n-GaN Surface Research Articles

Effect of oxygen plasma treatment on nonalloyed Al/Ti-based contact for high power InGaN/GaN vertical light-emitting diodes

InGaN/GaN vertical light emitting diodes (LEDs) with argon (Ar) and oxygen (O2) plasma-treated nonalloyed Al/Ti electrodes were fabricated on sapphire substrates. At the operating current of 350 mA, the forward voltage (VF) for O2 plasma-treated Al/Ti-based devices with dimensions 1360 × 1360 μm2 was improved, whose value was comparable or lower to that of nonalloyed Cr/Au-based devices. The Al/Ti electrodes resulted in improvement in optical output power of LEDs due to their high reflectivity (typically 10%–15% higher based on our data) compared to LEDs with conventional Cr/Au-based electrodes. The x-ray photoelectron spectroscopy showed the increase in Ga-O peak intensity during O2 plasma treatment. These results demonstrate that O2 plasma-treated Al/Ti electrodes reduced the contact resistance by forming a thin conductive GaOxN1−x layer at n-GaN surface.

Light-Extraction Enhancement by Cavity Array-Textured N-Polar GaN Surfaces Ablated Using a KrF Laser

The creation of KOH-etched pyramidal patterns on the N-GaN surface is considered to be the most effective texturing method for improving the light-extraction efficiency of GaN LEDs. Using KrF-laser ablation, concave downward cavities are formed on the N-GaN surface. This letter demonstrates that KrF-laser-ablated cavities enhance the light-extraction efficiency of the KOH-etched pyramidal N-GaN surface by 25%. With further etching by KOH, the curved-surface sidewall of laser-ablated cavities do not form pyramids; instead, relatively large inclined facet sidewalls are formed in the laser-ablated cavities. We believe that these inclined facet sidewalls in laser-ablated cavities further enhance the light-extraction efficiency of KOH-etched pyramidal N-GaN surfaces.

Improved Output Power of InGaN LEDs by Lateral Overgrowth on Si-Implanted n-GaN Surface to Form Air Gaps

In this paper, air gaps were embedded in the n-GaN layer to improve light output power of InGaN-based light-emitting diodes (LEDs). Si ions (Si±28) were implanted on the n-GaN surface, causing a lattice constant disorder. Therefore, the GaN grown on the Si-implanted areas had a lower growth rate than the implantation-free regions. Without using a dielectric thin film, lateral epitaxial overgrowth technique was used to form air gaps above the implanted regions and below the active layers of InGaN LEDs. We proposed the growth mechanisms of GaN layer on the Si-implanted GaN templates and characterized the InGaN-based LEDs with embedded air gaps array. With a 20-mA current injection, experimental results indicate that light output power (LOP) of the proposed LEDs was enhanced by 36%, compared with those of the conventional LEDs. This enhancement can be attributed to the light scattering at the textured GaN/gap interfaces to increase the effective light escape cone in the LEDs. Based on ray tracing simulation, if the height and the width of bottom of gaps were increased to 3 μm, the Lop could be enhanced over 70%.

Plasma-Induced Damage and Recovery on Au/n-GaN Schottky Diode in Different Processes

The effects of plasma-induced damage on deep traps in n-GaN have been investigated using current–voltage (I–V), capacitance–voltage (C–V), and photocapacitance (PHCAP) measurements. The Au/n-GaN Schottky barrier diodes were fabricated in an inductively coupled plasma ion etching (ICP-RIE) system. After mesa etching to achieve ohmic contact, the n-GaN surface, at which Schottky contacts are fabricated, is etched ∼100 nm by ICP-RIE with various Cl2/Ar ratios and RIE bias powers (PB), to introduce plasma damage. The electrical properties of the fabricated Shottky barrier diodes (SBDs) strongly dependent on the RIE gas composition and the bias power PB applied to the sample stage. In order to overcome the residue and plasma damage on the Schottky area, the samples were treated with HCl at 110 °C for 30 min. Several deep levels (1.8, 2.5, and 3.0 eV below the conduction band) were detected by PHCAP measurement. Improved electrical characteristics were achieved as a result of the HCl treatment and sintering process. The PHCAP measurement results also revealed the effectiveness of thermal and chemical treatments.

