P.V.V. Jayaweera

Uncooled infrared detectors for 3–5μm and beyond

Avoiding cryogenic cooling not only reduces the cost and weight but also simplifies the infrared detector system allowing widespread usage. Here an uncooled infrared detection using intravalence bands is reported. A set of three p-GaAs∕AlxGa1−xAs multiple heterojunction detector structures were used to demonstrate the concept experimentally. A preliminary detector showed peak responsivity of 0.29mA∕W at 2.5μm at 300K. The intravalence band approach can be used to cover various wavelength ranges by using different material systems giving rise to the possibilities of a dual band detector operating in atmospheric windows.

Dye-sensitized photoelectrochemical and solid-state solar cells: charge separation, transport and recombination mechanisms

Experiments on dye-sensitized (DS) photoelectrochemical cells made from SnO2, ZnO and comparison with similar cells based on TiO2 gives much insight into the nature of charge separation, transport and recombinations. It is shown that the trap mediated recombinations are sensitive to the effective electron mass and therefore explains difference between the cells made from TiO 2 and SnO2 or ZnO. Considering the trap mediated electron leakage from nanocrystallites, a theoretical model is constructed to explain quantitatively the effect of trapping on static and transient behavior of the cells. The model clearly demonstrate that trapping seriously affects the performance of the cell, when the electron leakage from traps is significant. The predictions of the model are compared with experimental data on transient measurements. The paper will also comment on the problem of recombinations in DS solid-state cells.

Room temperature nano- and microstructure photon detectors

The development of room temperature infrared (IR) detectors for wavelengths beyond NIR will open up many applications that are currently limited due to cooling requirements. Three approaches are discussed, which show promise for room temperature IR detection. Tunneling quantum dot (QD) detector, utilizes a tunneling barrier in order to block the dark current while permitting the photocurrent to pass through due to resonance effects, has shown room temperature response for a detector operating at 6 and 17μm. The PbS QDs in a dielectric medium utilizes electronic polarizability of QDs, sensing only the variations of the radiation intensity, operating at ambient temperature. This method allows narrow multiple response bands. A GaAs/AlGaAs heterojunction detector, utilizing light, heavy and split-off hole transition in a p-doped semiconductor, shows a threshold of 3.4μm operating up to 330K.

GaSb homojunctions for far-infrared (terahertz) detection

A GaSb based homojunction interfacial work function internal photoemission far-infrared (>30μm) detector is presented. Metal-organic vapor phase epitaxy grown p-GaSb∕GaSb samples show 9.7A∕W peak responsivity and a peak detectivity of 5.7×1011 Jones with effective quantum efficiency of 33% at 36μm and 4.9K. The detector exhibits a 97μm (∼3THz) free carrier response threshold wavelength. Results indicate that p-GaSb homojunction internal work function internal photoemission detectors are promising candidates to be a competitor for terahertz applications.

Dye-sensitized near-infrared room-temperature photovoltaic photon detectors

Dye molecules bonded to a semiconductor surface could inject carriers to a band on photoexcitation. This process known as dye-sensitization is used for extending the sensitivity of silver halide emulsions. More recently, dye-sensitization has been adopted to devise solar cells. A near-infrared (NIR) sensitive heterojunction n‐TiO2∕D∕p‐CuSCN (where D denotes a NIR absorbing dye) is developed to examine the possibility of using dye-sensitization for IR detection. Although the responsivity is lower and response slow compared to silicon detectors, dye-sensitized detectors would be cost effective, especially for large area devices. They are operable at room temperature and have the advantage of insensitivity to noise induced by band-gap excitations (providing high specific detectivity of ∼1011). Furthermore, the spectral response can be adjusted by choosing the appropriate dye.

Normal incidence detection of ultraviolet, visible, and mid-infrared radiation in a single GaAs/AlGaAs device.

A GaAs/AlGaAs detector is demonstrated showing multiple detection capabilities. This detector exhibits a broad spectral response in the 200-870 nm (ultraviolet-visible) range for forward bias and in the 590-870 nm (visible) range for reverse bias. In the mid-IR region, two peaks at 5 and 8.9 microm can be observed for low and high forward bias voltages, respectively. In addition, the peak at 8.9 microm is sensitive to the polarization of the incoming radiation.

1/f noise and dye-sensitized solar cells

The adsorbed molecular species, such as H2O and I2, that produce electron acceptor states on the TiO2 surface are found to generate 1/f noise in the electric current through nanocrystalline films of TiO2. It is suggested that the trapping and detrapping of electrons at the surface states is the cause of this noise. When the TiO2 film surface is coated with dyes, the passivation of the active surface sites suppresses 1/f noise. Implications of 1/f noise on the functioning of the dye-sensitized solar cell, notably the effect of adsorbed iodine in inducing recombinations, are discussed.

Semiconductor terahertz detectors and absorption enhancement using plasmons

As novel applications using terahertz radiation are developed, there is an increased demand for sensitive terahertz detectors. This has led to new approaches for enhancing the response of terahertz detectors. Results were recently reported on the terahertz response of a p-type AlGaAs/GaAs, n-type GaAs/AlGaAs, n-type GaN/AlGaN, and p-type GaSb/GaSb Interfacial Workfunction Internal Photoemission detectors. The use of surface plasmon coupling due to metal grids is one approach discussed here to enhance the performance of these terahertz detectors. Due to the greatly enhanced near fields of the plasmons, the absorption would be increased leading to improved detectors. Results are presented on the enhancement of absorption by plasmon effects in a thin film coupled with a metal grid.

Displacement currents in semiconductor quantum dots embedded dielectric media: A method for room temperature photon detection

It is shown that the high electronic polarizability of quantum dots can be utilized to devise photon detectors by embedding quantum dots in dielectric media to form capacitors. Modulated light generates displacement currents and an expression is obtained for responsivity in terms of the properties of the quantum dot, the dielectric, and the detector geometry. A model detector constituted of PbS quantum dots embedded in paraffin wax is devised to illustrate the principle, giving ∼0.6A∕W as an upper limit for the responsivity. As these systems sense only the variations of the light intensity, they could be operated at ambient temperature.

Analysis of Dark Current Mechanisms for Split-Off Band Infrared Detectors at High Temperatures

An analysis of dark current mechanisms has been performed on high-operating-temperature (up to 330 K) split-off (SO) band p+-GaAs/AlGaAs heterojunction infrared detectors (3-5 μm). In contrast to conventional 1-D current models due to carrier transport based on tunneling and/or thermionic emission mechanisms, a 2-D electrical model is used to explain nonuniformity degradation of zero-bias differential resistance (RoA) with temperatures as measured on SO detectors. The 2-D characteristic of carrier transport could have the limitation on high-temperature performances of detectors and, hence, needs optimizing. A theoretical model shows that this 2-D effect can be reduced by structural modifications such as using smaller mesa sizes, higher doping of the p+ -GaAs layer, and a higher potential barrier that prospectively provides better electrical uniformity for SO detectors working at high temperatures.