AlGaN Buffer Research Articles

Mixed Phases at the Bottom Interface of Si‐Doped AlGaN Epilayers of Optoelectronic Devices

This paper presents an analysis of crystalline structures of Si‐doped Al0.4Ga0.6N layers grown on not‐intentionally doped AlGaN buffer layer with an AlN nucleation layer by metal organic chemical vapor deposition. Weak cubic Al0.4Ga0.6N (002) and (103) reflection peaks are observed in high‐resolution XRD θ/2θ scans and cubic Al0.4Ga0.6N (LO) mode in Raman scattering spectroscopy. These cubic subgrains are localized at the bottom interface of Si‐doped layer due to the pulsed lower growth temperature and rich hydrogen atmosphere at the start of silane injection. Their appearance has no direct relationship with the buffer and nucleation layer. This study is helpful not only to understand fundamental properties of high aluminum content Si‐doped AlGaN alloys but also to provide specific guidance on the fabrication of multilayer optoelectronic devices where weak cubic subgrains potentially occur and exert complicated influences on the device performance.

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AlGaN/GaN field effect transistors for power electronics—Effect of finite GaN layer thickness on thermal characteristics

AlGaN/GaN heterostructure field effect transistors with a 150 nm thick GaN channel within stacked AlxGa1−xN layers were investigated using Raman thermography. By fitting a thermal simulation to the measured temperatures, the thermal conductivity of the GaN channel was determined to be 60 W m−1 K−1, over 50% less than typical GaN epilayers, causing an increased peak channel temperature. This agrees with a nanoscale model. A low thermal conductivity AlGaN buffer means the GaN spreads heat; its properties are important for device thermal characteristics. When designing power devices with thin GaN layers, as well as electrical considerations, the reduced channel thermal conductivity must be considered.

Comparison of the microstructural characterizations of GaN layers grown on Si (111) and on sapphire

Due to the large differences in the lattice constants and the thermal expansion coefficients between GaN and Si, GaN growth on a Si substrate usually leads initially to high defect densities and cracks. If high-quality GaN films on Si substrate are to be obtained, it is essential to understand the different growth characteristics of GaN layers grown on Si and on sapphire. In this study, the GaN specimens were grown on sapphire and Si (111) substrates with AlGaN and AlN buffer layers, respectively, by metalorganic chemical vapor deposition (MOCVD). Using transmission electron microscopy (TEM) and micro-Raman spectroscope, we carried out a comparative investigation of GaN growth by characterizing lattice coherency, defect density, and residual strain. These analyses revealed that the GaN layers grown on Si have much residual tensile strain and that strain has an effect on the formation of InGaN/GaN multiple quantum wells (MQWs) above the GaN layers.

Measuring the composition of AlGaN layers in GaN based structures grown on 150 mm Si substrates using (2 0 5) reciprocal space maps

A critical element for the successful growth of GaN device layers on Si is accurate control of the AlGaN buffer layers used to manage strain. Here we present a method for measuring the composition of the AlGaN buffer layers in device structures which makes use of a one-dimensional x-ray detector to provide efficient measurement of a reciprocal space map which covers the full compositional range from AlN to GaN. Combining this with a suitable x-ray reflection with low strain sensitivity it is possible to accurately determine the Al fraction of the buffer layers independent of their relaxation state.

Synergistic effect of reactor chemistry and compressive stress on dislocation bending during GaN growth

The synergistic effect of compressive growth stresses and reactor chemistry, silane presence, on dislocation bending at the very early stages of GaN growth has been studied using in-situ stress measurements and cross-sectional transmission electron microscopy. A single 100 nm Si-doped GaN layer is found to be more effective than a 1 μm linearly graded AlGaN buffer layer in reducing dislocation density and preventing the subsequent layer from transitioning to a tensile stress. 1 μm crack-free GaN layers with a dislocation density of 7 × 108/cm2, with 0.13 nm surface roughness and no enhancement in n-type background are demonstrated over 2 inch substrates using this simple transition scheme.

Growth and characterization of AlGaN/AlN/GaN/AlGaN double heterojunction structures with AlGaN as buffer layers

High electron mobility transistors (HEMTs) structures with GaN, Al0.025Ga0.975N and Al0.04Ga0.96N high resistivity (HR) buffers were grown on sapphire by metal organic chemical vapor deposition (MOCVD). The structural and electrical properties of these three samples were investigated and compared. By increasing Al composition of AlGaN buffer, full width at half maximum (FWHM) values of (0002) and (10-12) x-ray rocking curves for buffer increase, indicating higher threading dislocation density. Room temperature noncontact Hall measurements were performed, and the measured 2DEG mobility was 1828cm2/Vs for GaN buffer, 1728cm2/Vs for Al0.025Ga0.975N buffer, and 1649cm2/Vs for Al0.04Ga0.96N buffer, respectively. Combining the theoretical calculation with the experiments, it was demonstrated that the decrease of mobility was attributed to higher dislocation density in sample with higher Al composition of AlGaN buffer. Devices were fabricated and it was found that the double heterojunction (DH) HEMT with Al0.025Ga0.975N buffer could effectively reduce the buffer leakage current.

