AlGaN Buffer Research Articles

Origins of hillock defects on GaN templates grown on Si(111)

The origin of surface hillocks (also known as pancake defects) on GaN-on-Si wafers grown by MOVPE has been investigated. FIB/TEM observations confirmed that the appearance of the hillocks is due to the formation of Ga-rich precipitates within the AlGaN buffer layer. XRD (002) FWHM measurements also show that the surface hillocks are associated with a high degree of crystal tilt in the AlN nucleation layer. Two factors are considered to be the cause of such a phase separation: (1) a high density of surface steps associated with the regions of large crystal tilt which act as nucleation centers and (2) a lower mobility of Al adatoms at the growth surface compared with Ga, leading to a preferential incorporation of Ga in the precipitates. The impact of these precipitates on the wafer bow of the structures is considered.

Structural and optical investigations of AlGaN MQWs grown on a relaxed AlGaN buffer on AlN templates for emission at 280 nm

10-period Al0.57Ga0.43N/Al0.38Ga0.62N multi-quantum wells (MQWs) were grown on a relaxed Al0.58Ga0.42N buffer on AlN templates on sapphire. The threading dislocations and V-pits were characterized and their origin is discussed. The influence of V-pits on the structural quality of the MQWs and on optical emission at 280nm was analyzed. It was observed that near-surface V-pits were always associated with grain boundaries consisting of edge threading dislocations originating from the AlN/Al2O3 interface. Although the high density of V-pits disrupted MQWs growth, it did not affect the internal quantum efficiency which was measured to be ~1% at room temperature even when V-pit density was increased from 7×107cm−2 to 2×109cm−2. The results help to understand the origin, propagation and influences of the typical defects in AlGaN MQWs grown on AlN/Al2O3 templates which may lead to further improvement of the performance of DUV devices.

Structural and optical investigations of AlGaN MQWs grown on a relaxed AlGaN buffer on AlN templates for emission at 280 nm

Structural and optical investigations of AlGaN MQWs grown on a relaxed AlGaN buffer on AlN templates for emission at 280 nm

In-situ wafer bowing measurements of GaN grown on Si (111) substrate by reflectivity mapping in metal organic chemical vapor deposition system**Project supported by the National Natural Science Foundation of China (Grant Nos. 61274039 and 51177175), the National Basic Research Program of China (Grant No. 2011CB301903), the Ph.D. Programs Foundation of Ministry of Education of China (Grant No. 20110171110021), the International

In this work, the wafer bowing during growth can be in-situ measured by a reflectivity mapping method in the 3×2″ Thomas Swan close coupled showerhead metal organic chemical vapor deposition (MOCVD) system. The reflectivity mapping method is usually used to measure the film thickness and growth rate. The wafer bowing caused by stresses (tensile and compressive) during the epitaxial growth leads to a temperature variation at different positions on the wafer, and the lower growth temperature leads to a faster growth rate and vice versa. Therefore, the wafer bowing can be measured by analyzing the discrepancy of growth rates at different positions on the wafer. Furthermore, the wafer bowings were confirmed by the ex-situ wafer bowing measurement. High-resistivity and low-resistivity Si substrates were used for epitaxial growth. In comparison with low-resistivity Si substrate, GaN grown on high-resistivity substrate shows a larger wafer bowing caused by the highly compressive stress introduced by compositionally graded AlGaN buffer layer. This transition of wafer bowing can be clearly in-situ measured by using the reflectivity mapping method.

Response to “Comment on ‘Determination of alloy composition and strain in multiple AlGaN buffer layers in GaN/Si system’” [Appl. Phys. Lett. 106, 176101 (2015)

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Ultraviolet light emitting diodes by ammonia molecular beam epitaxy on metamorphic ([formula omitted]) AlGaN/GaN buffer layers

In this paper we demonstrate ultraviolet (UV) light emitting diodes (LEDs) grown on metamorphic AlGaN buffers on freestanding GaN (202¯1) substrates by ammonia assisted molecular beam epitaxy (MBE). Misfit and related threading dislocations were confined to the stress relaxed, compositionally graded buffer layers, and single quantum well devices emitting at 355, 310 and 274nm were grown on top of the graded buffers. The devices showed excellent structural and electrical (I–V) characteristics.

