GaN Contact Layer Research Articles

Comparative studies of GaN, n-GaN and n+-GaN contact layers on GaN/c-Al2O3 virtual substrates synthesized by PA MBE

An approach to create ohmic contacts to GaN layers without using the high-temperature annealing procedure is described in the paper. The results of comparative studies of structural, morphological, optical, and electrical properties of undoped GaN and silicon-doped n-GaN and n+-GaN doped contact layers are presented.The contact layer technology can be successfully realized using a plasma-assisted molecular beam epitaxy (PA MBE) setup of industrial type, where due to characteristic design features, the growth of nitride AIIIN compounds occurs using small streams of activated nitrogen, which significantly limits the possibility of varying the technological parameters.For the first time it is demonstrated that using the proposed technology for the creation of post-grown GaN, n-GaN and n+-GaN contact layers to GaN/c-Al2O3 virtual substrates in Ga-enriched conditions of epitaxial PA MBE growth at relatively low temperatures Ts = 700oC an effective filtration of dislocations threading from the buffer GaN layer of the virtual substrate formed by MOCVD can be realized.Calculations of in-plane εxx and out-of-plane εzz deformations, as well as residual biaxial stresses based on X-ray diffraction and Raman microspectroscopy data indicate at high structural quality of the formed contact layer regardless of the silicon doping level. At the same time PA MBE contact layers GaN, n-GaN and n+-GaN demonstrate close optical properties (refractive index and photoluminescence) in a wide range (400–850 nm), as well as minimal influence of defects associated with the formation of silicon clusters at high doping levels.The contact resistance reduced to the pad width determined using the transmission line method for the structure with n+-GaN contact layer was of ∼0.11 Ohm*mm and ∼0.5 Ohm*mm for n-GaN. Note that electrophysical characteristics of n-GaN and n+-GaN contact layers do not degrade under high-dose exposure of high-energy X-ray irradiation.The technique developed in our work to create ohmic contacts to GaN layers, as well as the obtained experimental results, have important implications for understanding the physics of AIIIN nanoheterostructures and will contribute to their potential applications in their fabrication.

Study of simulations of double graded InGaN solar cell structures

The performances of various configurations of InGaN solar cells are compared using nextnano semiconductor simulation software. Here, we compare a flat base-graded wall GaN/InGaN structure, with an InxGa1−xN well with sharp GaN contact layers, and an InxGa1−xN structure with InxGa1−xN contact layers, i.e., a homojunction. The doping in the graded structures is the result of polarization doping at each edge (10 nm from each side) due to the compositional grading, while the well structures and homojunctions are impurities doped at each edge (10 nm from each side) at levels equal to the polarization doping density in the graded structure with similar maximum indium concentration. The solar cells are characterized by their open-circuit voltage, Voc, short circuit current, Isc, solar efficiency, η, and energy band diagram. The results indicate that an increase in Isc and η results from increasing both the fixed and maximum indium compositions, while the Voc decreases. The maximum efficiency is obtained for the InGaN well with 60% In.

Improved on-state performance in AlGaN-channel heterojunction field-effect transistors with a quaternary AlGaInN barrier layer and a selectively grown n++-GaN contact layer

Al0.19Ga0.81N-channel metal-insulator-semiconductor heterojunction field-effect transistors (MIS-HFETs) with a quaternary AlGaInN barrier layer and a selectively regrown n++-GaN contact layer were fabricated using the self-alignment selective-area growth (SAG) technique. The self-alignment SAG process was accomplished by the local-area etching process and the subsequent refilling process with a highly-Si-doped n++-GaN layer by metalorganic chemical vapor deposition (MOCVD). It was confirmed that the ohmic contact resistance was drastically reduced from 10.5 Ωmm to 2.5 Ωmm via the self-alignment SAG contacts. Further, fabricated MIS-HFETs with a gate length of 1.5 μm and a drain-to-source length of 9.5 μm exhibited a high drain current density of over 300 mA/mm with a drain-to-source resistance of approximately 25 Ω mm. The fabricated devices also showed a high off-state breakdown voltage of 1220 V at a gate-to-drain length of 10 μm, corresponding to a breakdown field of 122 V/μm.