Plasma-Induced Damage and Recovery on Au/n-GaN Schottky Diode in Different Processes

The effects of plasma-induced damage on deep traps in n-GaN have been investigated using current–voltage (I–V), capacitance–voltage (C–V), and photocapacitance (PHCAP) measurements. The Au/n-GaN Schottky barrier diodes were fabricated in an inductively coupled plasma ion etching (ICP-RIE) system. After mesa etching to achieve ohmic contact, the n-GaN surface, at which Schottky contacts are fabricated, is etched ∼100 nm by ICP-RIE with various Cl2/Ar ratios and RIE bias powers (PB), to introduce plasma damage. The electrical properties of the fabricated Shottky barrier diodes (SBDs) strongly dependent on the RIE gas composition and the bias power PB applied to the sample stage. In order to overcome the residue and plasma damage on the Schottky area, the samples were treated with HCl at 110 °C for 30 min. Several deep levels (1.8, 2.5, and 3.0 eV below the conduction band) were detected by PHCAP measurement. Improved electrical characteristics were achieved as a result of the HCl treatment and sintering process. The PHCAP measurement results also revealed the effectiveness of thermal and chemical treatments.

Enhanced light extraction of GaN-based light emitting diodes via simultaneous ITO texturing and n-GaN nanorod formation using Al2O3 powder

The authors have fabricated the surface-textured GaN based light emitting diodes (LEDs) by incorporating a transparent powder onto an indium tin oxide (ITO) surface and exposed n-GaN surface to improve the light extraction efficiency by decreasing the total internal reflection and by achieving an angular randomization of the photons. The ITO surface and exposed n-GaN surface was simultaneously textured using a method known as natural lithography. The Al2O3 powders were coated onto the LED surface as a random mask for dry etching. After packaging, the light output powers of the LEDs with the textured ITO layer and with nanorods at n-GaN were enhanced about ∼ 11 and 15 %, respectively, compared with that of the conventional LED. Also, the light output power of the LED with both the textured ITO layer and the formed n-GaN nanorods increased by ∼ 24% compared with that of the conventional LED.

Low resistance and thermally stable Ti/Al-based Ohmic contacts to N-face n-GaN for vertical light-emitting diodes by using Ti(Ga) solid solution and TiN layers

The authors report on the formation of highly reliable Ti/Al-based ohmic contacts to N-face n-GaN for high-performance vertical light-emitting diodes by using Ti(Ga) solid solution and TiN layers. The Ti(Ga) solid solution layer is used to minimize the outdiffusion of Ga atoms from the n-GaN surface region. Unlike the Ti/Al contacts, the Ti(Ga)/Ti/Al and Ti(Ga)/TiN/Al samples exhibit ohmic behavior with contact resistivities of 3.9 – 4.8 × 10−4 Ωcm2 after annealing at 250 °C. It was further shown that unlike the Ti(Ga)/TiN/Al samples, the Ti/Al and the Ti(Ga)/Ti/Al samples are largely electrically degraded when annealed at 300 °C in an oven. Based on x-ray photoemission spectroscopy and secondary ion mass spectrometry results, ohmic formation and degradation mechanisms are briefly described and discussed.

High-power single-chip InGaN blue light-emitting diode with 3.3 W output power

The characteristics of a high-power single-chip blue light-emitting diode (LED) operating with >10 W input power are reported. The LED chip was fabricated in the form of a vertical-injection structure with chip dimensions of 1.8×1.8 mm. Electrode patterns at the n-GaN surface were designed to optimise the output power and operation voltage. Electrical and optical characteristics of the LED were measured up to 3 A injection current under pulsed operation condition. Output power and forward voltage at 3 A were obtained to be 3.3 W and 3.67 V, respectively, which demonstrates the wall-plug efficiency of >30% with 10 W input power. Even higher output power and efficiency are expected from the single-chip LED by reducing the efficiency droop of InGaN-based LEDs.

Formation of Low-Resistant and Thermally Stable Nonalloyed Ohmic Contact to N-Face n-GaN

We report on the formation of Ohmic contacts with low resistance and high thermal stability to N-face n-GaN for vertical structure light emitters using a Ti(50nm)/Pt(50nm)/Au(50nm) metal scheme, where the Pt layer is introduced as a blocking layer to suppress the diffusion of Au onto the N-face n-GaN surface. It is shown that unlike the conventional Ti/Al/Ti/Au contacts, the Ti/Pt/Au contacts exhibit an Ohmic behavior with a relatively low specific contact resistivity of 1.1×10−4 Ω·cm2 even after annealing at 350°C. X-ray diffraction (XRD) measurements by synchrotron radiation and Auger electron spectroscopy (AES) examination are performed to understand the effects of heat treatment.