AlGaN-based MQWs grown on a thick relaxed AlGaN buffer on AlN templates emitting at 285 nm

We report on the growth of Al0.57Ga0.43N/Al0.38Ga0.63N MQWs grown on a relaxed Al0.58Ga0.42N buffer on AlN template by Metal Organic Vapor Phase Epitaxy. The MQW structure is designed so that the strain in the quantum wells induced by their lattice mismatch with barriers is sufficient to enhance TE polarized emission (E-field ⊥ c). A 630-nm thick relaxed Al0.58Ga0.42N buffer grown on AlN template serves as a pseudo-substrate to release the strain in the barriers and to avoid related defects or composition fluctuation in the active region. Thin (< 2 nm) quantum wells allow preservation of the overlapping of electron and hole wavefunctions considering the strong quantum-confined Stark effect in AlGaN-based MQW structures. Scanning transmission electron microscopy (STEM) coupled to energy-dispersive X-ray spectroscopy (EDX) analysis is used to optimize the growth conditions and to determine the composition of wells and barriers. Optical characterizations of the grown structure reveal a well-defined band-edge emission peak at 285 nm. Based on macro-transmission measurements and simulations, the absorption coefficient of the wells is estimated to be 3 × 105 cm−1 (E-field ⊥ c), attesting that the oscillator strength is preserved for these AlGaN MQWs with high Al content, which is promising for efficient surface-emitting devices in the deep ultra-violet (DUV) region.

Improvement of breakdown and current collapse characteristics of GaN HEMT with a polarization-graded AlGaN buffer

A GaN HEMT with a polarization-graded AlGaN buffer is performed by two-dimensional analysis of drift-diffusion simulations. The bulk trap-induced current collapse of the proposed structure is effectively restrained in contrast to that of conventional HEMTs with a GaN or AlGaN buffer, resulting from the fact that the high and flat back-barrier altitude in the proposed structure prevents the two-dimensional electron gas (2DEG) from spilling over from the channel, with the reduction of hot carriers injecting into the buffer followed by trapping in deep acceptor-like levels. Simultaneously, the off-state breakdown voltage is remarkably enhanced, due to the strong electric breakdown field of the polarization-graded AlGaN buffer and the restraint of the buffer leakage current. In addition, the relationship between the off-state breakdown voltage and the thickness of the polarization-graded AlGaN buffer is analyzed.

Growth and characterization of AlGaN/GaN/AlGaN double-heterojunction high-electron-mobility transistors on 100-mm Si(111) using ammonia-molecular beam epitaxy

To improve the confinement of two-dimensional electron gas (2DEG) in AlGaN/GaN high electron mobility transistor (HEMT) heterostructures, AlGaN/GaN/AlGaN double heterojunction HEMT (DH-HEMT) heterostructures were grown using ammonia-MBE on 100-mm Si substrate. Prior to the growth, single heterojunction HEMT (SH-HEMT) and DH-HEMT heterostructures were simulated using Poisson-Schrödinger equations. From simulations, an AlGaN buffer with “Al” mole fraction of 10% in the DH-HEMT was identified to result in both higher 2DEG concentration (∼1013 cm−2) and improved 2DEG confinement in the channel. Hence, this composition was considered for the growth of the buffer in the DH-HEMT heterostructure. Hall measurements showed a room temperature 2DEG mobility of 1510 cm2/V.s and a sheet carrier concentration (ns) of 0.97 × 1013 cm−2 for the DH-HEMT structure, while they are 1310 cm2/V.s and 1.09 × 1013 cm−2, respectively, for the SH-HEMT. Capacitance-voltage measurements confirmed the improvement in the confinement of 2DEG in the DH-HEMT heterostructure, which helped in the enhancement of its room temperature mobility. DH-HEMT showed 3 times higher buffer break-down voltage compared to SH-HEMT, while both devices showed almost similar drain current density. Small signal RF measurements on the DH-HEMT showed a unity current-gain cut-off frequency (fT) and maximum oscillation frequency (fmax) of 22 and 25 GHz, respectively. Thus, overall, DH-HEMT heterostructure was found to be advantageous due to its higher buffer break-down voltages compared to SH-HEMT heterostructure.

