P-ohmic Contact Research Articles

The effect of humidity on the degradation mechanisms of GaN-based green laser diodes

The degradation behaviors of InAlGaN-based green laser diodes in humid ambients were studied. Nonhermetic laser diodes were aged at three different relative humidity conditions at constant current aging mode. For all nonhermetic devices failure occurred as a result of a large increase in the threshold current and sharp decrease of light output power. The reverse leakage current for the failures did not increase when the threshold current increased and slope efficiency decreased, indicating that the change in the threshold current was a result of a change in internal and/or mirror loss. Besides the threshold current increase, the photoluminescence intensity of the ridge region decreased more than 80% after aging compared with the non-aged companions. The moisture oxidized the p-Ohmic contact region of the semiconductor and caused the increase of the serial resistance. The mirror facet degradation was observed in scanning electron microscope. This led to change in reflectivity and light scattering and eventually to a large shift in the threshold current and slope efficiency of the device.

The controllable electronic characteristics and Schottky barrier of graphene/GaP heterostructure via interlayer coupling and in-plane strain

Establishing the desired heterostructures by assembling suitable semiconductor materials has shown significant potential for applications in next-generation micro-nano electronic devices. In this present contribution, we demonstrate the geometric structure and electronic properties of graphene/GaP heterostructure by first-principles calculations. It is found that the heterostructure is characterized by weak interlayer coupling accompanying the stable layer spacing of 3.40 Å and binding energy of −39.35 meV, meaning that the interlayer is dominated by van der Waals (vdW) force. The electronic band structure of free-standing graphene and GaP monolayers are preserved well. Meanwhile, a tiny bandgap of approximatively 20 meV at the Dirac point of graphene is opened, which is attributed to the breakdown of sublattice symmetry. In the ground state, the Schottky contact of p-type is present with the n-type and p-type SBH of 1.71 eV and 0.10 eV, respectively, which can be effectively induced by imposing interlayer coupling as well as in-plane strains. Especially, the transition of p-Schottky contact to p-Ohmic contact occurs when the layer spacing decreases to 3.20 Å or the strain increases to +2 %. These theoretical results may offer potential guiding principle in future electronic devices.

Influence of Thermal Annealing on the PdAl/Au Metal Stack Ohmic Contacts to p-AlGaN

In this study, a PdAl (20 nm)/Au (30 nm) metal stack scheme is used for forming low-ohmic-resistance contact on Mg-doped (1.5 × 1017 cm−3) p-type AlGaN at various annealing temperatures. Using a circular-transmission line model, the specific contact resistance (ρc) of PdAl/Au/p-AlGaN ohmic contact is determined via the current–voltage (I–V) characteristics. As-deposited contacts demonstrate non-linear behavior. However, the contact exhibits linear I–V characteristics with excellent ohmic contact of ρc = 1.74 × 10−4Ωcm2, when annealed at 600 °C for 1 min in a N2 atmosphere. The Ga and Al vacancies created at the PdAl/Au and p-AlGaN interfaces, which act as acceptors to increase the hole concentration at the interface. The out-diffusion of Ga as well as in-diffusion of Pd and Au to form interfacial chemical reactions at the interface is observed by X-ray photoelectron spectroscopy (XPS) measurements. The phases of the Ga–Pd and Ga–Au phases are detected by X-ray diffraction (XRD) analysis. Morphological results show that the surface of the contact is reasonably smooth with the root-mean-square roughness of 2.89 nm despite annealing at 600 °C. Based on the above experimental considerations, PdAl/Au/p-AlGaN contact annealed at 600 °C is a suitable p-ohmic contact for the development of high-performance electronic devices.

Surface hole gas enabled transparent deep ultraviolet light-emitting diode

The inherent deep-level nature of acceptors in wide-band-gap semiconductors makes p-ohmic contact formation and hole supply difficult, impeding progress for short-wavelength optoelectronics and high-power high-temperature bipolar electronics. We provide a general solution by demonstrating an ultrathin rather than a bulk wide-band-gap semiconductor to be a successful hole supplier and ohmic contact layer. Free holes in this ultrathin semiconductor are assisted to activate from deep acceptors and swept to surface to form hole gases by a large electric field, which can be provided by engineered spontaneous and piezoelectric polarizations. Experimentally, a 6 nm thick AlN layer with surface hole gas had formed p-ohmic contact to metals and provided sufficient hole injection to a 280 nm light-emitting diode, demonstrating a record electrical-optical conversion efficiency exceeding 8.5% at 20 mA (55 A cm−2). Our approach of forming p-type wide-band-gap semiconductor ohmic contact is critical to realizing high-efficiency ultraviolet optoelectronic devices.

