Fuh-Shyang Juang

GaN and InGaN Metal-Semiconductor-Metal Photodetectors with Different Schottky Contact Metals

The characterizations of n-type doped GaN, p-type doped GaN and n-type doped In0.2Ga0.8N Schottky metal-semiconductor-metal (MSM) photodetectors were reported. The epilayers were grown on sapphire by metalorganic chemical vapor deposition (MOVCD). Schottky contacts were fabricated using Au, Ti, Ni and Pt metals. The dark and illuminated current–voltage characteristics of GaN and InGaN MSM photodetectors with different Schottky metals were studied. The n-GaN MSM photodetectors with Au Schottky contacts showed better responsivity than those with other metals and they were also better than Au/p-GaN and Ti/n-In0.2Ga0.8N MSMs. The effects of the pitch width between the interdigitate fingers and the thickness of Schottky metals on the characteristics of photocurrents were also studied.

Optical, structural and electrical properties of tin doped indium oxide thin films prepared by spray-pyrolysis technique

Tin doped indium oxide (In2O3:Sn) or indium tin oxide (ITO) thin films have been successfully deposited by the low cost spray-pyrolysis method. Low sheet resistance and high mobility films were obtained when the films were deposited at the substrate temperature of 793 K. The direct optical bandgaps for the films deposited at 793 (a) and 753 K (b) were found to be 3.46 and 3.40 eV, respectively. Similarly, the indirect bandgaps for a- and b-type films were found to be 3.0 and 2.75 eV, respectively. The Burstein-Moss shift was observed in the films. The refractive index (n) and extinction coefficient (k) were found to be in the range of 2.1 to 1.1 and 0.6 to 0.01, respectively. The various scattering mechanisms such as lattice, ionized impurity, neutral impurity, grain boundary and alloy scattering due to variation of theoretical mobilities with temperature are discussed, in order to compare experimental results. In the lattice scattering mechanism, the quantum size effect phenomena were employed to estimate the energy dilation (EI). The a-type films exhibited SnO2 as secondary phase whereas b-type films showed single phase In2O3:Sn with high sheet resistance. The lattice constants were found to be 10.16 and 10.09 A for a- and b-type films, respectively.

Photoluminescence characteristics of Mg- and Si-doped GaN thin films grown by MOCVD technique

Abstract We have studied the optical, structural and surface morphology of doped and undoped GaN thin films. The p- and n-type thin films have been successfully prepared by low-pressure MOCVD technique by doping with Mg and Si, respectively. The different carrier concentrations were obtained in the GaN thin films by varying dopant concentrations. Photoluminescence (PL) studies were carried to find the defect levels in the doped and undoped GaN thin films at low temperature. In the undoped GaN thin films, a low intensity and broad yellow band peak was observed. The donor–acceptor pair (DAP) emission and its phonon replicas were observed in both the Si or Mg lightly doped GaN thin films. The dominance of the blue and the yellow emissions increased in the PL spectra, as the carrier concentration was increased. The XRD and SEM analyses were employed to study the structural and surface morphology of the films, respectively. Both the doped and the undoped films exhibited hexagonal structure and polycrystalline nature. Mg-doped GaN thin films showed columnar structure whereas Si-doped films exhibited spherical shape grains.

Liquid phase deposited SiO2 on GaN

Abstract An efficient and low cost approach to deposit uniform silicon dioxide layers on GaN by liquid phase deposition (LPD) near room temperature are described and discussed. The process is simple. GaN wafers are immersed into a H2SiF6 and H3BO3 solution to form the silicon dioxide layers. The deposition conditions and the properties of the SiO2 films will be characterized.

Photo-enhanced native oxidation process for Hg/sub 0.8/Cd/sub 0.2/Te photoconductors

Proposes an easy and reproducible vapor-phase photo surface treatment method to improve the device performance of the Hg/sub 0.8/Cd/sub 0.2/Te photoconductive detector. We explore the effect of surface passivation on the electrical and optical properties of the HgCdTe photoconductor. Experimental results, including surface mobility, surface carrier concentration, metal-insulator-semiconductor leakage current, 1/f noise voltage spectrum, the 1/f knee frequency, responsivity R/sub /spl lambda//, and specific detectivity D* for stacked photo surface treatment and ZnS or CdTe passivation layers are presented. These data are all directly related to the quality of the interface between the passivation layer and the HgCdTe substrate. We found that, by inserting a photo native oxide layer, we can shift the 1/f knee frequency, reduce the noise power spectrum, and achieve a lower surface recombination velocity S. A higher D* can also be achieved. It was also found that HgCdTe photoconductors passivated with stacked layers show improved interface properties compared to the photoconductors passivated only with a single ZnS or CdTe layer.