Effect of compositionally graded AlGaN buffer layer grown by different functions of trimethylaluminum flow rates on the properties of GaN on Si (111) substrates

In the present paper, compositionally graded AlGaN buffer layer was employed to compensate the tensile stress and improve the crystalline quality of GaN grown on silicon substrate. During the growth of the graded buffer layer, the flow rate of trimethylaluminum (TMAl) was gradually decreased according to a series of functions, while the flow rate of trimethylgalliumm (TMGa) was increased with the same linear function. Therefore, these graded AlGaN layers showed different distributions of Al, which was confirmed by the high-resolution X-ray diffraction (HR-XRD) and the secondary ion mass spectroscopy (SIMS). A linear compositionally graded AlGaN layer was obtained by using the convex parabolic TMAl flow rate, which leads to a large compressive stress in GaN layer due to a linearly gradual change of lattice parameters and thermal expansion coefficient (TEC) from AlN seeding layer to GaN layer. Using the linear compositionally graded buffer layer, a 3-μm-thick crack-free GaN film with a low dislocation density was grown on silicon substrate. The dislocation evolution associated with stress was investigated by the cross-sectional micro-Raman scattering and transmission electron microscopy (TEM).

Physics of GaN-based Power Field Effect Transistors

Unique properties of GaN/AlN/InN heterostructures make them superior for high power applications. The key issues in the device designs are achieving normally-off operation, controlling the electric field distribution in the device channel to prevent breakdown, eliminating or reducing non-ideal effects causing reliability issues, and providing for efficient heat dissipation. High electric fields at the gate edges lead to an additional strain and hot electron effects causing the current collapse and gate lag. Quantum well designs (e.g. incorporating an InGaN or GaN quantum well between the wide band gap AlGaN barrier layer and GaN or AlGaN buffer) might control the wave function penetration and the real space transfer and increase the breakdown voltage. Insulated gate heterostructure HFETs (MOSHFETs) demonstrated superior performance and reliability. Field plates, recessed and double recessed gates, drain field controlled electrodes have been used to control current collapse and improve device reliability. An emerging approach is Frequency Configurable Electronics that enables better electric field uniformity.

The Influence of Graded AlGaN Buffer Thickness for Crack-Free GaN on Si(111) Substrates by using MOCVD

GaN films with different thicknesses of Al composition graded AlGaN buffer are grown on substrates of Si(111) by metal-organic chemical vapor deposition (MOCVD). The thicknesses of graded AlGaN buffer are fixed at 200 nm, 300 nm, and 450 nm, respectively. Optical microscopy, atomic force microscopy, x-ray diffraction, and Raman spectroscopy are employed to characterize these samples. We find that the thickness of the graded AlGaN buffer layer plays a key role on the following growth of GaN films. The optimized thickness of the graded AlGaN buffer layer is 300 nm. Under such conditions, the GaN epitaxial film is crack-free, and its dislocation density is the lowest.

Effects of AlN interlayer on growth of GaN-based LED on patterned silicon substrate

GaN-based light emitting diodes (LEDs) were grown on 1 mm × 1 mm patterned 2-inch and 6-inch Si (111) substrates by metal–organic vapour phase epitaxy (MOVPE). AlN interlayers with different thicknesses were introduced between the composition-graded AlGaN buffer layer and the GaN seed layer in different LED structures. The crystalline quality, wavelength uniformity and crack density of the 2-inch wafer were improved by increasing the AlN interlayer thickness. With a 30 nm AlN interlayer, a crack-free, smooth and reflective 6-inch LED wafer was grown, the full width at half maximum (FWHM) of the XRD rocking curves of GaN (002) and GaN (102) planes were 384 and 432 arcsec, respectively. The standard deviation of thickness and dominant wavelength were 0.03 μm (average thickness was 3.84 μm) and 1.52 nm (average dominant wavelength was 457.9 nm), respectively. The AlN interlayer can change the growth mode of the GaN seed layer and subsequent n-GaN, which changes the density of dislocation and the residual tensile stress of the GaN film. A smaller residual tensile stress in the GaN film can help to reduce bowing of the wafer, improve the wavelength uniformity, and suppress the generation of cracks.

