GaN Contact Research Articles

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]

Carrier Transport Mechanism at the Interface between Metals and p-Type III–Nitrides Having Different Surface Electronic Structures

We have investigated the carrier transport mechanism for nonalloyed Ni/Au ohmic contacts to p-In0.15Ga0.85N/GaN superlattices (SLs) and Mg delta (δ)-doped GaN using current–voltage–temperature (I–V–T) and specific contact resistance–temperature (Rsc–T) data. The p-SL and Mg δ-doped GaN contacts show a linear I–V behavior, indicating that the Mg δ doping in the GaN matrix as well as p-SL is very effective in improving the electrical properties of p-ohmic contacts. From the Rsc–T and I–V–T measurements, the p-SL and Mg delta-doped GaN contacts show slight temperature-dependence characteristics at the temperatures of ≥293 K. These different behaviors can be described in terms of a reduced energy band-bending, an increased hole carrier density, and a reduced Schottky barrier height.

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.

Ultra-low resistance ohmic contacts to GaN with high Si doping concentrations grown by molecular beam epitaxy

Ti/Al/Ni/Au ohmic contacts were formed on heavily doped n+ metal-polar GaN samples with various Si doping concentrations grown by molecular beam epitaxy. The contact resistivity (RC) and sheet resistance (Rsh) as a function of corresponding GaN free carrier concentration (n) were measured. Very low RC values (<0.09 Ω mm) were obtained, with a minimum RC of 0.035 Ω mm on a sample with a room temperature carrier concentration of ∼5 × 1019 cm−3. Based on the systematic study, the role of RC and Rsh is discussed in the context of regrown n+ GaN ohmic contacts for GaN based high electron mobility transistors.

Investigation of Ni/Ag contact to p‐GaN with an O2 plasma treatment and its application to GaN‐based LEDs

AbstractIn this paper, the contact structure of Ni–Ag/p‐GaN with different O2 plasma treatment times for 1 and 5 min was utilized for investigation. From experimental results, the Ni–Ag contact to p‐GaN with different O2 plasma treatment for 1 and 5 min reveals Schottky behaviors. Based on the variation of the specific contact resistance with respect to temperature, the dominant transport mechanism of Ni–Ag/p‐GaN structure presented form thermionic emission to field emission as increasing time from 1 to 5 min. From the X‐ray photoelectron spectroscopy (XPS) results, the increase of the N vacancies and antisite defects (ON) would enhance the resistance of the treated p‐GaN underneath the contact, which would make the Ni–Ag/p‐GaN contact reveal Schottky behavior. With a 20 mA current injection, the operation voltage of light‐emitting diodes (LEDs) with Ni/Ag contact to p‐GaN through O2 plasma treatment 400 W–5 min was 3.13 V larger than the LEDs with Ni/Ag contact to p‐GaN through O2 plasma treatment 400 W–1 min. This is due to the larger specific contact resistance. The 3.13 V operation voltage is still good and acceptable. Furthermore, the largest output power among all devices can be achieved. Besides, the reliability is still good.

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)

Investigations on the Origin of the Ohmic Behavior for Ti/Al Based Contacts on n-Type GaN

This work reports of investigation on the origin of the ohmic behavior for Ti/Al basedcontacts on n-GaN. Indeed, last publications tend to prove that the ''nitrogen vacancy'' theory is notconfirmed by the experimental ground available. To find new answers, we first made electrical charac-terizations on some Al/Ti/n+-GaN contacts with different annealing conditions. Then, we performedSIMS depth profiles of annealed TiN/Ti/n+-GaN samples. Results developed in this paper tend todemonstrate that the ohmic behavior of Ti/Al based contact is mainly due to an increase of the dop-ing concentration. Moreover, the high doping level can be explained by an in-diffusion of Ti and anexo-diffusion of Si elements close to the metal / semiconductor interface.

The effect of ITO annealing on electrical characteristic of GaN based LED

In the recent years，more and more light-emitting diodes use indium tin oxide (ITO) as the current spreading layer. But if there is not any treatment, the electrical properties of light emitting diode are very poor. So to get excellent electrical properties of light emitting diode, annealing is an effective method to improve the electrical properties of light emitting diode using indium tin oxide as the current spreading layer. However, the annealing time and temperature can affect the electrical property of light emitting diode individually. In order to investigate this problem，we measured the series resistance and ideality factor of the light emitting diode got under different annealing time and annealing temperature. According to the model proposed by Jay M. Shah，we can inferred the characteristics of indium tin oxide and P-type GaN contact. The results showed that: the electrical properties of the light-emitting diode can reach an excellent value with increasing annealing temperature and time, and if continuing to increase in temperature or time, it can lead to a decline in light-emitting diode electrical properties. It is very helpful to optimize the annealing temperature and time and manufacture excellent electrical properties of devices.

