Showing papers by "Ho Won Jang published in 2004"

Mechanism for the increase of indium-tin-oxide work function by O2 inductively coupled plasma treatment

Abstract: The effects of O2 inductively coupled plasma (ICP) treatment on the chemical composition and work function of indium-tin-oxide (ITO) surface were investigated. Synchrotron radiation photoemission spectroscopy showed that the O2 ICP treatment resulted in the increase of the ITO work function by 0.8 eV. Incorporation of oxygen atoms near the ITO surface during the ICP treatment induced a peroxidic ITO surface, increasing the work function. The enhanced oxidation of a thin Ni overlayer on the O2-ICP-treated sample suggests that preventing the migration of oxygen atoms into the active region of organic light-emitting diodes is important for improving device lifetime.

Mechanism for ohmic contact formation of Ni∕Ag contacts on p-type GaN

Abstract: The mechanism for ohmic contact formation of Ni∕Ag contacts on p-type GaN was investigated using synchrotron photoemission spectroscopy. A low contact resistivity of 6.6×10−5Ωcm2 was obtained from Ni(50A)∕Ag(1200A) contact after annealing at 500°C in O2 ambient. Ni out-diffused to form a NiO and Ag in-diffused into the contact interface during the oxidation annealing. Out-diffused Ga atoms from GaN could dissolve in the Ag layer to form Ag–Ga solid solutions, leaving Ga vacancies below the contact. Ga vacancies could increase the net hole concentration and reduce the surface band bending, resulting in the ohmic contact formation.

P-type conductivity in bulk AlxGa1-xN and AlxGa1-xN/AlyGa1-yN superlattices with average Al mole fraction > 20%

Abstract: Synchrotron radiation photoemission spectroscopy reveals enhanced oxygen incorporation in AlxGa1−xN as the Al mole fraction increases. It is shown that the increased oxygen donor incorporation can result in a conductivity-type change from p-type to n-type in Mg-doped AlxGa1−xN. Consistent with the conductivity-type change, epitaxial Al0.20Ga0.80N films exhibit n-type conductivity despite heavy Mg doping. The p-type conductivity of bulk AlxGa1−xN with a high Al mole fraction can be improved by employing AlxGa1−xN/AlyGa1−yN superlattices (SLs). At 300 K, Mg-doped Al0.17Ga0.83N/Al0.36Ga0.64N SLs (average Al mole fraction of 23%) exhibit strong p-type conductivity with a specific resistance of 4.6 Ω cm, a hole mobility of 18.8 cm2/Vs, and an acceptor activation energy of 195 meV.

Incorporation of Oxygen Donors in AlGaN

Abstract: The chemistry of oxygen atoms at the surface of an AlGaN layer for Al 0.35 Ga 0.65 N/GaN heterostructures was investigated by scanning photoemission microscopy (SPEM) using synchrotron radiation. SPEM imaging and space-resolved photoemission spectroscopy showed that the oxygen atoms were preferentially incorporated into AlGaN rather than GaN due to the high reactivity of Al with oxygen. In situ annealing at 1000°C could lead to the outdiffusion of oxygen impurities from the bulk AlGaN, resulting in a significant increase in the intensity of Al-O bonds at the AlGaN surface. Therefore, it is suggested that the unintentional doping of oxygen impurities in AlGaN could yield a heavily doped n-type AlGaN layer, resulting in a drastic reduction in effective Schottky barrier heights of metal contacts on AlGaN/GaN heterostructures.

Low-resistance and high-reflectance Ni/Ag/Ru/Ni/Au ohmic contact on p-type GaN

Abstract: We report a metallization scheme of low-resistance, high-reflectance, and thermally-stable ohmic contact on p-type GaN. The specific contact resistivity as low as 5.2×10−5Ωcm2 and the high reflectance of 91% were simultaneously obtained from Ni(50A)∕Ag(1200A)∕Ru(500A)∕Ni(200A)∕Au(500A) contact annealed at 500°C in O2 ambient. The oxidation annealing promoted the outdiffusion of Ga atoms to dissolve in the Ag layer, leaving Ga vacancies below the contact. The Ru layer could act as a diffusion barrier for intermixing of the reflective Ag with upper layers of Ni and Au. Thus, suppression of the intermixing results in the high reflectance and good thermal stability of the contact.

Ohmic contacts for high power LEDs

Abstract: 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.

High-brightness GaN-based light-emitting diode with indium tin oxide based transparent ohmic contact

Abstract: We have fabricated GaN-based light-emitting diodes (LEDs) using Ni/Au with indium tin oxide (ITO) overlayer as a p-electrode. A Ni (20 A)/Au (30 A)/ITO (600 A) contact with pre-annealing at 500 °C under an O2 ambient before ITO deposition (O-annealed contact) showed lower contact resistivity compared to the contact with pre-annealing under a N2 ambient (N-annealed contact) and without the pre-annealing (nonannealed contact). The pre-annealing under the O2 ambient produced NiO, which acted as the diffusion barrier for out-diffusion of N and Ga atoms and in-diffusion of In during the subsequent postannealing. Thus, the formation of a Au–In solid solution was effectively suppressed, resulting in the decrease of contact resistivity and enhancement in thermal stability. The LED with the O-annealed contact as a p-electrode showed lower operation voltage at 20 mA, better thermal stability, and enhanced light output than the LED with the N-annealed or nonannealed contact. The low operation voltage and better ther...