Choochon Lee

Composition dependence of room temperature 1.54 μm Er3+ luminescence from erbium-doped silicon:oxygen thin films deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition

The composition dependence of room temperature 1.54 μm Er3+ photoluminescence of erbium-doped silicon:oxygen thin films produced by electron cyclotron resonance plasma enhanced chemical vapor deposition of SiH4 and O2 with concurrent sputtering of erbium is investigated. The Si:O ratio was varied from 3:1 to 1:2 and the annealing temperature was varied from 500 to 900 °C. The most intense Er3+ luminescence is observed from the sample with a Si:O ratio of 1:1.2 after a 900 °C anneal and the formation of silicon nanoclusters embedded in the SiO2 matrix. The high active erbium fraction, efficient excitation via carriers, and high luminescence efficiency due to the high quality SiO2 matrix are identified as key factors in producing the intense Er3+ luminescence.

TWO-DIMENSIONAL SIMULATION STUDY OF FIELD-EFFECT OPERATION IN UNDOPED POLY-SI THIN-FILM TRANSISTORS

We propose a simulation model to explain two‐dimensional field‐effect operation of undoped poly‐Si thin‐film transistors (TFTs) under small drain voltages. Our model includes both thermionic‐emission and drift‐diffusion conduction processes. We calculated grain‐boundary potential barriers, channel currents, and various device parameters depending on grain size and defect density. In order to validate our model, we compared calculated currents with experimental data for two types of poly‐Si TFTs. We could obtain good current fits simultaneously in both subthreshold and linear regions by adopting proper densities of states in the poly‐Si channels. We could also explain well the temperature‐dependent current changes and the current activation energy versus the gate voltage. Finally, we succeeded in modeling the drain current under small drain voltages by using the combined transport process in the two‐dimensional grain‐boundary structure.

Effect of the statistical shift on the anomalous conductivities of n‐type hydrogenated amorphous silicon

We calculated the shift of Fermi energy EF with temperature, using a model density of states for hydrogenated amorphous silicon (a‐Si:H) similar to that of deep level transient spectroscopy results, by a numerical method. The conductivity σ was calculated from the calculated EF as a function of temperature. It was found that some features of anomalous transport phenomena of n‐type a‐Si:H such as kinks or the continuous bending of log σ vs 1/T curves and the Meyer–Neldel‐type preexponential factors can be explained, at least in part, by the statistical shift alone.

Low temperature photoluminescence characteristics of carbon doped GaAs

We have analyzed low temperature (12 K) photoluminescence (PL) characteristics of carbon(C) doped GaAs epilayers. No traces of donor levels were observed in the PL spectra. This suggest that well‐behaved carbon is incorporated as an acceptor into the GaAs lattice. The measured peak energy of the PL intensity distribution shifts to lower energy and the full width at half maximum (FWHM) increases with increasing hole concentration. We have obtained empirical relations for FWHM of PL intensity distribution in two distinct hole concentration regions. These relations are considered to provide a useful tool to determine free hole concentration in C doped GaAs by low temperature PL measurements. As the hole concentration is increased above 2×1019 cm−3, a shoulder separated from the PL peak was observed in the PL spectra at Eg+EF, where  Eg is the band gap and EF is the Fermi energy. The shoulder became very prominent at 9.2×1019 cm−3.

Quantum size effects on the conductivity in porous silicon

The quantum confinement effect was examined by measuring dark conductivity and optical transmittance of free-standing porous silicon films of different porosities. With increasing porosity from 40 to 80%, the transmission spectrum shows a blue shift and the activation energy of the conductivity increases from 0.3 to 1.0 eV. Using the activation energies, confinement energies are estimated with respect to the crystallite size, determined from the given porosity, and the results are compared with previous works. Good agreement is obtained.

Selenium and silicon delta‐doping properties of GaAs by atmospheric pressure metalorganic chemical vapor deposition

We investigate selenium and silicon delta‐doping characteristics for GaAs layers grown by atmospheric pressure metalorganic chemical vapor deposition. Broadened capacitance‐voltage (C‐V) profiles and low doping activities are found in case of Se delta doping. In contrast, excellent delta‐doping characteristics result for silicon. The obtained C‐V full width at half maximum are in the range of 44–49 A for samples even with substrate temperatures of 700–750 °C, which are relatively high compared with the molecular‐beam epitaxy (MBE) substrate temperature. Hall mobility measurements show the mobility enhancement due to a screening of impurity charges. Also, Shubnikov–de Haas oscillations demonstrate the two‐dimensional nature of electrons confined in the delta‐doped layers. Therefore, our delta‐doped layers show no substantial differences in quality with MBE‐grown delta‐doped layers. In addition, preliminary delta field‐effect transistors having a gate length of 1.5 μm and a gate width of 300 μm are fabricated.