A comparative study of Ti and Cr based p-ohmic contacts on high power GaAs laser diodes

The present work involves a comparison of sequentially evaporated Ti/Pt/Au and Cr/Au p-ohmic contacts for high power laser diode fabrication. Subsequently, the contacts were annealed at 440 °C in furnace annealing under N2 ambient and their surfaces were evaluated by optical and electron microscopy. The specific contact resistivity of the contacts was determined by circular transfer length method (c-TLM). Secondary ion mass spectroscopy (SIMS) analysis of the annealed contacts indicates considerable intermixing of the metal and Ga atoms across the interface for Cr/Au contact. On the other hand, insignificant intermixing of atoms was observed in Ti based contacts. The electrical and optical characteristics of the laser diodes formed with Ti/Pt/Au ohmic contact suggest superior performance compared to the diode formed with Cr/Au contact.

Improved Turn-On and Operating Voltages in AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes

While good ohmic contact formation has been achieved on both p-GaN and n-AlGaN surfaces, the turn-on and operating voltages of AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) remain very high. We find that this critical problem is mainly caused by the large difference between the annealing temperatures required for ohmic contact formation on the p-GaN and high Al-fraction n-AlGaN surfaces. We studied the effects of the high-temperature annealing treatments required for n-ohmic contact formation on the subsequent p-ohmic contact formation process in DUV-LEDs. The results show that post-annealing treatment at high temperature is necessary to form an ohmic contact on n-Al0.7Ga0.3N, but a treatment temperature of 900°C or more could cause severe degradation of the specific contact resistivity and the bulk resistivity of p-GaN. We conclude that 900°C is the optimum temperature to form an ohmic contact on n-Al0.7Ga0.3N in DUV-LEDs, where p-GaN and n-Al0.7Ga0.3N act as the p- and n-ohmic contact layers, respectively. We also found that the specific contact resistivity of p-GaN can be reduced by an additional low-temperature annealing treatment after the high-temperature annealing step; this effect can be attributed to the enhancement of the hole concentration in the p-GaN surface contact region. Finally, DUV-LEDs that emit at 280 nm were fabricated using four different annealing treatments during processing. A considerable reduction in the series resistance and thereby in the operating voltage was confirmed using the annealing process proposed above, consisting of a high-temperature anneal at 900°C followed by a low-temperature anneal at 500°C for 3 min.

Enhanced performance characteristics of n-ZnO/p-GaN heterojunction light-emitting diodes by forming excellent Ohmic contact to p-GaN

We report significantly improved performance characteristics of n-ZnO/p-GaN heterojunction light-emitting diodes (LEDs) by forming excellent Ohmic contact to p-GaN. An elaborately designed fabrication procedure to suppress ZnO sputtering damage on p-GaN, i.e., a selective SiO2 passivation of the p-GaN surface, was shown to yield excellent Ohmic contact to p-GaN with a specific contact resistance of 9.8×10−3Ωcm2, while the reference contact formed on non-protected p-GaN showed non-Ohmic behavior. Heterojunction LEDs fabricated with the improved p-Ohmic contact had much better performance characteristics than did the reference LEDs, i.e., the forward voltages at 2mA were 5.6 and 16.2V, the series resistances were 404 and 1693Ω, and the optical output powers at 3mA were 56.4 and 19.9μW for the developed and reference LEDs, respectively.

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 Pt and Ti on Ni/Ag/(Pt or Ti)/Au p-ohmic contacts of GaN based flip-chip LEDs

In this paper, we report the dependence of Ni/Ag/diffusion barrier (D.B)/Au p-ohmic contact on PtD.B and TiD.B for GaN based flip-chip light emitting diodes (FC LEDs). It is shown that D.B metals have strongly influenced on the reflectance and contact resistivity of contacts. We present these results are caused by the variation of the morphology and atomic distribution due to D.Bs. The roles of PtD.B and TiD.B on Ni/Ag/D.B/Au p-GaN ohmic contacts are analyzed using the confocal laser scanning microscopy (CLSM) measurement and the secondary ion mass spectroscopy (SIMS) profiles in details.

Influence of penetrating V-pits on leakage current of GaN based p-i-n UV detector

The leakage mechanism of GaN-based p-i-n （p-AlGaN/i-GaN/n-GaN） UV detector has been investigated. With the same dislocation density, devices made from material with higher density of V-pits on surface produce larger leakage current. SEM images show that some V-pits penetrate into i-GaN layer, sometimes even the n-GaN layer. If p-ohmic contact metal （Ni/Au） deposits in the V-pits, Schottky contact would be formed at the interface of metal and i-GaN, or form ohmic contact at the interface of metal and n-GaN. The existence of parallel Schottky junction and ohmic contact resistance enhances the leakage current greatly.

