GaN Buffer Layer Research Articles

A secret luminescence killer in deepest QWs of InGaN/GaN multiple quantum well structures

This work suggests new alternative explanation why a single InGaN quantum well (QW) or the deepest QWs in the multiple quantum well (MQW) structures suffer with a high non-radiative recombination rate. According to SIMS results, positron annihilation spectroscopy and photoluminescence measurements we suggest that vacancy of Ga in complex with hydrogen atoms can play a dominant role in non-radiative Shockley-Read-Hall recombination of the deepest QWs in InGaN/GaN MQW structures. Vacancy of gallium originate dominantly in GaN buffer layers grown at higher temperatures in H2 atmosphere and are transported to the InGaN/GaN MQW region by diffusion, where they are very effectively trapped in InGaN layers and form complex defects with hydrogen atoms during epitaxy of InGaN layers. Trapping of gallium vacancies is another suggested mechanism explaining why the widely used In containing prelayers help to increase the luminescence efficiency of the InGaN/GaN MQW active region grown above them. Understanding the mechanism why the luminescence efficiency is suppressed in deeper QWs may be very important for LED community and can help to develop new improved technologies for the growth of InGaN/GaN MQW active region.

Optical Investigation of Proton‐Irradiated Metal Organic Chemical Vapor Deposition AlGaN/GaN High‐Electron‐Mobility Transistor Structures

About 2 MeV proton numerical calculation damage indicates that a quasiflat profile is introduced into the GaN buffer layer of high‐electron‐mobility transistor (HEMT) templates. A previous transport study of such HEMT structures showed increased breakdown voltage and reduced GaN buffer layer leakage after proton irradiation. Hyperspectral electroluminescence measurements detected the emission band in the spectral region between 700 and 800 nm. This emission is assumed to be associated with the generation of trap levels responsible for the device failure. To obtain insights into the nature of radiation‐generated traps, detailed low‐temperature photoluminescence (PL) and cathodoluminescence (CL) experiments are conducted in a set of virgin and proton‐irradiated device structure templates. Both the PL and CL spectra show large intensity variations of all emission bands, in the spectral range between 330 and 890 nm, with increasing proton dosage. This is consistent with the introduction of compensating and/or nonradiative recombination centers in the GaN buffer layer. Luminescence measurements carried out under various excitation conditions show different recombination rates for emission bands observed near 420 and 550 nm, indicating different free carrier capture rates. Spectral analysis suggests that these two emission bands have different dependences on proton irradiation dosage, consistent with different chemical natures.

Metalorganic Chemical Vapor Phase Epitaxy Growth of Buffer Layers on 3C‐SiC/Si(111) Templates for AlGaN/GaN High Electron Mobility Transistors with Low RF Losses

Herein, the interest of cubic silicon carbide as a template for the growth of AlGaN/GaN high electron mobility transistor (HEMT) heterostructures on silicon substrates for high‐frequency operation is shown. On the one hand, 0.6–0.8 μm‐thick 3C‐SiC grown by chemical vapor deposition on intrinsic silicon substrate having initial resistivity superior to 5 kΩ cm enables the metalorganic vapor phase epitaxy of GaN buffer layers with propagation losses below 0.4 dB mm−1 at 40 GHz and 0.5 dB mm−1 at 67 GHz. On the other hand, an HEMT heterostructure is grown on 1.5 μm‐thick 3C‐SiC on 4° off‐axis silicon substrate having an initial resistivity superior to 200 Ω cm that allows to keep a sufficiently resistive epilayer stack limiting the loss up to 0.78 dB mm−1 at 40 GHz. Device process developed on a piece of the 100 mm diameter wafer leads to the demonstration of DC transistor operation with low leakage currents. Compared with direct growth on silicon, these templates enable reduced radio frequency (RF) propagation losses that are very interesting for high‐frequency transistors and circuits operation.

Research on epitaxial of 250 nm high quality GaN HEMT based on AlN surface leveling technology

GaN HEMT epitaxial materials with different buffer layer thicknesses were grown on 4H-SiC substrates by metal-organic chemical vapor deposition (MOCVD). The interface quality between AlN and GaN was improved based on AlN surface planarization technology. A high quality 250 nm thick GaN epitaxial layer was grown. The (0 0 2) plane and the (1 0 2) plane rocking curve have a full width at half maximum of 81 arc seconds and 209 arc seconds, respectively, and the electron mobility of the heterojunction two-dimensional electron gas reaches 2238 cm2/V·s. The surface topography and electrical properties of the material were measured by atomic force microscopy (AFM) and non-contact Hall tester. It was found that the surface smoothness of the GaN buffer layer and the electron mobility of 2DEG did not change significantly, and the X-rays diffractometer was used to analyze the crystal quality and stress of the GaN epitaxial layer.

