Hyung Jae Lee

Stress relaxation in Si-doped GaN studied by Raman spectroscopy

We report the Si-doping-induced relaxation of residual stress in GaN epitaxial layers grown on (0001) sapphire substrate by the metalorganic vapor phase epitaxy technique. Micro-Raman spectroscopy is used to assess stress situation in the films with systematically modulated doping concentration from 4.0×1017 up to 1.6×1019 cm−3. As the Si-doping concentration increases, a monotonic decrease of the E2 phonon frequency is observed, which signifies gradual relaxation of the stress in the film. The layers are fully relaxed when electron concentration exceeds 1.6×1019 cm−3. The linear coefficient of shift in Raman frequency (ω) induced by the in-plane biaxial compressive stress (σ∥) is estimated to be Δω/Δσ∥=7.7 cm−1/GPa. We suggest that Si doping increases density of misfit dislocation, judging from linewidth of x-ray rocking curve.

Formation mechanism of stains during Si etching reaction in HF–oxidizing agent–H2O solutions

The mechanism of stain formation in the chemical etching reaction of silicon has been investigated in HF–oxidizing agent–H2O solutions. The chemical formula of the stain formed during the silicon etching reaction is K2SiF6. The concentration of holes on silicon surface increases with the increase of redox potential and the concentration of oxidizing agent used in manufacturing the etching solution. The increase in the hole concentration accelerates not only the etch rate but also the formation rate of K2SiF6. The etched silicon surfaces are covered with a K2SiF6 layer when redox potential and concentration of oxidizing agent are great at low HF concentrations. This happens because the formation rate of K2SiF6 is much greater than its dissolution rate by HF. Sufficiently high HF concentration in the etching solution is apparently essential to increase the etch rate without the formation of K2SiF6.

Effects of growth rate of a GaN buffer layer on the properties of GaN on a sapphire substrate

We studied the effects of the growth rate of a GaN buffer layer grown on a GaN epilayer. It was found that this growth rate plays a key role in improving the quality of the GaN film on a sapphire substrate and an optimum growth rate exists that yields the best crystal quality. A GaN film grown on a buffer layer with the optimum growth rate of 18.3 nm/min has an electron Hall mobility of 539 cm2/V s and a dislocation density of approximately 2×108 cm−2. These improvements of GaN film qualities are illustrated by the promotion of the lateral growth mode.

GROWTH MECHANISM OF 3C-SIC(111) FILMS ON SI USING TETRAMETHYLSILANE BY RAPID THERMAL CHEMICAL VAPOR DEPOSITION

We have used a rapid thermal chemical vapor deposition technique to grow epitaxial SiC thin films on Si wafers by pyrolyzing tetramethylsilane (TMS). The films were observed to grow along the (111) direction of 3C–SiC at temperatures above 1000 °C. The quality of the films was significantly influenced by the TMS flow rate in the gas mixture, the growth temperature, and the gas pressure in the reactor. Single-crystal SiC films were grown at TMS flow rates below 1.0 sccm with a H2 carrier gas flow rate of 100 sccm. The gas pressure in the reactor has a great influence on the crystallinity, morphology, and thickness of the SiC film grown. Gas phase analyses indicated that TMS dissociates into hydrogen, silicon atoms, and hydrocarbons such as CH4, C2H2, and C2H4 at the growth temperature. The chemical composition of the grown films was analyzed. The growth mechanism of the SiC film on the Si substrate without the carbonization process is discussed based on the experimental results.

Optical absorption and anomalous photoconductivity in undoped n-type GaN

Photoconductivity and optical absorption measurements were employed to analyze deep levels in undoped n-type GaN films grown on sapphire substrate by metalorganic chemical vapor deposition. At room temperature, the photoconductivity measurement exhibits a broad level at around 1.90 eV. Similarly, the optical absorption spectrum shows a deep level located at 1.87 eV within the band gap, which is best described by a transition from a donor charge-transfer level to the conduction band, according to Lucovsky theory. A persistent photoconductivity whose behavior is distinctive from that of previously reported work for n- or p-type GaN epitaxial films was observed. The photocurrent quenching and decreased dark current in the persistent photoconductivity effect suggest that metastable electron states are formed in the band gap to trap electrons which tunnel out the potential barrier with long recovery time.

