Tzu-Min Ou

Vertical organic triodes with a high current gain operated in saturation region

The authors report the fabrication of vertical organic triodes operated in the pronounced saturation regions by using two back-to-back pentacene Schottky diodes. The device with an optimized 0.7nm LiF hole injection enhancement layer and a 70nm pentacene emitter can achieve an ultrahigh common-emitter current gain of 48 in the saturation region at a low applied voltage VCE of −4V and a base current density of 0.25mA∕cm2. In addition, the device exhibits a high output current density of 12.1mA∕cm2 and a high on/off current ratio of 103.

All-organic hot-carrier triodes with thin-film metal base

In this letter, the authors investigate the promising vertical-type triodes based on small organic molecules and the related hot-carrier transport. The devices show transistorlike characteristics, in which output current can be modulated by demanding different input currents on their thin metal base electrodes. By using pentacene for the channel layer material and N,N′-di(naphthalen-l-yl)-N,N′-diphenyl-benzidine for the carrier energy-enhancing layer, the vertical-type hot carrier triodes exhibit a good current saturation with current gain of 2.38 for both the common-base and common-emitter configurations. The mechanism of operation is proposed and examined by the basic electrical measurements.

Pentacene-Based Planar- and Vertical-Type Organic Thin-Film Transistor

Pentacene-based planar- and vertical-type organic thin-film transistors (OTFTs) are investigated in this paper. High operation voltages are observed for the planar-type OTFTs with top source/drain electrodes, which results from the limitations of channel length and low material mobility. With a reduced channel length, a LiF hole-injection enhancement layer, and a thin metal gate, the vertical-type pentacene OTFTs exhibit a low-voltage operation of less than 5 V and a compatible on/off ratio of larger than 102. The smaller current gain observed from the device under current modulation is attributed to the increase of base recombination current under the common-emitter mode.

Temperature dependence of carrier dynamics for InAs∕GaAs quantum dot infrared photodetectors

Temperature-dependent micro-photoluminescence (μ-PL) spectra and the spectral response for the 30-period undoped InAs∕GaAs quantum-dot infrared photodetectors (QDIPs) are investigated. The competition of different transition levels within the spectra is observed. Also observed is the influence of dot size and density on the μ-PL characteristics. Consistently, the mid-infrared spectral response for the fabricated QDIPs exhibit the same energy position as the shifted PL spectra relative to the energy of wetting layer, which indicates the multi-transition mechanisms responsible for the QDIP spectral response.

Formation and Characterization of 1.5-Monolayer Self-Assembled InAs/GaAs Quantum Dots Using Postgrowth Annealing

In this paper, we report that low-density InAs/GaAs quantum dots (QDs) can be formed by postgrowth annealing the samples with 1.5-monolayer (ML) InAs coverage, which is thinner than the critical layer thickness for the Stranski-Krastanov growth. The annealing procedure was performed immediately after the deposition of the InAs layer. The effects of annealing time and annealing temperature on the dot density, dot size, and optical characteristics of the QDs were investigated. The optimum annealing conditions to obtain low-density QDs are longer than 60 s and higher than 500degC . Meanwhile, no luminescence can be observed for the wetting-layer, which may suggest that the postgrowth annealing will make the wetting layer thinner and thus reduce the effects of wetting layer on carrier relaxation and recombination. On the other hand, we observe that a decrease of the PL intensity at the annealing conditions of 60 s and 515degC , which is possibly due to the increasing surface dislocations resulted from the In adatom desorption at higher annealing temperature.

Transport mechanisms and the effects of organic layer thickness on the performance of organic Schottky diodes

Experimental results of static and dynamic characteristics for single-layer hole-only devices based on copper phthalcyanine (CuPc) and pentacene are observed in this article. The contribution to injection currents from electrode has been investigated by varying the thickness of the organic film. From the observation of current density versus bias voltage (J-V) characteristics, it is concluded that the space-charge-limited conductivity is the dominant transport mechanism for the organic Schottky diodes. Accordingly, an increase of the organic layer thickness will increase the trapping energy level. However, even with the thin CuPc film down to 50nm, the dynamic cut-off frequency of the device is still limited to 150Hz. Low hole mobility and large active area of the device are responsible for the phenomenon. Dramatic enhancement of cut-off frequency up to 11kHz can be obtained for the pentacene-based Schottky diodes.

The Surface Morphology and Optical Characteristics for InAs/GaAs Quantum Dots with Different Coverage

In this study, the influence of varied InAs coverage on InA/GaAs quantum dots (QDs) formation was investigated. With the thickest coverage of 2.6 monolayers (ML), two-group size distribution of QDs is observed. In this sample, extremely large and irregular shaped islands with height over 20 nm were formed, which could be produced by the coalescence of neighboring islands. And these islands were thought to be dislocated. However, other samples with lower coverage showed a more uniform distribution of QD size. And the red shift of ground state transition with increasing coverage can be attributed to the increasing height of QDs. QDs density decreases with increasing InAs coverage.