High-growth-rate AlGaN buffer layers and atmospheric-pressure growth of low-carbon GaN for AlGaN/GaN HEMT on the 6-in.-diameter Si substrate metal-organic vapor phase epitaxy system

In this study, we investigated the effects of growth pressure on incorporation of carbon in GaN and on the growth rate of AlGaN using a multiwafer (7×6in.) mass-production metal-organic vapor phase epitaxy (MOVPE) reactor. In the two-dimensional electron gas (2DEG) region of an AlGaN/GaN high electron-mobility transistor (HEMT) structure, a high GaN purity is required. The incorporation of carbon in GaN could be easily controlled over a carbon concentration range of three orders of magnitude by varying pressure. Thus, the reactor can be used for growth at both reduced pressure and atmospheric pressure. Furthermore, an AlGaN growth rate of over 1μm/h was demonstrated for Al composition in the range of 0.3–0.8 by suppressing the gas-phase prereaction between the precursor materials. An AlGaN/GaN HEMT structure was also demonstrated. The magnitude of wafer bowing was less than 50μm. The full widths at half maximum (FWHMs) of the X-ray rocking curve (XRC) of GaN were 570″ in the GaN (002) direction and 760″ in the GaN (102) direction. The sheet carrier density and Hall mobility were 1.056×1013cm−2 and 1550cm2/Vs, respectively.

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.

Transport characteristics of AlGaN/GaN/AlGaN double heterostructures with high electron mobility

The AlGaN/GaN/AlGaN double heterostructure (DH) with high electron mobility of 1862 cm2/Vs at room temperature and 478 cm2/Vs at 573 K high temperature was obtained by a combination of optimization schemes considering scattering mechanisms. First, a composite buffer layer structure, including GaN and AlGaN layer, was used to improve the crystal quality of the AlGaN/GaN/AlGaN DH. Second, interface roughness scattering was reduced by increasing the channel thickness, thus the two-dimensional electron gas mobility was further improved. Moreover, an ultrathin AlN interlayer was inserted between the GaN channel layer and the AlGaN buffer layer to decrease the alloy disorder scattering. The Hall effect measurements showed that the DH had better transport characteristics at high temperatures, and an electron mobility of 478 cm2/Vs was achieved at 573 K, which is twice larger than that of the conventional single heterostructure (∼200 cm2/Vs at 573 K). Therefore, AlGaN/GaN/AlGaN DH is more suitable for the applications in high temperature electronic devices.

Investigation of AlInN HEMT structures with different AlGaN buffer layers grown on sapphire substrates by MOCVD

We investigate the structural and electrical properties of AlxIn1–xN/AlN/GaN heterostructures with AlGaN buffers grown by MOCVD, which can be used as an alternative to AlInN HEMT structures with GaN buffer. The effects of the GaN channel thickness and the addition of a content graded AlGaN layer to the structural and electrical characteristics were studied through variable temperature Hall effect measurements, high resolution XRD, and AFM measurements. Enhancement in electron mobility was observed in two of the suggested AlxIn1−xN/AlN/GaN/Al0.04Ga0.96N heterostructures when compared to the standard AlxIn1–xN/AlN/GaN heterostructure. This improvement was attributed to better electron confinement in the channel due to electric field arising from piezoelectric polarization charge at the Al0.04Ga0.96N/GaN heterointerface and by the conduction band discontinuity formed at the same interface. If the growth conditions and design parameters of the AlxIn1−xN HEMT structures with AlGaN buffers can be modified further, the electron spillover from the GaN channel can be significantly limited and even higher electron mobilities, which result in lower two-dimensional sheet resistances, would be possible.

Breakdown voltage analysis of Al0.25Ga0.75N/GaN high electron mobility transistors with partial silicon doping in the AlGaN layer

In this paper, two-dimensional electron gas (2DEG) regions in AlGaN/GaN high electron mobility transistors (HEMTs) are realized by doping partial silicon into the AlGaN layer for the first time. A new electric field peak is introduced along the interface between the AlGaN and GaN buffer by the electric field modulation effect due to partial silicon positive charge. The high electric field near the gate for the complete silicon doping structure is effectively decreased, which makes the surface electric field uniform. The high electric field peak near the drain results from the potential difference between the surface and the depletion regions. Simulated breakdown curves that are the same as the test results are obtained for the first time by introducing an acceptor-like trap into the N-type GaN buffer. The proposed structure with partial silicon doping is better than the structure with complete silicon doping and conventional structures with the electric field plate near the drain. The breakdown voltage is improved from 296 V for the conventional structure to 400 V for the proposed one resulting from the uniform surface electric field.