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)

Characterization of silicon ion-implanted GaN and AlGaN

Electrical properties for Si-implanted n-type GaN and AlGaN layers and contact resistances of ohmic contacts (Ti/Al) formed on these layers were examined. Experimental results clearly show that ohmic contacts on n-type GaN and n-type AlGaN with a low specific-contact resistance of under 10 −7 and 10 −6 Ω cm 2 can be fabricated on the high concentration n-type layers, which have been formed by Si ion implantation (30 keV: 1 × 10 14, 5 × 10 14, 1 × 10 15 and 2 × 10 15/cm 2) and subsequent annealing at 1250 °C for 2 min using a Si 3N 4 as a surface protection layer.

Enhanced Output Power of InGaN-Based Light-Emitting Diodes With AlGaN/GaN Two-Dimensional Electron Gas Structure

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We demonstrate high-performance InGaN-based light-emitting diodes (LEDs) with tunneling-junction-induced 2-D electron gas (2DEG) at an AlGaN/GaN heterostructure, which is inserted in the middle of the <formula formulatype="inline"><tex Notation="TeX">$\hbox{P}^{+}$</tex></formula>-GaN contact layer of a conventional LED structure. The output power of a LED with a 2DEG insertion layer shows 17% enhancement compared to that of a conventional LED at 20 mA. This enhancement in output power for the LED with a 2DEG insertion layer could be attributed to both enhanced hole-injection efficiency and lateral current spreading by the presence of 2DEG at the AlGaN/GaN heterostructure. </para>

Laser-induced Zn doping in GaN based light-emitting diode

By laser-induced Zn doping, hole concentration in the p-type GaN contact layer of conventional GaN-based light-emitting diodes (LEDs) is increased and improvement of the LED property is confirmed. Compared with LED with no use of laser-induced doping, the forward voltage under 20 mA current is decreased from 3.33 to 3.13 V and the thermal resistance of the chip is decreased from 18.6 to 9.7 K/W. In addition, the lifetime of the device is increased about 41%. These results are attributed to the improvement of the p-type Ohmic contact due to laser-induced doping of Zn to the p-GaN contact layer.

Polarization-charge tunnel junctions for ultraviolet light-emitters without p-type contact

Polarization-charge tunnel junctions for ultraviolet light-emitters (λ&amp;lt;360 nm) are modeled using the k⋅p multiband quantum transmitting boundary method. It is shown that polarization-charge tunnel junctions can carry sufficient current to be viable for ultraviolet emitters. Sources of inefficiency in existing ultraviolet emitters are discussed, including poor hole-injection efficiency and optical absorption in the p-type GaN contact layers. It is demonstrated that polarization-charge tunnel junctions can deliver improvements in both areas, by eliminating the need for p-type contacts, and by enabling the use of a p-side-down structure, which is shown to have favorable characteristics for carrier transport compared to conventional p-side-up structures.

Properties of GaN(SiC)-(Ti, Zr)B x contacts subjected to rapid thermal annealing

The effect of rapid thermal annealing on the structural and physical properties of Au-(Ti, Zr)Bx- GaN(SiC) contacts and diode structures on their basis is investigated. The Xraydiffraction investigations and the layerbylayer Auger analysis showed that th e phase composition and structure of the GaN and SiC contacts are retained to the temperatures as high as 900 and 1000°C, respectively. The stability of interphase boundaries is confirmed by almost constant physical properties of contacts before and after the rapid thermal treatments. The Schottkybarrier height ϕb amounts to 0.89-0.9 eV for the contacts with GaN and 0.79- 0.83 eV for SiC; the ideality factor n of the I-V characteristic amounts to n = 1.2 for the Au-TiBx(ZrBx)- nGaN(SiC) contacts and n ≈ 1.12 for the Au-ZrB x -nn + �4 HSiC contacts. The structural investigations indicate that there are glassforming boron and metal oxides at an interphase boundary, which form a thin amorphous vitreous layer resistant against rapid thermal annealing and represent the diffusion barrier for the interphase transport.

