J. A. Zapien

Room-temperature single nanoribbon lasers

Using a single nano-object measurement methodology that enables the correlation between size/morphology/structure and photoluminescence (PL) characteristics, we show that nanoribbons are an excellent model system to study single nano-objects. In particular, we measure the PL characteristics of optically pumped individual single-crystal zinc–sulfide nanoribbons. Small collection angle measurements show that nanoribbons form excellent optical cavities and gain medium with high (full width at half maximum<0.1 nm) lasing modes free of PL background even for a low pumping power density of 9 kW/cm2. Large collection angles add a broad PL component and obscure the correct high-quality lasing of the nanowires/nanoribbons.

Manganese doping and optical properties of ZnS nanoribbons by postannealing

Manganese (Mn) doping of ZnS nanoribbons was achieved by simple thermal annealing. Upon heating ZnS nanoribbons with MnS powder up to 700°C, the intrinsic photoluminescence (PL) of the annealed nanoribbons disappeared and a new PL peak at 585nm gradually emerged. Significantly, the annealing process induced no detectable change in the morphology and uniform hexagonal wurtzite 2H structure of the single-crystal ZnS nanoribbons. The PL peak at 585nm is attributed to Mn dopant and confirms Mn incorporation in ZnS because (1) the peak appears only when ZnS ribbons were annealed with MnS, but does not appear without MnS, (2) its intensity increases with increasing annealing temperature, which is consistent with increased incorporation of Mn2+ ions, and (3) its position is similar to that of Mn-related emission in ZnS, and is independent of the measuring temperature and excitation power. This work demonstrates the capability of doping nanostructured materials by simple postannealing treatment.

Continuous near-infrared-to-ultraviolet lasing from II-VI nanoribbons

The authors show that II-VI nanoribbons are capable of room-temperature lasing covering the complete spectral range from near infrared (NIR) to ultraviolet (UV). This is accomplished by simply using nanoribbons of two ternary compositions, namely, CdSXSe1−X and ZnYCd1−YS. Under optical pumping, CdSXSe1−X nanoribbons lase from NIR (710nm) to green (510nm) as X changes from 0 to 1, while ZnYCd1−YS nanoribbons lase from green (510nm) to UV (340nm) as Y varies from 0 to 1. Furthermore, lasing control shows fine-tuning via composition changes that overlap thermally induced tuning. This demonstrates that II-VI materials can enable lasing at any selected wavelength between 710 and 340nm with continuous tuning capabilities.

Photoluminescence and photoconductivity properties of copper-doped Cd1−xZnxS nanoribbons

Copper-doped Cd1?xZnxS (x~0.16) nanoribbons were prepared by controlled thermal evaporation of CdS, ZnS, and CuS powders onto Au-coated silicon substrates. The nanoribbons had a hexagonal wurtzite structure, and lengths of several tens to hundreds of micrometres, widths of 0.6?15??m, and thicknesses of 30?60?nm. Cu doping and incorporation into the CdZnS lattice were identified and characterized by low-temperature photoluminescence (PL) and photoconductivity measurements. Temperature-dependent PL measurement showed that the PL spectra of both Cu-doped and undoped CdZnS nanoribbons have two emission peaks at 2.571 and 2.09?eV, which are assigned to band edge emission and deep trap levels, respectively. In addition, the Cu-doped nanoribbons present two extra peaks at 2.448 and 2.41?eV, which are attributed to delocalized and localized donor and acceptor states in the band gap of CdZnS resulting from Cu incorporation. Photoconductivity results showed the nanoribbons can be reversibly switched between low and high conductivity under pulsed illumination. The Cu-doped CdZnS nanoribbons showed four orders of magnitude larger photocurrent than the undoped ones. The current jumped from ~2 ? 10?12 to ~5.7 ? 10?7?A upon white light illumination with a power density of ~9?mW?cm?2. The present CdZnS:Cu nanoribbons may find applications in opto-electronic devices, such as solar cells, photoconductors, and chemical sensors.