A systematic study of MOCVD reactor conditions and Ga memory effect on properties of thick InAl(Ga)N layers: a complete depth-resolved investigation

Thick InAlN layers (In-molar fraction >0.37) on GaN buffer layers were prepared using a close-coupled showerhead metalorganic chemical vapor deposition (MOCVD) reactor.

Improvement of luminescence properties of n-GaN using TEGa precursor

Abstract The aim of this work is to compare and improve optical and structural properties of GaN layers prepared using TMGa or TEGa precursors. MOVPE grown GaN buffer layers on sapphire substrates are usually grown from TMGa precursor at the temperatures above 1000 °C. These layers contain deep and shallow acceptor levels which are responsible for blue and yellow defect bands in luminescent spectra. Both defect bands are detrimental for all major nitride device applications. Especially n-doped GaN layers suffer from strong yellow defect bands. In this work, it is shown that yellow band photoluminescence intensity can be suppressed by using TEGa precursor during the growth of n–doped GaN layers. Different kinds of growth parameters, such as growth temperature or growth rate, have been studied. It is also shown that the change of carrier gas (H2 or N2) has very strong influence on the layer quality. H2 carrier gas increased intensity of yellow band in sample grown from TEGa precursor while N2 carrier gas had the same effect for sample grown from TMGa precursor. Variable energy positron annihilation spectroscopy showed creation of single VGa in H2 atmosphere and clustering of VGa to big complexes ((VGa)3(VN)n) in N2 atmosphere.

Confirmation of the compensation of unintentional donors in AlGaN/GaN HEMT structures by Mg-doping during initial growth of GaN buffer layer

Effect of partial Mg doping on the compensation of unintentional donors at epilayer/template interface and in the GaN channel layer of AlGaN/GaN High Electron Mobility Transistor (HEMT) structures is investigated. Photoluminescence excitation spectroscopy and surface photovoltage spectroscopy measurements reveal the signature of high density of unintentional shallow donors and deep level defects throughout the GaN buffer layer along with a strong dominance at the GaN/Fe-GaN template interface. Mg doping during the initial growth of GaN buffer layer on Fe-GaN template is found to compensate unintentional donors which therefore reduces the density of deep defects. Hall measurements also confirm systematic improvement in the electronic transport properties of AlGaN/GaN HEMT for an optimized Mg doping. Further, Mg-doping driven enhancement of 2-dimensional electron gas confinement at AlGaN/GaN interface is also observed in the spectroscopy measurements. It is shown that the electrical and optical properties can be improved by an optimized Mg-doping of the initial part of GaN buffer layer in AlGaN/GaN HEMT structures.

Structural and morphological characteristics of GaN-based hybrid heterostructures grown on por-Si

GaN/Si(111) heterostructures were grown by plasma-assisted molecular beam epitaxy on usual Si(111) substrates and compliant por-Si/Si(111) substrates without using AlN buffer layer. The positive influence of the high-temperature nitridation step, as well as the usage of a compliant substrate on crystalline quality, was confirmed. We got a crack-free 850-nm-thick GaN layer at room temperature by using the low-temperature GaN buffer layer with nanocolumnar morphology.

Correlation of Threading Dislocations with the Electron Concentration and Mobility in InN Heteroepitaxial Layers Grown by MBE

The quantitative interdependencies of growth conditions, crystal defects and electrical/electronic properties of InN thin films, grown by plasma-assisted molecular beam epitaxy on GaN (0001) buffer layers have been investigated. InN epilayers with thickness near 700 nm, grown under different substrate temperature and/or growth rate, have been analyzed. Bulk electron concentration (Nbulk) and mobility values were extracted for each InN film using the inverted version of the multilayer Petritz model, subtracting the conductivity of a corresponding 120 nm InN film. The results indicate a significant reduction of the threading dislocation density by enhancing the diffusion length of indium adatoms during InN growth, through increase of substrate temperature and reduction of growth rate. The electrical characteristics deteriorate with increasing threading dislocation density. Assuming threading dislocations as exclusive sources of donors in InN, their charge state could be between +1 and +2 per c lattice constant length of dislocation line for Nbulk ≈ 4.0–5.7×1017 cm−3, and approximately +2 or larger for Nbulk ≈ 1.5–1.8×1018 cm−3. The scattering effect of threading dislocations is significantly weaker compared to reported theoretical calculations, i.e. it would correspond to an order of magnitude lower threading dislocation density than the experimentally observed density in the range of 1010 cm−2.

