Kyoung Kook Kim

High electron concentration and mobility in Al-doped n-ZnO epilayer achieved via dopant activation using rapid-thermal annealing

We report on the growth of very high-quality Al-doped n-type ZnO epilayers on sapphire substrates using a radio-frequency (rf) magnetron sputtering technique combined with a rapid-thermal annealing. Photoluminescence (PL) and Hall measurements show that both the optical and electrical properties of the ZnO layers are significantly improved with an increasing annealing temperature up to 900u2009°C. For example, the samples that are grown at 600u2009°C and a rf power of 100 W with an Ar∕O2 gas ratio of 1 give an electron concentration of 1.83×1020cm3 and a mobility of 65.6cm2∕Vs, when annealed at 900u2009°C for 3 min in a nitrogen ambient. Furthermore, x-ray diffraction measurements show that both the as-grown and annealed samples are of excellent crystallinity.

Formation of low resistance nonalloyed Al/Pt ohmic contacts on n-type ZnO epitaxial layer

We have investigated nonalloyed Al/Pt ohmic contacts on n-type ZnO:Al (nd=2.0×1018u2009cm−3). It is shown that the as-deposited Al/Pt contacts produce a specific contact resistivity of 1.2×10−5u2009Ωu200acm2. Auger electron spectroscopy and x-ray photoelectron spectroscopy depth profile results show interdiffusion between oxygen and aluminum, resulting in an increase of carrier concentrations near the ZnO surface. The increase of the carrier concentration at the surface region of ZnO is attributed to the low resistance of the nonalloyed Al/Pt contact.

Highly low resistance and transparent Ni/ZnO ohmic contacts to p-type GaN

We report on a promising Ni (5 nm)/Al-doped ZnO (AZO) (450 nm) metallization scheme for low resistance and transparent ohmic contacts to p-GaNu200a(5×1017u2002cm−3). It is shown that the as-deposited Ni/AZO contact shows a nonohmic characteristic due to the insulating nature of the as-deposited AZO. However, annealing the contacts at 450 and 550u200a°C for 2 min in air ambient results in linear current–voltage characteristics, giving a specific contact resistance of 1.01×10−5 and 8.46×10−6 Ωu200acm2, respectively. It is further shown that annealing the contact at 550u200a°C for 5 min produces a specific contact resistance of 6.23×10−6 Ωu200acm2. The light transmittance of the contacts annealed at 550u200a°C for 2 min is measured to be higher than 76% at wavelengths in the range of 400–550 nm. It is shown that the Ni/AZO contact could be a suitable scheme for high-performance optical devices.

Design of high-efficiency GaN-based light emitting diodes with vertical injection geometry

The authors report on the design and fabrication of high-efficiency GaN-based light emitting diodes (LEDs) with vertical-injection geometry. Based on the analyses of LED test patterns fabricated with various n-electrode dimensions, a design rule for vertical LEDs is proposed. It is found that the suppression of the vertical current under n electrodes and the efficient injection of the spreading current across the n layers are essential to fabricate high-efficiency LEDs. Introduction of the current blocking layer along with well-designed branched n electrodes results in a large enhancement of power efficiency by a factor of 1.9, compared with that of reference LEDs.

Effect of rapid thermal annealing on Al doped n-ZnO films grown by RF-magnetron sputtering

High-quality Al-doped n-type ZnO (n-ZnO:Al) epilayers have been grown by an rf-magnetron sputtering technique combined with a rapid thermal annealing (RTA) process. The electrical and optical properties of as-deposited samples are considerably improved upon annealing at 900°C for 3 min in nitrogen ambient. The improvement is attributed to the deoxidation of Al-oxides, i.e., the activation of Al dopants. The samples annealed at 900°C produce a mobility of 65.5 cm2/Vs and a carrier concentration of 1.03×1020 cm-3. It is also shown that the sample surface becomes significantly smoother after annealing. The results show that the RTA process effectively improves the electrical and optical properties of the Al-doped ZnO films.

