Sang Ho Kim

Effect of hydrogen peroxide treatment on the characteristics of Pt Schottky contact on n-type ZnO

We report on the formation of good Pt Schottky contacts on the Zn-terminated n-type ZnO (0001) surfaces (∼2×1017cm−3) using surface treatment with a hydrogen peroxide solution. The Pt contacts on organic solvent-cleaned ZnO (0001) show leaky behavior with a high leakage current of ∼−0.05A under −5V reverse bias voltage, whereas the hydrogen peroxide-treated contacts show Schottky behavior with very low leakage current of ∼−6.5×10−8A under −5V reverse bias voltage. Schottky barrier heights estimated from current-voltage and capacitance-voltage characteristics are 0.89 and 0.93eV, respectively. Room-temperature photoluminescence results show that the hydrogen peroxide treatment is fairly effective in removing deep-level defects near the ZnO surface region. In addition, the preliminary deep-level transient spectroscopy result is also presented.

Electrical characteristics of Pt Schottky contacts on sulfide-treated n-type ZnO

We have investigated the effect of sulfide treatment on the electrical characteristics of Pt contacts on (000-1) n-type ZnO(∼5×1015cm−3) single crystals. The Pt contact on conventionally cleaned ZnO surface shows an ohmic behavior. However, the contact produces a Schottky behavior, when the ZnO surface is etched in a boiling (NH4)2Sx solution. Measurements show that the Schottky barrier height, ideality factor, and leakage current at −5V of the Pt contact on the sulfide-treated ZnO are 0.79eV, 1.51, and 3.75×10−10A, respectively. Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) examinations indicate the formation of ZnS phase at the Pt∕ZnO interface. Based on the capacitance–voltage, AES, and XPS results, a possible mechanism for the formation of good Schottky contacts is given.

Growth of nominally undoped p-type ZnO on Si by pulsed-laser deposition

We report on the growth of nominally undoped p-type ZnO films on Si(111) substrates by pulsed-laser deposition. Hall effect measurements show that the undoped ZnO films change from n-type to p-type material when the oxygen pressure changes from 6×10−5to3×10−4Torr during growth. Ti∕Au contacts produce ohmic behavior to n-type ZnO (∼1017cm−3), but leaky Schottky behavior to p-type ZnO (∼1018cm−3). Electrical and photoluminescence results indicate that native defects, such as oxygen and zinc vacancies, could play an important role in determining the conductivity of these nominally undoped ZnO films.

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.

Formation of vanadium-based ohmic contacts to n-GaN

We investigate vanadium (V)-based ohmic contacts on n-GaN (Nd=2.0×1018 cm−3) as a function of annealing temperature. It is shown that the V (60 nm) contacts become ohmic with specific contact resistances of 10−3–10−4 Ω cm2 upon annealing at 650 and 850 °C. The V (20 nm)/Ti (60 nm)/Au (20 nm) contacts produce very low specific contact resistances of 2.2×10−5 and 4.0×10−6 Ω cm2 when annealed at 650 and 850 °C, respectively. A comparison shows that the use of the overlayers (Ti/Au) is very effective in improving ohmic property. Based on Auger electron spectroscopy and glancing-angle x-ray diffraction results, possible explanations for the annealing temperature dependence of the ohmic behavior of the V-based contacts are described and discussed.

Zn/Au ohmic contacts on n-type ZnO epitaxial layers for light-emitting devices

We have investigated thermally stable and low resistance Zn/Au ohmic contacts on Al-doped n-type ZnO layers. The Zn/Au contacts produce linear current-voltage behaviors when annealed at temperatures in the range of 300 to 600°C for 1 min in a nitrogen ambient. The samples annealed at 500°C yielded contact resistivity as low as 2.36 3 10−5 V cm2. However, annealing the samples at 600°C degraded their electrical characteristics with contact resistivity of 1.01 3 10−2 V cm2. Based on the X-ray diffraction and X-ray photoemission spectroscopy results, the possible formation and degradation mechanisms of the Zn/Au contacts are described and discussed.