Ultra-high silicon doped N-polar GaN contact layers grown by metal-organic chemical vapor deposition

We report thin, high quality n+ type doped N-polar GaN contact layers grown using metal-organic chemical vapor deposition with carrier concentration as high as 3.5 × 1020 cm−3 and an electron mobility of 80 cm2 V−1 s−1 at room temperature resulting in a low sheet resistance of 57.3 Ω/□ and specific contact resistance of 1.7 × 10−7 Ω.cm2. These results were obtained via silicon doping of () N-polar GaN grown on 4° miscut sapphire substrates using a flow modulation growth scheme at a deposition temperature of 850 °C.

Selective area epitaxy of n+-GaN layers on SiO2 patterned GaN/c-Al2O3 templates by PA MBE

The n+-GaN epilayers were synthesised by PA MBE on the SiO2 patterned GaN/c-Al2O3 templates, grown by MOCVD. Formation of the polycrystalline GaN atop of the SiO2 mask during PA MBE was observed. It was found that macroscopic voids at the interface polycrystalline GaN/SiO2/n-GaN template appeared during the PA MBE process. The polycrystalline GaN film was completely removed by etching in hot aqueous KOH solution. Hall measurements have shown that the value of electron concentration in n+-GaN contact layer is about ne∼4.6×1019 cm-3.

On the elastic properties of INGAN/GAN LED structures

In this study, three InGaN/GaN light-emitting diode (LED) structures with five periods are investigated grown by metal organic chemical vapor deposition (MOCVD) technique. During growth of these three samples, active layer growth temperatures are adjusted as 650, 667 and 700 °C. These structures are grown on sapphire (Al2O3) wafer as InGaN/GaN multi-quantum wells (MQWs) between n-GaN and p-AlGaN + GaN contact layers. During growth, pressure and flux ratio of all sources are kept constant for all samples. Only temperature of InGaN active layer is changed. These structures are analyzed with high-resolution X-ray diffraction (HR-XRD) technique. Their surface morphologies are investigated with atomic force microscopy (AFM). Reciprocal space mapping (RSM) is made different from classical HR-XRD analyses. Using this method, mixed peaks belonging to InGaN, AlGaN and GaN layers are seen more clearly and their full width at half maximum (FWHM) values is determined with better accuracy. With FWHM gained from RSM and Williamson–Hall (W–H) method based on universal elastic coefficients of the material, particle size D (nm), uniform stress σ (GPa), strain e and anisotropic energy density u (kJ m−3) parameters for the samples are calculated. The results are compared with literature. On the other hand, to have an idea about the accuracy of the results AFM images are examined. Parameters calculated showed differences but it is seen that the largest particle size is gained for GaN and the smallest is gained for AlGaN. For all parameters, it is seen that they increase for GaN layer and decrease for AlGaN layer with increasing temperature. For InGaN layer parameters, they showed both increasing and decreasing or decreasing and increasing behavior harmonically with an increase in temperature. Results showed that they are compatible with literature. Results gained from Scherrer and W–H are very near to each other.

Effect of GaN substrate thickness on the optical field of InGaN lasers diodes

In this work the influence of GaN substrate thickness on the near and far-field patterns of InGaN lasers structures is analyzed. In simulation a conventional separate confinement heterostructure of InGaN-MQW / GaN / AlGaN is considered. A fluctuating behavior is found, showing that for some values of the substrate thickness the near and far- field patterns can be optimized and there are critical values of the substrate thickness that produce the lowest values of the confinement factor and the higher values of the full wide half-maximum of the far field. It is also shown that by substituting the GaN contact layer by an AlxGa1-xN layer with a parabolic variation of the Al content it is possible to reduce the optical field leakage to substrate. Results indicated that properly choosing the thickness of the substrate and replacing the n-GaN contact layer by a graded-index (GRIN) AlxGa1-xN layer it is possible to improve both the confinement factor and Far field pattern in nitride lasers.

Influence of residual carbon impurities in a heavily Mg-doped GaN contact layer on an Ohmic contact.

The influence of residual carbon impurities incorporated into a heavily Mg-doped GaN layer has been studied systematically according to the relation between the carbon concentration and specific contact resistance. Furthermore, the results of temperature-dependent current-voltage characteristics and the photoluminescence spectra indicate that a proper concentration of residual carbon impurities can improve the performance of Ohmic contact by introducing deep-level defects to enhance the variable-range-hopping conduction.