Effects of Initial Thermal Cleaning Treatment of a Sapphire Substrate Surface on the GaN Epilayer

We investigated the effects of the in situ thermal cleaning treatment of a (0001) sapphire substrate surface in hydrogen ambient on the structural, optical, and electrical properties of epitaxial GaN films grown by metalorganic chemical vapor deposition (MOCVD). Hall effect, X-ray diffraction (XRD), and photoluminescence measurements of GaN films grown at 1040°C clearly indicate that the film quality is strongly affected by the thermal cleaning treatment of the substrate surface. GaN films under the optimized thermal cleaning treatment at 1070°C for 10 min showed minimum full-widths at half maximum (FWHMs) of 273 arcsec and 728 arcsec for (002) and (102) XRD peaks, respectively. In addition, the FWHM of the band-edge emission peak was as narrow as 28.3 meV, and the intensity ratio between the band edge emission and the yellow band emission was as high as 100 at room temperature. It was also found that the roughness of sapphire surface was reduced after the thermal treatment.

Effects of Experimental Parameters on Void Formation in the Growth of 3C‐SiC Thin Film on Si Substrate

The effects of growth parameters have been examined for the epitaxial growth of a void-free SiC film on a Si substrate. Experiments were performed under various growth conditions by pyrolyzing tetramethylsilane (TMS) in a rapid thermal chemical vapor deposition reactor. Void-free single crystalline SiC films were grown when the Si substrate was heated after the flow of TMS. The increase of TMS flow rate produced void-free SiC films but the crystallinity of the films varied from single crystalline to polycrystalline. The growth of void-free single crystalline SiC films was observed at substrate temperatures below 1000°C. The outdiffusion of Si atoms from the Si substrate surface and the void formation in the silicon side of the SiC/Si interface were investigated using various experimental techniques. The mechanism of the void formation is briefly discussed in this work.

Growth law of silicon oxides by dry oxidation

A theoretical description of the kinetic mechanism of thermal oxidation in silicon is proposed by complementing the Deal - Grove model. The relationship of the classical linear - parabolic growth law is generalized to the logarithmic growth law which provides a complete description for the whole regime of oxide films. In particular, the enhanced oxidation rate in the thin regime may be attributed to the diffusion length, which is characterized by the difference between the activation energies of the diffusion process and the reaction process. Our fitting of the logarithmic growth law to several experimental results shows excellent agreement and the fitting parameters also provide activation energies of 1.50 and 2.49 eV for the interfacial reaction and diffusion in oxide respectively.

The study on the growth and properties of Mg doped and Mg–Si codoped p-type GaN

Abstract We have investigated the doping characteristics of Mg doped GaN films grown by metalorganic chemical vapor deposition. We have found that Mg doped GaN layer shows high electrical conductivity and good surface morphology, simultaneously, when the [Mg]/[Ga] ratio in gas phase is 7.6×10−3. If [Mg]/[Ga] ratios exceed an optimum value of 7.6×10−3, surface morphologies and electrical conduction properties become poor and blue emissions, considered as deep donor-to-acceptor-pair transitions in photoluminescence spectra, are dominant. Moreover, the Mg–Si codoping characteristics was explained effectively taking advantage of the concept of competitive adsorption between Mg and Si during the growth. Based on the experimental results, we suggest the methods to get a p-GaN showing high conductivity using Mg-Si codoping.

Formation Mechanism and Pore Size Control of Light-Emitting Porous Silicon

The mechanisms of silicon dissolution and pore formation during the formation of porous silicon layers (PSLs) are investigated in the HF-water electrolyte solution. As HF concentration increases in the electrolyte, the depth of pores increases while the pore diameter decreases. It is found that hydroxide ion ( OH-) plays an apparently significant role in the reaction with silicon. The formation of silicon oxide is observed on the silicon surface during PSL formation. The dissolution mechanism of silicon is presented, based on spectroscopic analyses for the porous silicon surface. The area of initially formed silicon oxide on the silicon wafer surface determines the pore diameter. With increasing amount of OH- ions, or equivalently decreasing HF concentration, the thickness of the silicon walls between pores is reduced, revealing the blue shift of photoluminescence energy due to the quantum size effect.