Sung-Gi Ri

Leakage current analysis of diamond Schottky barrier diode

The current-voltage characteristics of non-punch-through-type diamond Schottky barrier diodes (SBDs) are analyzed by using thermionic and thermionic-field emission (TFE) models. Diamond SBD with defects such as nonepitaxial crystallites (NCs) shows shunt path conductance both under forward and reverse bias conditions. However, SBD without NCs shows a low reverse leakage current density of less than 1×10−11A∕cm2, which is more than 12 orders of magnitude smaller than the forward current density. From the fitting of the reverse leakage current of SBD without NCs, TFE current dominates when the reverse electric field is larger than 1.2MV∕cm and its current density value reaches 10−6A∕cm2 even at 1.6MV∕cm, which is lower than the avalanche limit.

High performance of diamond p+-i-n+ junction diode fabricated using heavily doped p+ and n+ layers

A good ideality factor and rectification ratio were obtained in a p+-i-n+ diamond diode with p+ and n+ doping levels of ∼1020 cm−3, where the hopping conduction mechanism dominates in the bulk p+ and n+ layers. The diode characteristics show a rectification ratio of 108 at ±10 V and an ideality factor of n=1.32. This diode showed ruggedness with a large current density of over 15 000 A/cm2 at +35 V. These results indicate the possibility of large-current devices.

High-Efficiency Excitonic Emission with Deep-Ultraviolet Light from (001)-Oriented Diamond p–i–n Junction

We have realized high-efficiency excitonic emission with deep-UV light at room temperature for a (001)-oriented diamond light-emitting diode with an intrinsic diamond layer as an active region. The boron-doped p-type, non-doped intrinsic, and phosphorus-doped n-type ( p–i–n) junction diode structure was formed by applying an optimized homoepitaxial growth technique based on microwave plasma-enhanced chemical vapor deposition. High-performance p–i–n junction characteristics were confirmed from current–voltage and capacitance–voltage properties. A strong UV light emission at around 240 nm due to free exciton recombination was observed at a forward current of over 6 mA, while the broad visible light emission from deep levels was significantly suppressed compared to that of reported electroluminescence in diamond p–n junctions.

Device-grade homoepitaxial diamond film growth

We have successfully synthesized homoepitaxial diamond films with atomically flat surface by the microwave plasma chemical vapor deposition (CVD) using an extremely low CH 4 /H 2 ratio of CH 4 /H 2 gas less than 0.15% CH 4 /H 2 ratio and Ib(0 0 1) substrates with low misorientation angle (θ off ) less than 1.5. It was found that surface morphologies of the films strongly depend on a growth condition of CH 4 /H 2 ratio and θ off of the substrate and was suggested that the hydrogen etching and the θ off played an important role for the epitaxial diamond film growth with an atomically flat surface. On the other hand, from the cathodoluminescence spectra and Schottky junction properties of these diamond films with atomically flat surface, it has been clarified that these films have actually a high potentiality for electronic devices.

Strong Excitonic Emission from (001)-Oriented Diamond P?N Junction

We have succeeded in fabricating (001)-oriented diamond p?n junctions with good diode characteristics and realized UV light emission by current-injection at room temperature. As p?n junctions, a phosphorus-doped n-type layer was formed on (001)-oriented boron-doped p-type one by applying an optimized homoepitaxial growth technique based on micro-wave plasma-enhanced chemical vapor deposition. Current?voltage characteristics showed a rectification ratio of 106 at ?30 V at room temperature. The existence of the space-charge layer through the p?n junction was confirmed from capacitance?voltage characteristics. A strong UV light emission at 235 nm was observed at forward current over 20 mA and is attributed to free exciton recombination.