Changes of micro zone luminescent properties and stress of GaN-based light emitting diode film grown on patterned silicon substrate, induced by the removal of the substrate and AlN buffer layer

At present, there are mainly two kinds of methods to prevent crack and reduce tensile stress of the silicon substrate GaN based light emitting diode (LED) epitaxial films: one is to use the patterned silicon substrate and the other is to grow a thick AlGaN buffer layer. The two kinds of methods have their own advantages and disadvantages. Although the patterned silicon substrate GaN based LED has industrialized and is gradually accepted by the market, there remain many scientific and technical problems, to be resolved, and a lot of research gaps worth studying deeply. Among these problems, to clearly investigate the different micro zone photoluminescence and the stress states in a single-patterned GaN based LED film grown on patterned silicon substrate. The studies of the stress interaction between the buffer layer and the quanturn well layer and the effect on the luminescent properties have important guiding significance for improving the quality and performance of the devices. Different micro zone photoluminescence (PL) properties in single-patterned GaN-based LED films grown on patterned silicon substrates, nondestructive free-standing LED thin film after removing away the silicon substrate, and the free-standing LED films after removing away the AlN buffer layer are studied. The variations of the bending degree of the free-standing LED thin films before and after removing away AlN buffer layer are inverstigated by using fluorescence microscopy and scanning electron microscopy. The results show as follows. 1) After removing away the silicon substrate, the free-standing LED film bends to the substrate direction in a cylindrical bending state. After removing away the AlN buffer layer, the LED film bends into flat. 2) For LED thin films on silicon substrates or off silicon substrates, their PL spectra have significant differences in different micro zones for the same pattern. When the AlN buffer layer is removed from the substrate its PL spectrum tends to be consistent in the different micro zones of the same pattern. When the patterned silicon substrate GaN-based LED thin film is removed from the silicon substrate, the PL spectrum is redshifted in each micro zone. After AlN buffer layer is removed from the substrate, the PL spectra present different degrees of blueshift in each micro zone. 3) The LED films before and after removing away the AlN buffer layer show some differences in droop effect.

Changes of stress and luminescence properties in GaN-based LED films before and after transferring the films to a flexible layer on a submount from the silicon epitaxial substrate

Due to the lack of GaN substrates, hetero-epitaxial growth of GaN thin films is usually carried out on a foreign substrate. There are three kinds of substrate for GaN: sapphire, silicon carbide, and silicon; the sapphire substrate is the chief one, currently. Due to the availability of large scale and low cost of Si substrates, in recent years, extensive research has been devoted to the development of gallium nitride (GaN) optoelectronic devices on silicon substrates. Because of the large lattice mismatch and thermal-expansion cofficient difference between Si and GaN, it is difficult to grow thick enough crack-free GaN LED film on Si substrates. The two main kinds of methods for overcoming the crack problem are using the patterned Si substate and the thick AlGaN buffer layer. Although the two techniques could solve the problem of crack by cooling after growth, they will lead to an increase in tensile stress for GaN on Si. When making vertical-structured LED devices by transferring the GaN-based LED thin films from Si substrate to a new submount, this tensile stress will be partially released; but few researches have been made about the stress change before and after the transfer of the film, although the stress in GaN is an important factor that alters the energy band structure and may influence the vibrational properties. In this paper, we grow the crack-free GaN-based LED films on patterned Si(111), then light-emitting diode (LED) thin films are successfully transferred from the original Si (111) substrate to the submount with a flexible layer, and then the LED films without the influence of the submount and substrate are fabricated. In the following experiments, the strain-stress variation of the LED film is determined by using nondestructive high resolution X-ray diffraction (HRXRD) in detail, and the variation of photoluminescence (PL) properties of the film is studied too. Results obtained are as follows: 1) When the LED film is transferred to the flexible submount, the huge tensile stress will turn into compressive stress, and the latter in the InGaN layers will increase. 2) The In concentration in the (InGaN/GaN) MQW (multi-quantum well) systems can be evaluated with the help of reciprocal space maps (RSM) around the symmetric (0002) and asymmetric (1015) Bragg reflections. The In concentration in (InGaN/GaN) MQW will reduce when the GaN-based LED film is transferred to the flexible submount. 3) The PL spectra of the LED films will obviously appear red shift, after they are transferred to the flexible submount.