Emission Properties of Heavily Doped Epitaxial Indium-Nitride Layers

The results of studies of the spontaneous photoluminescence and stimulated emission spectra of epitaxial n-InN layers with a concentration of free electrons of ~1019 cm–3 are reported. The layers are grown by molecular-beam epitaxy with the plasma activation of nitrogen on sapphire substrates with AlN and GaN buffer layers. It is found that, as the InN layers are grown under conditions with enrichment with nitrogen at a growth temperature elevated to 470°C close to the beginning of the decomposition of InN, the crystal quality of the layers is improved and the stimulated-emission threshold is lowered. As the conditions of growth change to conditions with enrichment with the metal, two emission bands separated by an energy of 100 meV are observed in the spontaneous-photoluminescence spectra of InN. For such layers, a substantial increase in the stimulated-emission threshold and, in some cases, the lack of a transition to stimulated emission are observed. In the study, an interpretation of the observed emission bands is given and some inferences as to their nature are made.

Growth of millimeter wave AlN/GaN heterostructures by MOCVD

The influences of growth temperature on crystalline quality and electrical properties of thin barrier AlN/GaN heterostructures were studied by metal organic chemical vapor deposition (MOCVD). It is found that the high growth temperature (≥1100 °C) is beneficial to improve the crystalline quality of the AlN barrier layer, but increases the defect density in the GaN buffer layer. The AlN/GaN hetero-interface quality was deteriorated when high-temperature AlN barrier layer was grown upon low-temperature GaN buffer layer, resulting in a reduction of electron mobility. In addition, the growth temperature of the GaN cap layer affects the stress evolution of wafer during the cooling process, which influences the wafer bowing and electrical properties. By optimizing the growth temperature of each functional layer, the GaN/AlN/GaN heterostructure with a low defect density and a smooth surface is obtained. The electron mobility at room temperature is 1549 cm2/(V·s) with a sheet resistance value of 504.8 Ω/□, and the root mean square (RMS) of surface roughness is 0.322 nm. Based on the as-grown GaN/AlN/GaN heterostructure, a GaN power MMIC was fabricated, offering an output power of 15 dBm (31.6 mW) and power added efficiency (PAE) of 9.3% at 146 GHz.

Negative transconductance effect in p-GaN gate AlGaN/GaN HEMTs by traps in unintentionally doped GaN buffer layer**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFB0402900), the National Natural Science Foundation of China (Grant No. 61634002), the Scientific Research Foundation of Graduate School of Nanjing University, China (Grant No. 2016CL03), and the Key Project of Jiangsu

We investigate the negative transconductance effect in p-GaN gate AlGaN/GaN high-electron-mobility transistor (HEMT) associated with traps in the unintentionally doped GaN buffer layer. We find that a negative transconductance effect occurs with increasing the trap concentration and capture cross section when calculating transfer characteristics. The electron tunneling through AlGaN barrier and the reduced electric field discrepancy between drain side and gate side induced by traps are reasonably explained by analyzing the band diagrams, output characteristics, and the electric field strength of the channel of the devices under different trap concentrations and capture cross sections.

Leakage current and rectification behavior of Au / TiO2 / GaN junctions with TiO2 interlayer oxygen deposition pressure

Au/TiO2/GaN junctions with a TiO2 interlayer were deposited at various oxygen pressures from 5 mTorr to 80 mTorr, and their surface, microstructural, and electrical properties were investigated. Compared with a single 5 μm-Si-doped GaN film on an (0001) Al2O3 substrate, the 0.2 μm-Si-doped GaN film with an undoped GaN buffer layer demonstrated improved properties for the application to metal/semiconductor junctions. Atomic force microscope and tunneling electron microscope measurements suggested that the TiO2 interlayer deposited at room temperature exhibited a distinctive change above the oxygen deposition pressure of 40 mTorr. In contrast to the small rectification ratio of 101-102 for Au/GaN junctions, the Au/TiO2/GaN junctions with the TiO2 interlayers displayed a large rectification ratio of 106-107 when the oxygen pressure during the deposition of TiO2 interlayer was maintained at 40 mTorr. These results suggest that the leakage current and the rectification behavior in a metal/oxide/semiconductor junction can be effectively controlled using the oxygen deposition pressure for an oxide interlayer.

Insights into the off-state breakdown mechanisms in power GaN HEMTs

We analyze the off-state, three-terminal, lateral breakdown in AlGaN/GaN HEMTs for power switching applications by comparing two-dimensional numerical device simulations with experimental data from device structures with different gate-to-drain spacing and with either undoped or Carbon-doped GaN buffer layer. Our simulations reproduce the different breakdown-voltage dependence on the gate-drain-spacing exhibited by the two types of device and attribute the breakdown to: i) a combination of gate electron injection and source-drain punch-through in the undoped HEMTs; and ii) avalanche generation triggered by gate electron injection in the C-doped HEMTs.