Effect of nanopillar sublayer embedded with SiO2 on deep traps in green GaN/InGaN light emitting diodes

The effect of a layer of GaN nanopillars with SiO2 nanoparticles inserted into the n+-GaN contact Layer on the electrical properties, electroluminescence (EL) and photoluminescence (PL), admittance spectra, and deep trap spectra of green multi-quantum-well GaN/InGaN light emitting diodes (LEDs) grown by metalorganic chemical vapor deposition (MOCVD) on patterned sapphire substrates is reported. The PL and EL intensities for these SiO2 LEDs are measurably enhanced compared with reference to LEDs without the nanopillar sublayer. This correlates with the decrease in the SiO2 LEDs of the concentration of 0.25 eV electron traps and 0.45 eV hole traps, both located in the InGaN QWs.

(Invited) Electrically Pumped Lasers and Light Emitting Diodes in the Ultraviolet-C Band with AlGaN Nanowires

Ultraviolet (UV) light sources, including light emitting diodes (LEDs) and lasers, with emission wavelengths less than 280 nm (UV-C band) are critically important for a wide range of applications including water purification, disinfection, data storage, medical diagnosis, and bio/chemical sensing. To date, the development of such light sources with aluminum gallium nitride (AlGaN) has been severely limited by the presence of large dislocation densities and the extremely inefficient p-type doping. In addition, the intrinsic TM light polarization of Al-rich AlGaN prohibits the light extraction from conventional c-plane wurtzite structures. In this context, we have investigated the molecular beam epitaxial (MBE) growth and characterization of nearly defect-free AlGaN nanowires on Si. We have demonstrated, for the first time, electrically injected lasers in the UV-C band, which exhibit a threshold current density of 0.2 kA/cm2. By exploiting the efficient light scattering among nanowire arrays, we have also demonstrated high efficiency AlN nanowire LEDs with emission wavelength ~ 207 nm. In this experiment, the AlGaN nanowires were spontaneously formed on Si substrate by radio-frequency plasma-assisted MBE without using any foreign metal catalyst. The laser device structure consisted of n-GaN contact layer, n-AlGaN cladding layer, AlGaN active region, p-AlGaN cladding layer, and p-GaN contact layer. The optical cavity was formed by the Anderson localization of light, due to the recurrent multiple scattering of photons in random nanowire arrays. The fabrication process involved standard optical lithography and contact metallization techniques. The lasers operated under continuous wave (CW) biasing conditions. The electroluminescence emission was collected from the device top surface by a CCD camera coupled to a high-resolution spectrometer. A lasing peak around 262 nm was clearly measured at 77 K, with a threshold current density of 0.2 kA/cm2. This is the first demonstration of electrically injected semiconductor lasers in the deep UV wavelength range. Detailed studies further showed that the self-organized AlGaN nanowires exhibited extensive atomic scale compositional modulation. The extremely low threshold current is directly related to the three-dimensional quantum confinement of charge carriers by the quantum dot/dash-like nanostructures in nearly defect-free AlGaN nanowires. We have further investigated deep UV LEDs based on self-organized AlGaN nanowires. In this study, the device structure consisted of n-GaN contact layer, n-Al(Ga)N, i-Al(Ga)N, p-Al(Ga)N, and p-(Al)GaN contact layers. Strong electroluminescence at 207 nm was measured at room-temperature from LEDs fabricated with AlN. Furthermore, detailed angle dependent electroluminescence studies indicated that the dominant light emission direction was from the c-axially grown nanowire top surface, due to the efficient light scattering amongst nanowires, in spite of TM light polarization in AlN. Moreover, an electrical efficiency of > 85% was realized from such AlN nanowire LEDs, which can be largely ascribed to the hole hopping conduction in the impurity band hopping. In addition, our detailed studies further indicated that the performance of AlN nanowire LEDs was largely limited by electron overflow.

Isotype InGaN/GaN heterobarrier diodes by ammonia molecular beam epitaxy

The design of isotype InGaN/GaN heterobarrier diode structures grown by ammonia molecular beam epitaxy is presented. On the (0001) Ga-polar plane, a structure consisting of a surface n+ GaN contact layer, followed by a thin InGaN layer, followed by a thick unintentionally doped (UID) GaN layer, and atop a buried n+ GaN contact layer induces a large conduction band barrier via a depleted UID GaN layer. Suppression of reverse and subthreshold current in such isotype barrier devices under applied bias depends on the quality of this composite layer polarization. Sample series were grown under fixed InGaN growth conditions that varied either the UID GaN NH3 flow rate or the UID GaN thickness, and under fixed UID GaN growth conditions that varied InGaN growth conditions. Decreases in subthreshold current and reverse bias current were measured for thicker UID GaN layers and increasing InGaN growth rates. Temperature-dependent analysis indicated that although extracted barrier heights were lower than those predicted by 1D Schrödinger Poisson simulations (0.9 eV–1.4 eV for In compositions from 10% to 15%), optimized growth conditions increased the extracted barrier height from ∼11% to nearly 85% of the simulated values. Potential subthreshold mechanisms are discussed, along with those growth factors which might affect their prevalence.