Aging and Stability of GaN High Electron Mobility Transistors and Light-Emitting Diodes With $\hbox{TiB}_{2}$- and Ir-Based Contacts

There is interest in developing more stable contacts to a variety of GaN-based devices. In this paper, we give two examples of devices that show improved thermal stability when boride or Ir diffusion barriers are employed in ohmic-contact stacks. AlGaN/GaN high electron mobility transistors (HEMTs) were fabricated with Ti/Al/X/Ti/Au source-drain ohmic (where X is or Ir) contacts and were subjected to long-term annealing at 350 . By comparison with companion devices with conventional Ti/Al/Ni/Au ohmic contacts, the HEMTs with boride-based ohmic metal showed superior stability of both source-drain current and transconductance after 25 days of aging at 350 . The gate current for standard HEMTs increases during aging, and the standard ohmic contacts eventually fail by shorting to the gate contact. Similarly, InGaN/GaN multiple quantum well light-emitting diodes (MQW-LEDs) were fabricated with either Ni/Au//Ti/Au or Ni/Au/Ir/Au p-ohmic contacts. Both of these contacts showed superior long-term thermal stability compared to LEDs with conventional Ni/Au contacts.

Characteristics of Multileaf Holey Light-Emitting Diodes for Fiber-Optic Communications

An oxide-confined multileaf holey light-emitting diode (LED) in the 780 nm range is reported. The device is consisted of bottom distributed Bragg reflector (DBR), quantum wells (QWs), and top DBR, with a multileaf holey structure within the p-ohmic contact ring for light extraction. The spontaneous emission from under the etched holes and internally reflected lights can be extracted and collimated out of the smaller etched leaf holes. High-resolution imaging studies indicate that the device emits with smaller beams mainly through the multileaf etched holes made it suitable for fiber-optic communications.

High Temperature Stable Contacts for GaN HEMTs and LEDs

AbstractThere is continued interest in developing more stable contacts to a variety of GaN-based devices. In this paper we give two examples of devices that show improved thermal stability when boride, nitride or Ir diffusion barriers are employed in Ohmic contact stacks. AlGaN/GaN High Electron Mobility Transistors (HEMTs) were fabricated with Ti/Al/X /Ti/Au source/ drain Ohmic (where X is TiB2, ZrN, TiN, TaN or Ir) contacts and subjected to long-term annealing at 350°C. For GaN layers with an electron concentration of ∼3×1017 cm-3, the minimum specific contact resistance achieved is 6×10-5 Ω cm2 for Ti/Al/TiN/Ti/Au after annealing at 800°C. The specific contact resistance was found to strongly depend on the doping level, suggesting that tunneling is the dominant mechanism of current flow. By comparison with companion devices with conventional Ti/Al/Ni/Au Ohmic contacts, the HEMTs with boride-based Ohmic metal showed superior stability of both source-drain current and transconductance after 25 days aging at 350°C. The gate current for standard HEMTs increases during aging and the standard Ohmic contacts eventually fail by shorting to the gate contact. Similarly, InGaN/GaN multiple quantum well light-emitting diodes (MQW-LEDs) were fabricated with either Ni/Au/TiB2/Ti/Au or Ni/Au/Ir/Au p-Ohmic contacts. Both of these contacts showed superior long-term thermal stability compared to LEDs with conventional Ni/Au contacts.

Improved long-term thermal stability of InGaN∕GaN multiple quantum well light-emitting diodes using TiB2- and Ir-based p-Ohmic contacts

In Ga N ∕ Ga N multiple quantum well light-emitting diodes (MQW-LEDs) were fabricated with either Ni∕Au∕TiB2∕Ti∕Au or Ni∕Au∕Ir∕Au p-Ohmic contacts and annealed at 200 and 350°C for 45days. By comparison with companion devices with conventional Ni∕Au Ohmic contacts fabricated on the same wafer, MQW-LEDs with TiB2- and Ir-based Ohmic metallization schemes showed superior long-term thermal stability as judged by the change in turn-on voltage, leakage current, and output power, a promising result for applications where reliable operation at high temperature is required.