Detection of misfit dislocations at interface of strained Si/Si0.8Ge0.2 by electron-beam-induced current technique

Electron-beam-induced current(EBIC) has been employed to investigate misfit dislocations (MDs) at the interface of strained Si/Si0.8Ge0.2 , which are located within the depletion region of Schottky contact. The MDs are intentionally introduced by growing the strained-Si layer to a thickness larger than the critical thickness. Two orthogonal sets of weak dark lines and some weak dark dots are observed with low electron-beam energy at a low temperature. These dark lines and dark dots correspond to the MDs and threading dislocations (TDs), respectively. The MDs and TDs are found to be nearly electrically inactive at room temperature and increase their activities at lower temperature, indicating that they are accompanied by shallow levels and free from metallic contamination. Comparisons with the chemical etched pattern reveal that each of the EBIC dark lines corresponds to a bundle of MDs.

Surface conductive layers on oxidized (111) diamonds

Surface conductive layers (SCL) on oxidized (111) diamonds with smooth surfaces after exposure to air were detected and characterized by Hall effect measurements. Hall effect measurements show that the conductivity is p type with sheet hole concentrations around of 1012cm−2 and Hall mobilities between 5 and 130cm2∕Vs. The SCL vanishes by thermal annealing at a temperature higher than 460K in He atmosphere, and recovers in air. These characteristics are similar to those generated by hydrogen termination. The experiments revealed that these SCLs are present on boron doped (111) and undoped (111) diamond films with smooth surfaces and natural IIa (111) diamonds, but not on (111) diamond films with rough surfaces and not on (100) diamonds.

Fermi level pinning-free interface at metals/homoepitaxial diamond (111) films after oxidation treatments

Schottky barrier heights of metal (Al, Au, Ni, and Pt) contacts on boron (B)-doped (111) homoepitaxial diamond films are investigated as a function of surface oxidation treatments before metal deposition [after wet-chemical oxidation (WO), WO followed by annealing in Ar atmosphere (WO-AN) and air oxidation]. It is found that the Schottky barrier height (ϕB) depends on the metal work function (ϕM) (Sϕ≡dϕB∕dϕM>0.3) on surfaces only after WO-AN, indicating that the interface at metals/B-doped (111) films after WO-AN has fewer defect states which do not pin the Fermi level. Experimental results and the appearance of a p-type surface conductive layer (SCL) are discussed and it is concluded that the pinning-free nature of the Fermi level is related to the appearance of a SCL.

Epitaxial growth of nonpolar ZnO and n-ZnO/i-ZnO/p-GaN heterostructure on Si(001) for ultraviolet light emitting diodes

Nonpolar a-plane ZnO-film and n-ZnO/i-ZnO/p-GaN heterostructure LEDs were grown epitaxially by pulsed laser deposition and metal?organic chemical vapor deposition on Si(001) using AlN and MnS as buffer layers. X-ray diffraction pole figures showed an epitaxial relationship of ZnO() ? AlN() ? MnS(001) ? Si(001). A near band-edge emission from ZnO was observed at 378 nm in photoluminescence measurements. Electroluminescence of nonpolar n-ZnO/i-ZnO/p-GaN LEDs displayed UV emission at 390 nm under forward and reverse bias. Successful growth of nonpolar n-ZnO/i-ZnO/p-GaN heteroepitaxial on Si provides an attractive solution for integrating nonpolar ZnO-based optoelectronic devices with Si substrates for various applications.

Exciton-derived Electron Emission from (001) Diamond p – n Junction Diodes with Negative Electron Affinity

Electron emission current was observed from a hydrogen-terminated n-layer diamond surface of forward biased diamond p–n junction diodes, while there was no electron emission from the same surface after oxidization, suggesting that the phenomenon is related to negative electron affinity (NEA) of the hydrogen-terminated diamond surface. Since electrons in the n-layer flow toward to the p-layer due to forward bias, they cannot directly contribute to the emission current from the n-layer surface. In view of our previous result that the exciton-derived photoelectron emission was observed from the NEA diamond surface by total photoelectron yield spectroscopy, the phenomenon can be explained as electron emission due to exciton diffusion at the forward bias.