Electrical properties of GaN/poly(3-hexylthiophene) interfaces

Interfaces between wide-bandgap semiconductors and polymeric electronic materials are model systems for geometrically more complicated interfaces formed in nanostructured composite electronic, photonic, and photovoltaic devices. The wide-bandgap semiconductor GaN is readily available with well-defined electronic and structural properties, including reproducible control of doping and conductivity type, and can ideally serve as the inorganic side of the model system. Electron transport through a GaN/poly(3-hexylthiophene) (P3HT) semiconductor heterojunction depends on the conductivity type of the GaN and on the doping level in the polymer. The total contact resistance of a planar P3HT film with GaN contacts in a symmetric p-GaN/P3HT/p-GaN structure is consistent with the contribution of reversed-biased junction at one of the GaN/P3HT interfaces. An n-GaN/P3HT/n-GaN structure has a lower total resistance than the p-GaN structure, possibly arising from band-to-band tunneling at the interface. Doping the P3HT layer with iodine greatly reduced the contact resistance for interfaces with both conductivity types of GaN.

Intermixing and chemical structure at the interface between n-GaN and V-based contacts

The interface between n-type GaN and V-based contacts was characterized by soft x-ray spectroscopy. We have investigated the chemical interface structure before and after a rapid thermal annealing (RTA) step, which is crucial for the formation of an Ohmic contact. X-ray photoelectron and x-ray excited Auger electron spectra suggest that RTA induces an accumulation of metallic Ga at the surface. Using x-ray emission spectroscopy, we find that the probed nitrogen atoms are in a VN-like environment, indicating that vanadium interacts with nitrogen atoms from the GaN to form VN.

Barrier inhomogeneity and electrical properties of Pt∕GaN Schottky contacts

The temperature dependence of the electrical properties of Pt∕GaN Schottky barrier was studied. In particular, a Schottky barrier height of 0.96eV and an ideality factor of 1.16 were found after a postdeposition annealing at 400°C. Nanoscale electrical characterization was carried out by the conductive biased tip of an atomic force microscope both on the bare GaN surface and on the Pt∕GaN contacts. The presence of a lateral inhomogeneity of the Schottky barrier, with a Gaussian distribution of the barrier height values, was demonstrated. Moreover, GaN surface defects were demonstrated to act as local preferential paths for the current conduction. The temperature dependent electrical characteristics of the diodes were discussed in terms of the existing models on inhomogeneous barriers and correlated to the nanoscale electrical characterization of the barrier. In this way, the anomalous electrical behavior of the ideality factor and of the Schottky barrier and the low experimental value of the Richardson’s constant were explained.

Deep-Level Defects in MBE-Grown GaN-Based Laser Structure

In this paper we present results of deep-level transient spectroscopy (DLTS) investigations of defects in a GaN-based heterostructure of a blue-violet laser diode, grown by plasma-assisted molecular beam epitaxy on n + -type, dislocation free, bulk GaN substrate synthesized by a high-pressure high-temperature method. The heterostructure contained multiple InGaN quantum well as the active region, which was embedded between two, n-type (Si-doped) and p-type (Mg-doped), GaN wave-guiding layers and two, nand p-type, AlGaN cladding layers, and was covered with a highly Mg-doped GaN contact layer; see [1] for the exact sequence of the layers. The use of a p-n junction of the diode allowed for investigation of both majorityand minority-carrier traps with DLTS. In addition, the kinetics for capture of charge carriers into the defect states, measured by means of recording the dependence of the DLTS-signal amplitude on the filling-pulse duration, provided information on the defect geometry, which allowed distinguishing point defects from extended defects, such as dislocations. Two majority-carrier traps, called T1 and T2, with the activation energies of 0.28 eV and 0.33 eV, respectively, have been revealed in the DLTS spectra measured in the temperature range 80–400 K under typical bias conditions, i.e. under reverse quiescent bias, which was decreased during the filling pulse. On the other hand, DLTS measurements performed under injection conditions, i.e. under zero quiescent voltage and forward-bias filling pulse, revealed one minority-carrier trap, T3, with the activation energy of 0.20 eV. The two majority-carrier traps have been revealed in the DLTS spectra measured under different reverse bias conditions, suggesting that they are present in different parts of the laser-diode heterostructure. In addition, the two traps represent different charge-carrier capture behaviours. The T1 trap, which exhibits the logarithmic capture kinetics, is tentatively attributed to electron states of dislocations in the n-type wave-guiding layer of the structure. In contrast, the T2 trap displays the exponential capture kinetics and is assigned to point defects in the p-type wave-guiding layer of the structure. Additional information on the nature of electronic states of deep levels related to dislocations has been obtained from analysis of the dependence of DLTS-line shape of the T1 trap on the filling-pulse duration. This analysis allowed us to specify the type of electronic states associated with dislocations, which, owing to translational symmetry along dislocation lines, are expected to form one-dimensional energy bands rather than isolated localized electron states. According to the model recently proposed by Schroter et al. [2], which takes into account the rate at which the states reach their internal electron equilibrium within the band, we classify the revealed dislocation-related T1 trap to so-called band-like states in the cores of dislocations.