Influence of a deep-level-defect band formed in a heavily Mg-doped GaN contact layer on the Ni/Au contact to p-GaN**Project supported by the National Natural Science Foundation of China (Grant Nos. 61474110, 61377020, 61376089, 61223005, and 61176126), the National Science Fund for Distinguished Young Scholars of China (Grant No. 60925017), One Hundred Person Project of the Chinese Academy of Sciences, and

The influence of a deep-level-defect (DLD) band formed in a heavily Mg-doped GaN contact layer on the performance of Ni/Au contact to p-GaN is investigated. The thin heavily Mg-doped GaN (p++-GaN) contact layer with DLD band can effectively improve the performance of Ni/Au ohmic contact to p-GaN. The temperature-dependent I–V measurement shows that the variable-range hopping (VRH) transportation through the DLD band plays a dominant role in the ohmic contact. The thickness and Mg/Ga flow ratio of p++-GaN contact layer have a significant effect on ohmic contact by controlling the Mg impurity doping and the formation of a proper DLD band. When the thickness of the p++-GaN contact layer is 25 nm thick and the Mg/Ga flow rate ratio is 10.29%, an ohmic contact with low specific contact resistivity of 6.97× 10− 4 Ω·cm2 is achieved.

Demonstration of low resistance ohmic contacts to p-type (202̄1̄) GaN

We demonstrate low resistance Ohmic contacts to ( semipolar p-type GaN using a thin p++–GaN contact layer. This layer was optimized by varying the V/III ratio, Cp2Mg flow, thickness, and growth rate which produced low forward voltage devices with specific contact resistivities estimated to be 4 × 10−4 Ω cm2 and 5 × 10−4 Ω cm2 for palladium contacts and indium tin oxide (ITO) contacts respectively. Forward voltages of 2.89 V and 3.47 V were obtained at 20 A cm−2 and 800 A cm−2 respectively for light emitting diodes (LEDs) emitting at 435 nm with palladium contacts. LEDs with ITO contacts had forward voltages of 2.94 V and 3.55 V at 20 A cm−2 and 800 A cm−2 respectively.

Reducing forward bias voltage of InGaN/GaN-based light emitting diodes by using (In)GaN contact layer

The electrical properties of GaN-based light emitting diodes (LEDs) fabricated with indium–tin-oxide (ITO) p-contacts were investigated as functions of the thickness of the (In)GaN contact layers and the In content. The LEDs with the GaN contact layers showed lower forward voltages (in the range of 3.48–3.03 V) than the LEDs with ITO-only contacts (4.2 V); the forward voltages of the LEDs decreased with increasing contact layer thickness (from 1–4 nm) and increasing In content. However, the output power linearly decreased with increasing In content, whereas that of the GaN contact layer LEDs became saturated at a thickness of 2 nm. The X-ray photoemission spectroscopy (XPS) Ga 2p core level for the samples with the contact layers was shifted toward lower binding energies by 0.11–0.22 eV compared with that of the sample without the contact layer. However, the energy shift decreased with increasing In content. Unlike the contact-layer samples, the sample without the contact layer experienced outdiffusion of N atoms. Based on the XPS and atomic force microscopy results, the contact-layer-induced electrical improvement was described and discussed.

Effect of Argon Ion Irradiation on Ohmic Contact Formation on n-type Gallium Nitride

This paper describes electrical properties at the interface between n-type GaN and Ti contact layers formed by RF magnetron sputter deposition under various time of Ar irradiation. The conductance increases proportionately to the extension of the Ar irradiation time, indicating that Ar irradiation enhances the formation of nitrogen vacancies and consequently Schottky barrier width is narrowed. On the other hand, extensive Ar irradiation for 3600 s and longer does not show further increase of conductance. Extensive selective sputtering of nitrogen atoms out of GaN has induced the phase transformation from GaN to Ga at the surface of GaN. The Ga phase enhances the formation of Ga­Ti compounds during the deposition of Ti, and it increases the height and width of the Schottky barrier. [doi:10.2320/matertrans.MD201217]