Characteristics of unannealed ZnMgO∕ZnO p-n junctions on bulk (100) ZnO substrates

Zn 0.9 Mg 0.1 O ∕ Zn O p-n junctions were grown by pulsed laser deposition at ⩽500°C on bulk n-type, (100), nonpolar, a-plane ZnO substrates. No postgrowth annealing was performed, with the P-doped ZnMgO showing p-type conductivity (hole density ∼1016cm−3, mobility ∼6cm2V−1s−1) in the as-grown state. Front-to-back p-n junctions were fabricated with Ni∕Au used as the p-Ohmic contact and Ti∕Au as the backside n-Ohmic contact. The p contacts showed improved characteristics after annealing up to 400°C, but the n contacts were Ohmic as deposited. The junctions showed rectifying behavior up to 200°C. The forward turn-on voltage was ∼6.5V at 25°C. The simple, low-temperature growth and processing sequence show the promise of ZnO for applications in transparent electronics and UV light emitters.

Ohmic contacts for high power LEDs

We report novel metallization schemes of transparent p-ohmic contacts for conventional LEDs and reflective p-ohmic contacts for flip-chip and vertical-structure LEDs. Thermally stable and low-resistivity Ru/Ni/ITO ohmic contacts on p-type GaN resulted in the low contact resistivity of 2 x 10 -4 Ω cm 2 and the high transmittance of 92% at 470 nm wavelength. The light output power of the LED with the Ru/Ni/ITO p-contact was increased by 50% compared to the LED of a Ni/Au transparent p-contact. Using a newly developed Ni/Ag/Ru/Ni/Au reflective p-ohmic contact, the low contact resistivity of 5 x 10 -5 Ω cm 2 could be achieved. The light reflectance of the contact was as high as 90% at 470 nm wavelength.

Zn 0.9 Mg 0.1 O ∕ ZnO p – n junctions grown by pulsed-laser deposition

The electrical characteristics of Zn0.9Mg0.1O∕ZnOp–n junctions grown by pulsed-laser deposition on bulk, single-crystal ZnO substrates are reported. The forward turn-on voltage of the junctions was in the range 3.6–4V for Pt∕Au metallization used for the p-Ohmic contact on Zn0.9Mg0.1O.The reverse breakdown voltage is as high as 9V, but displays a small negative temperature coefficient of −0.1–0.2VK−1 over the range 30–200°C. The achievement of acceptable rectification in the junctions required growth of an n-type ZnO buffer on the ZnO substrate prior to growth of the p-type, phosphorus-doped Zn0.9mg0.1O.Without this buffer, the junctions showed very high leakage current.

Single Wall Carbon Nanotubes for p-Type Ohmic Contacts to GaN Light-Emitting Diodes

Homogeneous films of pure single wall carbon nanotubes (SWNTs) sufficiently thin to be optically transparent in the visible range of the spectrum were employed as p-Ohmic contacts on GaN-InGaN quantum-well light-emitting diodes. The specific contact resistance of the SWNT films on the p-GaN was 1.1 x 10 -2 Ω cm 2 after annealing at 700°C for 60 s under N 2 , which was a factor of 3 lower than standard Ni/Au contacts on the same p-GaN. The SWNT-contacted LEDs showed bright blue emission centered at 434 nm and demonstrate that the SWNT films provide a new class of electrically conducting, p-type, transparent electrode for use with photonic devices.

Effect of thermal annealing of Ni/Au ohmic contact on the leakage current of GaN based light emitting diodes

The effect of thermal annealing on current–voltage properties of GaN light emitting diodes (LEDs) has been studied. At annealing temperatures above 700 °C, the p–n junction of the diodes became very leaky and Ga-contained metallic bubbles were observed on the surface of Ni/Au p-ohmic contact. Transmission electron microscopy and energy dispersive x-ray spectrometer studies revealed that these metallic bubbles resided directly on top of the threading dislocations in GaN and both Ni and Au were indiffused into the LED structure along the cores of the TDs. The conducting paths formed by the metal containing dislocation cores are believed to be the cause for the observed short circuit behavior of p–n junctions at high annealing temperatures.

Comparison of ohmic contact properties on n-GaN/p-SiC and n-AlGaN/p-SiC heterojunctions

GaN/SiC and Al 0.25Ga 0.75 N/SiC heterojunction diodes were fabricated using Al/Ti for p-ohmic contact to the SiC and Ti/Al/Pt/Au for n-ohmic contact to the GaN and AlGaN. Annealing at 850 °C for 20 s (GaN) or 120 s (AlGaN) was required for achieving specific contact resistances in the 10 −6 Ω cm 2 range. The reverse breakdown voltage showed a negative temperature coefficient in both types of sample, with value ∼5.5±2.5×10 −3 V/K. The I– V characteristics of both heterojunctions show evidence of tunneling via defect states.