Carbon doping characteristics of GaAs and Al0.3Ga0.7As grown by atmospheric pressure metalorganic chemical vapor deposition using CCl4

Abstract This paper presents the results of a Van der Pauw-Hall analysis of the carbon doping characteristics of GaAs and Al 0.3 Ga 0.7 As epilayers grown by atmospheric pressure metalorganic chemical vapor deposition (AP-MOCVD) using carbon tetrachloride (CCI 4 ). It also describes the effects of rapid thermal annealing (RTA) on the electrical properties of CCI 4 -doped GaAs. Heavily CCI 4 -doped GaAs and Al 0.3 Ga 0.7 As epilayers with a wide range of hole concentrations were obtained by varying the growth temperature from 500 to 750°C and by varying the V/III (AsH 3 /TMG) ratio from 5 to 20 while keeping the flow rate of CCI 4 constant. CCI 4 -doped GaAs epilayers with hole concentrations between 3.5 × 10 17 and 8.95 × 10 19 cm -3 were obtained under the above growth conditions with constant TMG flow rate. CCI 4 -doped Al 0.3 Ga 0.7 As epilayers have higher hole concentrations than CCI 4 -doped GaAs epilayers for various growth temperatures at the same V/III ratio and CCI 4 flow rate, and the hole concentration increases with decreasing growth temperature. The CCI 4 -doped GaAs epilayers have nearly the same Hall mobilities as GaAs C-doped by V/III ratio control, but higher mobilities than Zn-doped GaAs. Our results show that the free hole concentration is heavily CCI 4 -doped GaAs epilayers increases after RTA at 790°C. This can be attributed to the decrease in the concentration of the hydrogen atoms caused by the breaking of the C-H or C-H x bonds. After longer annealing at 790°C the hole concentration decreases and the hole mobility increases. It is suggested by the increase in free hole concentration by RTA that the activation efficiency of the incorporated carbon in the as-grown CCI 4 -doped GaAs epilayers is incomplete.

Characteristics of heavily carbon-doped GaAs by LPMOCVD and critical layer thickness

Abstract The carbon-doping characteristics of GaAs epilayers have been investigated by varying the growth parameters - V/III ratio and growth temperature. We have obtained the highest 1.4 × 10 20 cm -3 hole concentration with hole mobility 42 cm 2 /V·s without using intentional doping source gases. The carbon incorporation increased with increasing growth temperature up to the critical temperature and above this temperature the carbon incorporation decreased. This critical temperature depends on input TMG flow rate. The peak energy of the PL intensity distribution was shifted to lower energy (red shift) and the full width at half maximum (FWHM) increased with increasing hole concentration. The (400) diffraction peak of the carbon-doped GaAs epilayer was split from the substrate GaAs peak. The split angle was 191.25 arc for the carbon-doped GaAs with 9 × 10 19 cm -3 hole concentration. We also have discussed the relationship between carbon-doping concentration and critical layer thickness ( L C ) by an excess stress model. The excess stress at which partial strain relief became observable was σ exc / μ = 0.0021. Thus we identified this excess stress as the critical excess stress for carbon-doped GaAs epilayers. If the excess stress was greater than this, the epilayers showed the surface cross-hatching.

Performance of the plasma deposited tungsten nitride barrier to prevent the interdiffusion of Al and Si

It is proposed that the plasma enhanced chemical vapor deposited tungsten nitride (PECVD‐W–N) thin film is used as a diffusion barrier to prevent the interdiffusion between Al and Si during postannealing process. The atomic concentration of N in W–N film deposited with NH3/WF6 partial pressure ratio of 0.5 is about 33 at. % and its resistivity is 90–110 μΩ cm. The Rutherford backscattering spectrometry, Auger electron depth profiles, x‐ray diffraction, and scanning electron micrographs show that 900 A PECVD‐W67N33 film interposed between Al and Si is more impermeable than PECVD‐W film due to N atoms and it also keeps its chemical integrity during the postfurnace annealing at 600 °C for 30 min in Ar ambient.

Voltage dependence of off current in a-Si:H TFT under backlight illumination

Abstract We have studied the off currents of a-Si:H TFT at negative gate voltages. The voltage dependence of the off current in a-Si:H TFT is explained as the hole current limited by two competing components, channel conductance due to hole accumulation and the drain junction conductance under backlight illumination. When enough carriers are injected from the source and drain regions by photogeneration, two quite different gate voltage dependences appeared. In the dark condition the increasing off current with negative gate bias is the hole current injected from the drain junction region, and the hole generation seems to originate from the thermally-assisted tunneling via the localized states in a-Si:H near the drain junction because of its small activation energy of 0.1 ≈ 0.2 eV.