Effects of W diffusion barrier on inhibition of AlN formation in Ti/W/Al ohmic contacts on N-face n-GaN

We investigate the effect of W diffusion barrier in Ti/W/Al ohmic contacts formed on N-face n-GaN. The contacts exhibit contact resistivity of as low as 2.3 × 10−4 Ω cm2 and better thermal stability than Ti/Al contacts. Cross-sectional transmission electron microscopy micrographs reveal that in-diffused Al atoms on the n-GaN surface react with N atoms to form an AlN layer in Ti/Al contacts, resulting in upward band bending, and consequently, a high contact resistivity. The use of a 10-nm-thick W layer suppresses the in-diffusion of Al atoms to n-GaN, thereby preventing the formation of AlN and enhancing the thermal stability of Ti/W/Al contacts.

Non-alloyed Cr/Au Ohmic contacts to N-face and Ga-face n-GaN

Non-alloyed Cr/Au Ohmic contacts on N-face and Ga-face n-GaN were studied. The specific contact resistances ( ρ c) of Cr/Au contacts onto the N-face and Ga-face n-GaN were as low as 2.4 × 10 −4 Ω cm 2 and 2.4 × 10 −5 Ω cm 2, respectively. Native oxide formed on the n-GaN surface was believed to be the key factor for higher ρ c. The results of X-ray photoelectron spectroscopy confirmed that n-GaN samples with different surface polarities or treated by different chemical solutions exhibited significant differences in gallium oxide content on the surface, which led to a marked difference in the ρ c of non-alloyed Cr/Au Ohmic contacts to GaN films.

The Impact of Bulk Defects, Surface States, and Excitons on Yellow and Ultraviolet Photoluminescence in GaN

The quantitative analysis of the in uence of deep bulk levels, surface states and excitons on yellow, green and ultraviolet photoluminescence from n-type GaN was performed. The theoretical calculations of recombination rates in the bulk and at n-GaN surface versus UV-excitation intensity were done numerically using nite element method basing on drift-di usion model assuming point deep levels, continuous energetic distribution of surface states, as well as excitons. The obtained results of the photoluminescence intensity were compared with experimental data (measured within the range from 10 to 10 photon cm−2 s−1) for n-GaN samples with various surface passivating layers (Al2O3, SiO2).

Stability of Al/Ti/Au contacts to N-polar n-GaN of GaN based vertical light emitting diode on silicon substrate

The electrical characteristics of Al/Ti/Au contact to N-polar n-GaN on Si substrate are investigated. It was found that a pre-treatment to the surface with Ar plasma could significantly enhance the stability of Al/Ti/Au contact. Forward voltage of the pre-treated sample was stabilized at about 3.23 V upon 1000 h aging under 900 mA and room temperature. In contrast, forward voltage of the untreated sample increased from 3.52 V to 4 V after 24 h aging. Those differences between the Ar plasma treated sample and untreated sample were attributed to the increase of the VN concentrate near surface of n-GaN by the Ar plasma treatment.

Optimized ICP etching process for fabrication of oblique GaN sidewall and its application in LED

Inductively coupled plasma (ICP) etching of GaN is systemically investigated by changing ICP power/RF bias power, operating pressure, and Cl2/BCl3 gas mixing ratio. The hexagonal etch pits related to screw dislocation existing along GaN epitaxial layer were observed on the etched GaN surface after ICP etching. The intensity of band-edge emission is significantly reduced from the etched n-GaN surface, which reveals that plasma-induced damage are generated after ICP etching. The oblique sidewall is transferred into GaN using a combination of Cl2/BCl3 plasma chemistry and hard mask SiO2. By adjusting ICP etching process parameters, oblique sidewalls with various oblique angles can be formed, allowing for conformal metal lines coverage across the mesa structures, which can play an important role in the interconnection of multiple microchips for light emitting diodes (LEDs) fabrication.

2D degenerate electron gas at Ba/n-AlGaN and Ba/n-GaN interfaces

Ba/n-GaN(0001) and Ba/n-AlGaN(0001) interfaces were investigated for the first time by means of ultraviolet photoelectron spectroscopy. The spectra of the photoemission from a valence band along with the spectra of the core levels of Ga 3d, Al 2p, and Ba 4d were studied. The formation of a 2D degenerate electron gas (an accumulated layer on the n-GaN and n-AlGaN surfaces during adsorption of Ba atoms) was revealed.