Effect of high‐temperature AlN buffer on anisotropy of semi‐polar (11‐22) GaN with two pressure growth stages

The low-temperature GaN (LT-GaN) and high-temperature AlN (HT-AlN) buffer layers were grown on m -plane sapphire by metal–organic chemical vapour deposition. The following semi-polar (11-22) GaN thin films were deposited under high- and low-pressure growth stages. Anisotropy of (11-22) GaN grown on HT-AlN buffer along two in-planar directions ([11-2-3] and [-1100]) were clearly suppressed, the maximum variation of the X-ray rocking curve full width at half maximum of (11-22) GaN on LT-GaN buffer was 0.2498°, that of (11-22) GaN on HT-AlN buffer was 0.0488°. The X-ray diffraction results of on-axis and off-axis both indicated that the crystal quality of the epitaxial GaN layer with HT-AlN buffer was obviously improved, and surface morphology was much smoother.

Undoped High-Resistance GaN Buffer Layer for AlGaN/GaN High-Electron-Mobility Transistors

It is shown that intentionally undoped high-resistance GaN buffer layers in AlGaN/GaN heterostructures with high electron mobility for transistors can be formed by ammonia molecular beam epitaxy. The GaN growth conditions have been optimized using calculations of the background impurity and point defect concentrations at different ratios of the gallium and ammonia fluxes.

Improved RF and DC performance in AlGaN/GaN HEMT by P-type doping in GaN buffer for millimetre-wave applications

In this paper, the RF and DC performance of AlGaN/GaN HEMT having p-type doping in GaN buffer layer was analysed. Novelty of this work lies in the inclusion of P-type doping in GaN buffer of proposed GaN-HEMT. The result shows the significance of p-type doping at GaN buffer layer for enhancing the RF characteristics of the AlGaN/GaN HEMT. The use of p-type doping in GaN buffer layer increase the conduction band minima in the channel and thereby improving the transconductance of the device. The proposed HEMT achieves a peak transconductance of 330 mS/mm at VGS = −2 V and a maximum drain current of 850 mA/mm. The unity-gain cut-off frequency, small signal capacitance and conductance were also extracted to demonstrate its influence on RF performance. The proposed GaN HEMT has exhibited a unity current gain cut-off frequency of 45 GHz, which is 40% higher than that of conventional HEMT. The improved RF and DC performance of proposed HEMT makes it suitable for millimetre-wave applications.

Control of yellow photoluminescence in AlGaN/GaN heterostructures

Photoluminescence with the peak corresponding to yellow color of the visible spectrum (so-called yellow luminescence) originates from deep levels in the GaN buffer layers of heterostructures and depends on heterostructure growth conditions. In turn deep levels affect the resistance of Ohmic contacts of microwave transistors fabricated from these heterostructures. This determines the reliability of GaN microwave transistor operation. Two types of units for control of photoluminescence with the peak in the yellow visible spectral region have been designed with the aim to control the quality of AlGaN/GaN/SiC and AlGaN/GaN/Al2O3 heterostructures. One of the units is used for fast control of yellow photoluminescence and the other for photoluminescence mapping on heterostructure wafer surfaces. Examples of photoluminescence maps for structures grown on different substrates have been given.

Improved bandwidth of a 2 THz hot-electron bolometer heterodyne mixer fabricated on sapphire with a GaN buffer layer

We report on the signal-to-noise and gain bandwidth of a niobium nitride (NbN) hot-electron bolometer (HEB) mixer at 2 THz fabricated on a sapphire substrate with a GaN buffer layer. Two mixers with different DC properties and geometrical dimensions were studied and they demonstrated very close bandwidth performance. The signal-to-noise bandwidth is increased to 8 GHz in comparison to the previous results, obtained without a buffer-layer. The data were taken in a quasi-optical system with the use of the signal-to-noise method, which is close to the signal levels used in actual astrophysical observations. We find an increase of the gain bandwidth to 5 GHz. The results indicate that prior results obtained on a substrate of crystalline GaN can also be obtained on a conventional sapphire substrate with a few micron MOCVD-deposited GaN buffer-layer.

Improvement of emission efficiency with a sputtered AlN buffer layer in GaInN-based green light-emitting diodes

We examined a sputtered AlN (sp-AlN) buffer layer to improve the emission efficiency of GaInN-based green LEDs. Both the electroluminescence and photoluminescence emission efficiencies are improved in a LED with a sp-AlN buffer layer compared to that with a conventional low-temperature GaN buffer layer. To identify the origin of the improvements, we carried out the analysis including X-ray diffraction, surface morphology, initial nucleation growth, and temperature-dependent PL. Through analysis and consideration, we showed that the improvements are mainly caused by less strain and a lower degree of carrier localization in quantum wells (QWs), that is highly influenced by the grain diameter of the GaN island at initial growth. These results indicate that a sp-AlN buffer layer induces less compositional pulling that results in less strain and alloy fluctuation in QWs, simultaneously.