Determination of alloy composition and strain in multiple AlGaN buffer layers in GaN/Si system

A method for absolute evaluation of alloy composition and strain in AlGaN buffer layers in GaN/Si system is described, which can be applied to any III-nitride alloys hetero-epitaxially grown on Si (111) or sapphire (0001) substrate. Absolute measurements of reciprocal space maps of (10-14) nitride layers are performed with the help of Si (313) reciprocal lattice points. The alloy composition and strain-relaxation state in all layers are directly calculated from the absolute values of reciprocal lattice points referenced to the Si substrate. The proposed method of analysis using Si substrate is more accurate than currently favored method in which an epitaxial GaN layer is used as the reference. The knowledge of strain state and alloy composition with greater accuracy would be useful in designing and growing III-nitride devices.

Polarization Engineering of Al(Ga)N/GaN HEMT Structures for Microwave High Power Applications

The aim of this paper is to optimize the epitaxial layer structure of an AlGaN/GaN high electron mobility transistor (HEMT) for high power density at high frequency. The idea is to play on the polarization engineering with the different layers of the epitaxial stack. The influence of the cap and barrier layer thicknesses, the aluminum content in the barrier and the insertion of an AlGaN buffer layer are studied through the electron gas density, electron mobility and sheet resistance. This permits to find out the best trade-off in order to satisfy the requirements for high performances.

Self-Heating in GaN Transistors Designed for High-Power Operation

DC and transient self-heating effects are investigated in normally off AlGaN/GaN transistors designed for a high-power operation. Electrical and optical methods are combined with thermal simulations; 2-μs-long voltage pulses dissipating about 4.5 W/mm are applied on four different transistor structures combining GaN or AlGaN buffer on an n-type SiC substrate with or without Ar implantation. Transistors with only 5% Al mass fraction in the buffer show almost a threefold increase in the transient self-heating if compared with devices on the GaN buffer. On the other hand, 2-μs-long pulses were found not to be long enough for the Ar-implanted SiC substrate to influence the device self-heating unless AlGaN composition changes. In the dc mode, however, both the buffer composition and Ar implantation significantly influence the self-heating effect with the highest temperature rise for the transistor having the AlGaN buffer grown on the Ar-implanted SiC. We point on possible tradeoffs between the transistor high-power design and the device thermal resistance.

Effects of lift-off and strain relaxation on optical properties of InGaN/GaN blue LED grown on 150 mm diameter Si (111) substrate

We report on the effect of strain relaxations induced by lift-off in an InGaN/GaN multiple quantum wells (MQWs) blue light-emitting diode (LED) structure grown by metal-organic chemical vapor deposition (MOCVD) on a 150-mm diameter Si (111) substrate. To manage the lattice strain and, meanwhile, filter the threading dislocations, an AlN nucleation layer followed by a fourfold-step-graded AlGaN buffer together with multiple low-temperature AlN interlayers hasbeen employed, which results in a crack-free growth of 4.9-µm GaN on Si (111) in the studied LED structure. High-resolution x-ray diffraction (HRXRD) and micro-Raman spectroscopy reveal that MQWs are coherently grown on the GaN layer while the GaN layer is tensile strained on the Si (111) substrate. Lift-off by wet-chemical etching results in phonon softening of both the top-layer GaN and the InGaN within the QWs; meanwhile, a red-shift of ~11meV in the photoluminescence (PL) from the MQWs accompanied by a significant decrease (>4.9 times) in the PL-peak decay time is observed at room temperature. These observations are discussed and associated with the lift-off induced strain relaxations, which change the wafer curvature that in turn results in redistribution of in-plane stresses along the growth direction in the GaN-on-Si LED structure.

Optical Properties of Gallium Nitride Heterostructures Grown on Silicon for Waveguiding Application

Gallium nitride (GaN) on silicon (Si) is governed by the possibility to use this family of semiconductor for novel optoelectronic devices. GaN layers are deposited by MOCVD on silicon Si (111) using AlGaN buffer layer. We have studied the microstructure quality of the films. From SEM, TEM and AFM observations, we have observed that the films exhibit a good quality: the films are highly oriented (0001) with a smooth surface morphology (roughness of 12nm). We have completely characterized the optical properties using the prism coupling technique.