384 nm laser diode grown on a (202¯1) semipolar relaxed AlGaN buffer layer

We demonstrate an electrically injected semipolar (202¯1) laser diode grown on a partially relaxed AlGaN buffer layer. The coherency stresses are relaxed by misfit dislocations at the GaN/AlGaN heterointerface which form by glide of preexisting threading dislocations along the (0001) basal plane. The defects are confined to the heterointerface which allows the growth of high aluminum composition films with threading dislocation densities of less than 108 cm−2. The lasing wavelength was 384 nm with a threshold current density of 15.7 kA/cm−2. UV lasers grown on semipolar relaxed AlGaN buffers provide an alternative to devices grown on AlN or sapphire.

TiAl Ohmic contact on GaN, in situ high or low doped or Si implanted, epitaxially grown on sapphire or silicon

AbstractIn this work, the Ti/Al Ohmic contact quality on n‐type gallium nitride (GaN) films has been studied as a function of different process parameters such as surface cleaning procedure, etching, thickness of the deposited layers or annealing conditions. GaN epilayers, with uniform doping concentration from 1 × 1016 to 5.8 × 1018 at./cm3 were grown on sapphire or silicon substrates using AlN and/or AlGaN buffer layers. Electrical characterizations were made using circular transfer length method (cTLM) patterns with a four‐probe equipment. Specific contact resistance (SCR) was then extracted from current–voltage (I–V) characteristics, for all the process conditions. Contact structures depending on experiment parameters were studied by means of (scanning) transmission electronic microscopy (STEM‐TEM). Our results reveal that process parameters such as surface treatment have a lower impact than annealing temperature or metal thickness and annealing duration. Finally, SCR values of 1 × 10−6 Ω cm2 can be reproducibly achieved. Moreover, good Ohmic contacts have been obtained on etched surfaces or on low‐doped layers implanted with Si. This low value demonstrates a good Ohmic contact and this large parameter process window is of high interest for future device fabrication based on GaN (planar or mesa structures).

Electromodulation spectroscopy of optical transitions and electric field distribution in GaN/AlGaN/GaN transistor heterostructures with various AlGaN layer thicknesses

AbstractGaN/AlGaN/GaN field effect transistor (FET) heterostructures with various thicknesses of AlGaN layer (10, 20, and 30nm) were investigated by electromodulation spectroscopy [i.e. contactless electroreflectance (CER) and photoreflectance (PR)]. In PR spectra optical transitions in GaN cap, AlGaN and GaN buffer layers were clearly observed whereas in CER spectra the optical transition in GaN buffer layer was not observed due to the screening of band bending modulation in CER spectroscopy by the two dimensional electron gas at the AlGaN/GaN interface. The transition related to GaN‐cap was observed ∼0.1 eV above the energy gap of GaN due to the quantum confinement in the surface GaN quantum well. The built‐in electric field in AlGaN layer was determined from the analysis of AlGaN‐related Franz‐Keldysh oscillation. For the sample with 10, 20, and 30nm thick AlGaN layer the electric field has been determined from CER(PR) measurements to be 0.30(0.27), 0.46(0.39) and 0.78(0.63) MV/cm, respectively. The smaller electric field, which is obtained from PR measurements, results from the photovoltaic effect which is eliminated in CER spectroscopy. The electric field distribution in the full heterostructure (GaN(cap) layer, AlGaN layer, and near the AlGaN/GaN interface in GaN(buffer) layer) has been found comparing theoretical calculations for the full heterostructure with the electric field for AlGaN layer obtained from CER measurements. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of In incorporation into the quantum well active region on the efficiency of AlGaN‐based ultraviolet light‐emitting diodes

AbstractImproving the internal quantum efficiency is still a major challenge for realizing efficient III‐nitride‐based light‐emitting diodes (LEDs) with emission wavelengths below 365 nm. Here, we report on the influence of a very low In amount of less than 1% on the luminescence properties of single Ga(In)N/AlGaN quantum well (QW) structures and the performance of AlGaN‐based LEDs with an emission wavelength of about 350 nm. The samples were grown by metalorganic vapor phase epitaxy on low defect density AlGaN buffer layers on (0001) sapphire substrates. The temperature dependence of the QW emission energy reveals a clear “S‐shape” behaviour indicating a significant carrier localization for In containing QWs. The electroluminescence efficiency of LEDs with a GaInN QW active region could be improved by a factor of 4 compared to LEDs with GaN QW, resulting in a continuous wave (cw) output power of 8.2 mW at 40 mA. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)