Dark Currents in HgCdTe Photodiodes Passivated with ZnS/CdS

Experimental and theoretical results are presented for current-voltage and dynamic resistance-voltage characteristics of Hg 1-x Cd x Te ion-implanted p-n junction photodiodes with x 0.22 passivated with ZnS/CdS layers. By measuring the temperature dependence of the dc characteristics in the temperature range 25-140 K, the dark current mechanisms are studied and the validity of the modeling is confirmed. It was found that the dark currents can be represented with three current components over a broad range of voltage and temperature. At high temperature (>90 K) and in low reverse bias region, the diffusion current dominates. On the other hand, at medium temperature (40-80 K) and medium reverse bias (< -0.15 V), trap-assisted tunneling plays an important role. At low temperature (<40 K) and in the medium reverse bias region (< -0.15 V), band-to-band tunneling is the key leakage current source. However, when the temperature is further lowered to 25 K and the applied reverse bias is very small (-0.15 to 0 V), the band-to-band tunneling current will be ruled out and the trap-assisted tunneling mechanism dominates again.

1/f noise and specific detectivity of HgCdTe photodiodes passivated with ZnS-CdS films

1/f noise in HgCdTe photodiodes has been measured as a function of temperature, diode bias, and dark current. The dependence of 1/f noise on dark current was measured over a wide temperature range. At low temperatures, where surface generation and leakage current were predominant, a linear relationship between 1/f noise and dark current was observed. At higher temperatures, where diffusion current is predominant, the correlation no longer holds. The temperature dependence of 1/f noise was also determined. The temperature dependence of the 1/f noise was found to be the same as that for the surface generation and leakage currents. All the data obtained in these experiments could be fit with theoretical predictions by a simple relationship between 1/f noise and dark current. The 1/f noise in the HgCdTe photodiode varies with diode bias, temperature, and dark current only through the dependence of the surface current on these devices. The maximum specific detectivity (D*) value and the maximum signal-to noise ratio are approximately 3.51/spl times/10/sup 10/ cm/spl middot/Hz/sup 1/2//W and 5096 at 50 mV reverse bias, respectively.

The red shift of ZnSSe metal-semiconductor-metal light emitting diodes with high injection currents

The reliable n/sup +/-ZnSSe metal-semiconductor-metal (MSM) blue-green light emitting diodes (LEDs) have been fabricated. The contact metal was CuGe/Pt/Au. The current transport mechanisms agree very well with the back to back tunneling diodes. The kink phenomena were observed in the MSM current-voltage curves. In the metal-semiconductor interface, the element Zn in ZnSSe can be replaced by Cu results in some acceptor levels as radiative recombination centers in the MS interface. The peak wavelength in the LED electroluminescent (EL) spectra was strongly dependent on the injection currents from 5 to 40 mA. The peak wavelength and full width at half maximum are 510 and 10 nm, respectively, at 10 mA injection current. When the injection current increases to 15 mA, the peak wavelength shifted to 530 nm due to different recombination centers. Further increasing the injection currents, the peak wavelength shifted slightly to the long wavelength side.

MOVPE-Grown Ultrasmall Self-Organized InGaN Nanotips

It has been demonstrated that self-organized InGaN nanotips can be vertically grown via metal-organic vapor phase epitaxy (MOVPE) and thermal annealing. It was found that typical height of these nanotips is 20 nm with an average width of 1 nm. It was also found that the local density of vertically grown self-organized InGaN nanotips could reach 1.6 times 1013 cm -2. Furthermore, the overall uniformity in both height and width of nanotips were also demonstrated. These small sized vertical nanotips are potentially useful for field emission devices, blue light emitters and near-field microscopy.

Effects of Transparent Conductive Layers on Characteristics of InGaN-Based Green Resonant-Cavity Light-Emitting Diodes

InGaN-based green resonant-cavity light-emitting diodes (RCLEDs) with indium–tin oxide (ITO) and Ni/Au transparent conductive layers (TCLs) have been fabricated on Si substrates by laser lift-off and wafer bonding techniques. The RCLED structure consisted of an InGaN/GaN multiple-quantum-well active layer between the top (5 pairs) and bottom (7.5 pairs) dielectric TiO2/SiO2 distributed Bragg reflectors. It was found that the cavity mode of the RCLED with an ITO TCL shows a linewidth of 4 nm at the main emission peak at 494 nm. The electroluminescence intensity of the ITO-RCLED sample is 1.73 times higher in magnitude than that of the Ni/Au-RCLED one. It was found that the quality factor of the InGaN RCLED structure increased from 84 to 120 when the Ni/Au TCL was replaced by ITO. The improvements in both the optical output and the quality factor could be attributed to the higher optical transmittance of the ITO TCL enhancing the spontaneous emission at its resonant wavelength.