T-Shaped Gate GaN HFETs on Si With Improved Breakdown Voltage and &lt;inline-formula&gt; &lt;tex-math notation="TeX"&gt;$f_{\rm MAX}$ &lt;/tex-math&gt;&lt;/inline-formula&gt;

In this letter, we have fabricated GaN heterostructure field-effect transistors (HFETs) with submicrometer T-shaped gate on Si substrate to realize compact solid-state power amplifiers used in Ku-band and higher. An AlGaN buffer layer was incorporated under the GaN channel layer, which was expected to enhance the breakdown voltage without decreasing the gain. The fabricated GaN HFETs with a gate length (L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</sub> ) of 0.16 μm showed a three-terminal breakdown voltage (BV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) of 98 V and a peak maximum oscillation frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">MAX</sub> ) of 226 GHz, with a simultaneous current gain cutoff frequency ( f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) of 63 GHz at VDS = 15 V. In addition, a minimum noise figure (N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Fmin</sub> ) of 1.01 dB with an associated gain of 16 dB at 14 GHz was obtained at V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> = 5 V. A 0.1-mm-wide GaN HFETs exhibited a saturated power density of 3.82 W/mm at V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> = 30 V, indicating the successful reduction of current collapse. To the best of our knowledge, this is the first demonstration of a GaN HFETs on Si, which combines high BV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , high f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">MAX</sub> , high power density, and low NF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">min</sub> at the Ku-band.

Reduction in leakage current in AlGaN/GaN HEMT with three Al-containing step-graded AlGaN buffer layers on silicon

AlGaN/GaN high-electron-mobility transistor (HEMT) structures with two and three Al-containing step-graded AlGaN buffer layers (BLs) were grown on silicon (111) substrates by metal organic chemical vapor deposition. Considerable tensile stress was observed in the GaN grown with only two 0.8 µm AlGaN BLs, while a large in-plane compression in GaN grown with three 2.3 µm AlGaN BLs. The reverse gate leakage current in the HEMT with three AlGaN BLs was approximately 0.1 µA/mm, which was more than one order of magnitude smaller than that for the HEMT with two AlGaN BLs. A three-terminal off-state breakdown voltage of 265 V and a vertical gate-to-substrate breakdown voltage of 510 V were obtained in the HEMT with three AlGaN BLs. Detailed analysis was performed on the basis of the structural properties of AlGaN/GaN heterostructures.

Crack-free ultraviolet AlGaN/GaN distributed Bragg reflectors grown by MOVPE on 6H-SiC(0 0 0 1)

Ultraviolet AlGaN/GaN distributed Bragg reflectors (DBRs) were grown on 2in. Si-face 6H-SiC(0001) by metal organic vapor phase epitaxy (MOVPE). Two samples with single AlGaN buffer layer and AlN/AlGaN double buffer layers were introduced to grow AlGaN/GaN DBRs. The optical microscope images show that there are plenty of cracks on the surface of the DBR with single AlGaN buffer. While for DBR with AlN/AlGaN double buffer layer are free of cracks. A 30 period of Al0.2Ga0.8N/GaN DBR was obtained with measured reflectance of over 92%. The crack-free DBR has a stop-band centered around 395nm with a FWHM at 14nm.

Electro-optic and converse-piezoelectric properties of epitaxial GaN grown on silicon by metal-organic chemical vapor deposition

We report the measurement of the (r13, r33) Pockels electro-optic coefficients in a GaN thin film grown on a Si(111) substrate. The converse piezoelectric (d33) and electro-absorptive coefficients are simultaneously determined. Single crystalline GaN epitaxial layers were grown with a AlGaN buffer layer by metal organic chemical vapor deposition, and their structural and optical properties were systematically investigated. The electro-optic, converse piezoelectric, and electro-absorptive coefficients of the GaN layer are determined using an original method. A semi-transparent gold electrode is deposited on the top of the GaN layer, and an alternating voltage is applied between top and bottom electrodes. The coefficients are simultaneously and analytically determined from the measurement of the electric-field-induced variation ΔR(θ) in the reflectivity of the Au/GaN/buffer/Si stack, versus incident angle and light polarization. The method also enables to determine the GaN layer polarity. The results obtained for a Ga-face [0001] GaN layer when using a modulation frequency of 230 Hz are for the electro-optic coefficients r13 = +1.00 ± 0.02 pm/V, r33 = +1.60 ± 0.05 pm/V at 633 nm, and for the converse piezoelectric coefficient d33 = +4.59 ± 0.03 pm/V. The value measured for the electro-absorptive variation at 633 nm is Δko/ΔE = +0.77 ± 0.05 pm/V.