Effect of interfacial reaction on electrical conduction across the interface between n-type gallium nitride and contact materials

The present paper reports electrical properties at the interface between n-type GaN and Ti-based contact layers formed by radio-frequency magnetron sputter deposition under various conditions. TiN contacts deposited using N2 gas are non-ohmic, i.e., formation of TiN adjacent to GaN is not a necessary condition for developing ohmic properties. On the other hand, Ti contacts deposited using Ar gas are ohmic in the as-deposited state, even though a layer of Ti2N is formed between GaN and Ti during the deposition. It is also shown that Ti deposition on undoped GaN produces ohmic contacts. The nitrogen vacancies increased in the sub-interface of GaN are essential for developing ohmic properties. The interfacial reaction between Ti and GaN to form nitrogen vacancies is affected by the partial pressure of N2 during the deposition.

Enhancement‐mode m ‐plane AlGaN/GaN HFETs with regrown n+‐GaN contact layer

AbstractHigh performance enhancement‐mode m ‐plane AlGaN/GaN heterojunction field‐effect transistors were reported in this letter. +3 V threshold voltages, 230 mA/mm of maximum drain‐source current (Ids(max)), and 58 mS/mm of maximum transconductance (gm(max)) were obtained. High Ids(max) and gm(max) were due to the regrown n+‐GaN contact layer. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Over 3.0 $\hbox{GW/cm}^{2}$ Figure-of-Merit GaN p-n Junction Diodes on Free-Standing GaN Substrates

This letter describes a new two-step electrode process on p-GaN and characteristics of GaN p-n junction diodes on free-standing GaN substrates with low specific ON-resistance Rοn and high breakdown voltage VB. We develop a two-step process for anode electrodes in order to avoid plasma damage to the p+-GaN contact layer during the sputtering process. The specific ON-resistance is further improved due to a new low-damage process. The breakdown voltage of the diodes with the field-plate (FP) structure is over 1100 V, and the leakage current was low, i.e., in the range of 10-9 A. The specific ON-resistance of the diodes of 50 μm in diameter with the FP structure was 0.4 mΩ · cm2. Baliga's figure of merit (VB2/Ron) of 3.0 GW/cm2 is obtained. These are the best values ever reported among those achieved by GaN p-n junction diodes on free-standing GaN substrates.

High-Power InGaN-Based LED With Tunneling-Junction-Induced Two-Dimensional Electron Gas at AlGaN/GaN Heterostructure

We demonstrate high-performance InGaN-based light-emitting diodes (LEDs) with tunneling-junction-induced 2-D electron gas (2-DEG) at n-AlGaN/GaN heterostructure, inserted in the middle of the p++-GaN contact layer of a conventional LED structure. The LED with a 2-DEG layer exhibits about 20% enhancement in output power, as compared with that of the conventional LED at 350 mA, which is believed to be due to enhanced hole-injection efficiency and better lateral current spreading by the presence of 2-DEG at the AlGaN/GaN heterostructure.

Reliability and performance of pseudomorphic ultraviolet light emitting diodes on bulk aluminum nitride substrates

AbstractHigh quality, bulk aluminum nitride substrates were used to obtain pseudomorphic AlxGa1‐xN layers with low dislocation density, smooth surfaces, and high conductivity. These layers were fabricated into mid‐ultraviolet light emitting diodes with peak wavelengths in the range of 240‐260 nm. From transmission electron microscope images, it was confirmed that the dislocation density in the n‐type Al0.7Ga0.3N layer is low with no dislocations in the field of view. The low dislocation density continued through the quantum wells and electron blocking layer. However, the lattice mismatch between the pseudomorphic AlxGa1‐xN layers and the p‐type GaN contact layer is high (approximately 2.4%) and the pseudomorphic growth could not be achieved for thick layers. Instead, a network of misfit dislocations was formed at the GaN/AlxGa1–xN interface, which relieved strain in the GaN layer. The threading dislocations did not appear to propagate into the active region and thus enable the achievement of high internal (IQE) and external quantum efficiencies (EQE) from the devices fabricated on low dislocation density pseudomorphic active layers. Values of EQE as high as 1.44% for a device emitting at 248 nm operated in pulsed mode were achieved. Devices were packaged in lead frame packages and achieved output powers of 45 mW at 1.5 A for a 257 nm device with a peak EQE of 0.8% at 200 mA when operated in pulsed mode. The thermal properties of the packaged devices limited high current performance in CW mode. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)