Efficient Wet Etching of GaN (0001) Substrate with Subsurface Damage Layer

Photoenhanced chemical (PEC) etching is applicable for processing an n-GaN (0001) surface rapidly. In this process, the surface oxidation is enhanced by photo-generated holes and the resulting oxide can dissolve into solutions. In current work, we conduct bias-assisted PEC etching in a KOH solution with a positively biased wafer, to remove the crystallographically highly damaged layer. The employed substrate was mechanically polished with diamond slurry of sub-micrometer particle size. Without the positive bias, the rate of PEC etching was quite low because the photogenerated holes were quickly depleted by the recombination process at the crystallographic defects and they could not contribute to the oxidation. On the other hand, in the case where the bias was applied, the photogenerated holes and electrons are separated forcibly in the band-bended surface, which effectively contributed to surface oxidation. As a result, a high removal rate was realized even on the damaged surface.

Electrical Characterization of GaN p–n Junctions Grown on Freestanding GaN Substrates by Metal–Organic Chemical Vapor Deposition

The electrical characteristics of GaN p–n junctions grown on freestanding GaN substrates by metal–organic chemical vapor deposition were investigated. The current–voltage (I–V) characteristics of the GaN p–n diode showed relatively low values and, little temperature dependence of the reverse leakage current. The breakdown voltage of the GaN p–n diode was 900 V and the leakage current at 600 V was 1 µA/cm2 or less. The inverse of squared capacitance–voltage (1/C2–V) data showed a linear relationship, indicating an abrupt junction at the p–n interface. Little temperature dependence of the C–V curve was also observed. Plasma etching before the growth of the p-GaN layer induced a large leakage current and a lower breakdown voltage, probably due to the disorder of atomic bonds and the nitrogen depletion at the plasma-etched surface. The insertion of a regrown n-GaN layer between the p-GaN and the plasma-treated n-GaN surface was effective in suppressing the leakage current and recovering the breakdown voltage.

Electrical Characterization of GaN p–n Junctions Grown on Freestanding GaN Substrates by Metal–Organic Chemical Vapor Deposition

The electrical characteristics of GaN p–n junctions grown on freestanding GaN substrates by metal–organic chemical vapor deposition were investigated. The current–voltage (I–V) characteristics of the GaN p–n diode showed relatively low values and, little temperature dependence of the reverse leakage current. The breakdown voltage of the GaN p–n diode was 900 V and the leakage current at 600 V was 1 µA/cm2 or less. The inverse of squared capacitance–voltage (1/C2–V) data showed a linear relationship, indicating an abrupt junction at the p–n interface. Little temperature dependence of the C–V curve was also observed. Plasma etching before the growth of the p-GaN layer induced a large leakage current and a lower breakdown voltage, probably due to the disorder of atomic bonds and the nitrogen depletion at the plasma-etched surface. The insertion of a regrown n-GaN layer between the p-GaN and the plasma-treated n-GaN surface was effective in suppressing the leakage current and recovering the breakdown voltage.

含有p-GaN 纳米阵列的InGaN/GaN 双异质结太阳能电池的制作

A method with p-GaN nanorod arrays is proposed to enhance the external quantum efficiency (EQE) of p-GaN/i-In- GaN/n-GaN double heterojunctional solar cells. Inductively coupled plasma ethcing is utilized to form the p-GaN nanorod arrays with self-assembled Ni cluster as the etching mask. To form a smooth n-GaN surface for subsequent metal deposition, we demonstrate two-step etching of n-GaN mesa. The peak EQE of solar cells with p-GaN nanorod arrays reaches 55%, which shows an enhancement of 10% as compared with the conventional device with p-GaN film.

Fabrication of vertical thin-GaN light-emitting diode by low-temperature Cu/Sn/Ag wafer bonding

Vertical thin-GaN LED was successfully fabricated on the GaN LED epi-layers grown on the patterned-sapphire substrate with the pyramidal pattern by low-temperature Cu/Sn/Ag wafer bonding at 150 °C. An inverted pyramidal pattern formed on the n-GaN surface after the GaN epi-layer was transferred onto Si wafer, which resulted from the pyramidal pattern on the patterned-sapphire substrate. The inverted pyramidal pattern has an equivalent function with roughening the n-GaN surface. With higher inverted pyramidal pattern coverage, the light extraction efficiency can be greatly enhanced. In addition, we found that the 4-fold increase (from 13.6% to 53.8%) in the pyramidal pattern coverage on patterned-sapphire substrate only gives the GaN LED epi-layer about 5.7% enhancement in the internal quantum efficiency.