffer layers grown on (0001) sapphire substrates

Low-temperature (LT) Al x Ga1−xN (0.1 < x < 0.8) films, 0.4 μm in thickness, were prepared on (0001) sapphire substrates at 500,∘C by alternate supply of Ga and Al alkyls and ammonia (NH3). Al composition in the solid phase was identified based on the shift of the (0002) Bragg angle of X-ray diffraction. A series of high temperature (HT) GaN films, 1.0 μm in thickness, were also grown at 1000,∘C on the LT-Al x Ga1−xN coated (0001) sapphire substrates with buffer layer thickness ranging from 7.5 to 20 nm. It was found that the optimized LT-Al x Ga1−xN buffer layer thickness decreases linearly with the Al-content. As-grown HT-GaN films having LT-LT-Al0.43Ga0.57N buffer layers show smooth surface based on optical microscopic (OM) observations. Transmission electron microscopy (TEM) confirms the mono-crystalline nature of the HT-GaN films. The quenched near band-edge photoluminescence (PL) emissions and an apparent yellow luminescence of the HT-GaN films are attributed to the LT-Al x Ga1−xN buffer layer induced mosaic microstructure and bonding defects in the films.

Growth of optically‐active InN with AlInN buffer by plasma‐assisted molecular beam epitaxy

InN films have been grown by plasma-assisted MBE on Al2O3(0001) substrates using different buffer fabrication techniques: thermo-activated In diffusion into nitridated sapphire during InN buffer annealing and deposition of 20 nm AlxIn1−xN buffer layers with x = 0.1–0.5. Different In/N flux ratio conditions are employed. The InN epilayers demonstrate IR photoluminescence in the 0.6–1.0 μm range, with an ambiguous dependence of the PL intensity on structural quality of the films. The highest quality 0.5 μm thick InN layer having (0002) XRD Θ-rocking curve width below 400 arcsec and smooth continuous interface with the substrate does not show any PL. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of GaN epitaxial layers on SiC substrates with Al xGa 1− xN buffer layers

High quality GaN epitaxial layers were obtained with Al x Ga 1− x N buffer layers on 6H–SiC substrates. The low-pressure metalorganic chemical vapor deposition (LP-MOCVD) method was used. The 500 Å thick buffer layers of Al x Ga 1− x N (0≤ x≤1) were deposited on SiC substrates at 1025°C. The FWHM of GaN (0004) X-ray curves are 2–3 arcmin, which vary with the Al content in Al x Ga 1− x N buffer layers. An optimum Al content is found to be 0.18. The best GaN epitaxial film has the mobility and carrier concentration about 564 cm 2 V −1 s −1 and 1.6×10 17 cm −3 at 300 K. The splitting diffraction angle between GaN and Al x Ga 1− x N were also analyzed from X-ray diffraction curves.

Effect of buffer layer and substrate surface polarity on the growth by molecular beam epitaxy of GaN on ZnO

We present results on the effect of substrate surface polarity, oxygen and zinc faces, on the quality of GaN epitaxial layers grown on ZnO(0001) substrates by reactive ammonia molecular beam epitaxy. The possible effects dealing with the disparity in surface preparation of the two faces have been eliminated. Photoluminescence and reflectivity measurements demonstrate that the oxygen face leads to higher quality GaN on ZnO compared to the zinc face. We also present optical data obtained by using different low-temperature AlN, GaN, and InxGa1−xN buffer layers. The best result has been obtained with lattice-matched In0.20Ga0.80N buffer layer.