Electrical and Interfacial Properties of Nonalloyed Ti/Au Ohmic and Pt Schottky Contacts on Zn-Terminated ZnO

We report on the electrical and interfacial properties of nonalloyed Ti/Au ohmic and Pt Schottky contacts on Zn-terminated n-ZnO (1.5×1017 cm-3). Nonalloyed Ti/Au and Pt contacts on the Zn-terminated ZnO respectively exhibit ohmic and Schottky behavior owing to different work functions and out-diffusion characteristics. The nonalloyed Ti/Au contact reveals very linear current–voltage behavior with a specific contact resistivity of 2.2×10-5 Ω cm2. However, Pt contact shows Schottky behavior with Schottky barrier heights (SBHs) of 0.62 eV and 0.78 eV, obtained from current–voltage (I–V) and capacitance–voltage (C–V) measurements, respectively. Using Auger electron spectroscopy (AES), we correlated the electrical properties of the nonalloyed Ti/Au ohmic and Pt Schottky contacts with the properties of the interface between the metal and ZnO.

Effects of Oxygen Partial Pressure on the Electrical and Optical Properties of Pulsed-Laser-Deposited Sb-Doped SnO2 Films

We have investigated the electrical and optical properties of pulsed-laser-deposited Sb-doped SnO 2 films (250 nm thick) as a function of the oxygen partial pressure. The SnO 2 films were grown on glass substrates using a SnO 2 target containing 5 atom % Sb. The distance between the substrate and the target was 7 cm and working pressure varied from 1.1 to 13.3 Pa. The target was ablated using KrF excimer laser with energy density of 3.75 J/cm 2 . It is shown that the electrical and optical properties of the films grown at 480°C with 3000 pulses sensitively depend on the oxygen pressure. The electron concentration is maximum (5.6 X 10 20 cm -3 ) at 4 Pa, the electron mobility is maximum (8.5 cm 2 V -1 s -1 ) at 9.3 Pa, and the resistivity is minimum (2.5 X 10 -3 Ω cm) at 4 Pa. The transmittance is shown to depend on the oxygen pressures. X-ray diffraction results show that the crystalline quality of the films becomes improved with decreasing oxygen pressure. It is further shown that the sample grown at 1.1 Pa contains oxygen-deficient phases. The UV absorption edge of the films shifts toward the shorter wavelengths with decreasing oxygen pressure, which is attributed to Burstein-Moss shift.

Low-resistance and thermally stable Pd/Re ohmic contacts to p-type GaN

We have investigated a low-resistance thermally stable Pd/Re ohmic contact on moderately doped p-GaN:Mg (1.1 × 1017 cm−3). It is shown that the I–V characteristic of the as-deposited sample is improved upon annealing at 550 °C for 1 min under N2 ambient. However, annealing the sample at 650 °C results in the degradation of the I–V behaviour. Specific contact resistance as low as 8.7 × 10−4 Ω cm2 is obtained from the Pd(20 nm)/Re(25 nm) contact annealed at 550 °C. It is also shown that the contact exhibits thermal stability during annealing at 550 °C. Auger electron microscopy and glancing angle x-ray diffraction studies are carried out to characterize interfacial reactions between the Pd/Re contacts and the GaN.

Low Resistance Ni-Mg Solid Solution/Pt Ohmic Contacts to p-Type GaN

We investigate Ni-Mg solid solution (4 nm)/Pt (6 nm) metallization schemes for use in forming high-quality ohmic contacts to p-GaN:Mg The as-deposited Ni-Mg solid solution/Pt contact reveals nonlinear current-voltage characteristics. Annealing the contacts at 450 and 550°C for 1 min in air ambient, however, improves electrical behaviors significantly. For example, specific contact resistance as low as is obtained, when the contacts are annealed at 450°C. Based on the I-V measurement, Auger electron spectroscopy, and X-ray photoemission spectroscopy results, possible ohmic formation mechanisms for the Ni-Mg solid solution/Pt contacts are discussed.