High-Rate Dry Etching of ZnO in BCl3/CH4/H2 Plasmas

High-rate dry etching characteristics of aluminum-doped zinc oxide (AZO) have been investigated in inductively coupled plasma (ICP) using BCl3/CH4/H2 plasma chemistry. Etch rates were measured as a function of BCl3 flow rate in BCl3/CH4/H2 mixture and dc-bias voltage. Measurement of etch rate, and etched sidewall profile were performed using a stylus profilometer and scanning electron microscopy, respectively. The highest AZO etch rate about 310 nm/min, could be obtained near 80% BCl3 and at dc-bias voltage of -350 V.

Thermally Stable and Low Resistance Ru Ohmic Contacts to n-ZnO

We report on a promising metallisation scheme for high-quality ohmic contacts to n-ZnO:Al (nd=3×1018 cm-3. The as-deposited contact yields a specific contact resistance of 2.1×10-3 Ω·cm2. However, annealing of the contact at 700°C for 1 min results in a resistance of 3.2×10-5 Ω·cm2. The prolonged annealing treatment causes negligible degradation of electrical and thermal properties. These results indicate that the Ru film would be a suitable scheme for the fabrication of high-performance ZnO-based optical and high-temperature devices.

Thermally Stable and Low Resistance Re/Ti/Au Ohmic Contacts to n ­ ZnO

We report on the formation of thermally stable ohmic contacts on n-type ZnO:Al (n d = 2 X 10 1 8 cm - 3 ) using a Re/Ti/Au metallization scheme. It is shown that the as-deposited Re/Ti/Au contact is ohmic with a contact resistivity of 2.1 X 10 - 4 Ω cm 2 . The electrical characteristics of the samples are further improved upon annealing, namely, the sample produces a specific contact resistance of 1.7 X 10 - 7 Ω cm 2 when annealed at 700°C for 1 min in a nitrogen ambient. Atomic force microscopy results show that the surface of the samples is reasonably smooth with a root-mean-square roughness of 6.0 nm when annealed at 700°C. The carrier transport mechanism is described using the relationship between temperature and specific contact resistance. Based on X-ray diffraction and X-ray photoemission spectroscopy results, the ohmic mechanism for the annealed samples is also described and discussed.

Light extraction enhancement of GaN-based light emitting diodes using MgF2/Al omnidirectional reflectors

We report on the enhancement of the light extraction of GaN-based light emitting diodes (LEDs) by using MgF2/Al omnidirectional reflectors (ODRs). The ODRs consisting of a quarter-wavelength-thick MgF2 having a refractive index of 1.39 and Al metal produce a high-angle-integrated reflectivity of 96.6%. To optimize the electrical injection and light reflection, the MgF2/Al ODRs are combined with Pd/Ag metallic reflectors using mesh configuration. Compared to reference LEDs, LEDs fabricated with the MgF2/Al ODRs show an enhanced output power by 23% and a slight increase in the forward voltage by 0.18 V, leading to the improvement in power efficiency by 17%.

Fabrication and NO2 gas sensing performance of TeO2-core/CuO-shell heterostructure nanorod sensors

TeO2-nanostructured sensors are seldom reported compared to other metal oxide semiconductor materials such as ZnO, In2O3, TiO2, Ga2O3, etc. TeO2/CuO core-shell nanorods were fabricated by thermal evaporation of Te powder followed by sputter deposition of CuO. Scanning electron microscopy and X-ray diffraction showed that each nanorod consisted of a single crystal TeO2 core and a polycrystalline CuO shell with a thickness of approximately 7xa0nm. The TeO2/CuO core-shell one-dimensional (1D) nanostructures exhibited a bamboo leaf-like morphology. The core-shell nanorods were 100 to 300xa0nm in diameter and up to 30xa0μm in length. The multiple networked TeO2/CuO core-shell nanorod sensor showed responses of 142% to 425% to 0.5- to 10-ppm NO2 at 150°C. These responses were stronger than or comparable to those of many other metal oxide nanostructures, suggesting that TeO2 is also a promising sensor material. The responses of the core-shell nanorods were 1.2 to 2.1 times higher than those of pristine TeO2 nanorods over the same NO2 concentration range. The underlying mechanism for the enhanced NO2 sensing properties of the core-shell nanorod sensor can be explained by the potential barrier-controlled carrier transport mechanism.PACS61.46. + w; 07.07.Df; 73.22.-f