Multichannel ellipsometer for real time spectroscopy of thin film deposition from 1.5 to 6.5 eV

A rotating polarizer multichannel ellipsometer has been optimized for operation well into the ultraviolet (UV) spectral range. With this instrument, 132 spectral points in the ellipsometric parameters (ψ, Δ) over the photon energy range from 1.5 eV (827 nm) to 6.5 eV (191 nm) can be collected in a minimum acquisition time of 24.5 ms, corresponding to one optical cycle of the rotating polarizer. Averages over two and 80 optical cycles (obtained in 49 ms and 1.96 s, respectively) give standard deviations in (ψ, Δ) of less than (0.04°, 0.08°) and (0.007°, 0.015°), respectively, for the energy range from 3.5 to 6.0 eV, as determined from successive measurements of a stable thermally oxidized silicon wafer. Key modifications to previous instrument designs include: (i) a tandem in-line Xe/D2 source configuration for usable spectral output from 1.5 to 6.5 eV; (ii) MgF2 Rochon polarizers for high transmission in the UV without the need for optical activity corrections; (iii) a spectrograph with a grating blazed a...

Ultraviolet-extended real-time spectroscopic ellipsometry for characterization of phase evolution in BN thin films

Real-time spectroscopic ellipsometry with an ultraviolet-extended spectral range (1.5–6.5 eV) has been applied to investigate the sputter deposition of boron nitride (BN) thin films with high cubic content in terms of a two-layer optical model. In this model, the inner and outer layers represent sp2- and sp3-bonded BN (hBN and cBN), respectively. The thickness evolution of the two layers as well as their dielectric functions over the extended spectral range have been determined.

Multichannel ellipsometry from 1.5 to 6.5 eV for real time characterization of wide band gap materials : phase identification in boron nitride thin films

Abstract A recently developed real time spectroscopic ellipsometer with an ultraviolet-extended spectral range (1.5–6.5 eV) has been applied to investigate the sputter deposition of BN films with high cBN content. Based on the optical contrast between the two phases (sp2 and sp3 bonded BN) above 4.5 eV, it has been possible to characterize the growth of these films using a two-layer isotropic optical model. In this model, the innermost layer represents the combined contribution from the sp2 bonded material (aBN and hBN) denoted collectively as hBN, whereas the outermost layer represents the contribution of the predominantly sp3 bonded material denoted as cBN. The BN films were grown on crystalline silicon substrates using two processes: (i) r.f. magnetron sputtering of a BN target with pulsed d.c. substrate biasing; and (ii) pulsed d.c. sputtering of a B4C target with r.f. substrate biasing. In both cases, the thickness evolution of the hBN and cBN layers as well as their dielectric functions over the extended spectral range have been determined. The cBN content deduced from the two-layer analysis is in good agreement with estimates from ex situ infrared transmission spectroscopy.

Real-time spectroscopic ellipsometry from 1.5 to 6.5 eV

Abstract A rotating polarizer multichannel ellipsometer has been optimized for operation well into the ultraviolet (UV) spectral range. Key modifications to previous instrument designs include (i) a tandem in-line Xe/D 2 source configuration for usable spectral output from 1.5 (827 nm) to 6.5 eV (191 nm), (ii) MgF 2 Rochon polarizers for high transmission in the UV and (iii) a spectrograph with a grating blazed at 250 nm and two stages of internally-mounted order-sorting filters. The resulting multichannel ellipsometer provides 132 point spectra in the ellipsometric parameters ( ψ , Δ) over the range from 1.5 to 6.5 eV with a minimum acquisition time of 24.5 ms. As an example of the application of this instrument, real-time spectroscopic ellipsometry (SE) is reported for the analysis of a hexagonal boron nitride thin film deposition by radio-frequency magnetron sputtering onto a silicon wafer substrate.

Synthesis of Silicon Nanowires

We present alternative routes for the production of silicon nanowires (SiNWs). We have successfully synthesized SiNWs by plasma enhanced chemical vapor deposition, high temperature annealing, and thermal evaporation. Selective growth is achieved by using a patterned catalyst. Bulk production of SiNWs can be achieved by thermal evaporation.

Real time characterization of wide band gap thin film growth using UV-extended spectroscopic ellipsometry: applications to cubic boron nitride

We report the application of the UV-extended multichannel ellipsometer in studies of the growth and layered structure of cBN films deposited on c-Si using pulsed dc sputtering of a B/sub 4/C target with rf substrate bias.