Doped Contact Regions Research Articles

Design of thermophotovoltaics for tolerance of parasitic absorption.

With advances in thermophotovoltaic (TPV) cells enabling recycling of sub-bandgap photons, a key barrier to reaching high prototype efficiencies has become radiative losses to parasitic high-emissivity regions, such as heavily doped contact regions, defects in coatings, and inactive areas. Here, we examine the impact of such radiative losses on the performance of various candidate cell materials, including GaAs, Si, InGaAsP, InGaAs, GaSb, and InGaAsSb. The ability of a TPV design to resist this performance loss is termed "radiation-sink tolerance" (RST). We show that RST is directly proportional to the spectral overlap between the absorptance profile of the cell and the emission profile of the emitter, which can be improved by adding a lower-bandgap absorber, increasing the emitter temperature, and utilizing a selective emitter. Our RST expressions can be used to estimate the efficiency of a prototypical TPV generator based on a component-level measurement.

Effects of contact space charge on the performance of quantum intersubband photodetectors

Highly non-uniform electric field exists in the active region of quantum intersubband devices, primarily due to the presence of PN junctions forming between heavily doped contact regions and non-intentionally doped barriers. Using a combination of experiments and theoretical simulations, we investigate the effect of this non-uniform internal electric field on the photodetector operation. Three quantum dots-in-a-well (DWELL) photodetectors have been fabricated with top spacer, bottom spacer, and no spacer around the active region, respectively, to demonstrate the effect of the non-uniform field. Drift-diffusion based calculations of the electric field provide further insight into the device operation.

Growth of Znx,Cd(1−x′)Se∕ZnxCdyMg(1−x−y)Se–InP quantum cascade structures for emission in the 3–5μm range

The molecular beam epitaxial growth and electroluminescence (EL) properties of Zn0.48Cd0.52Se∕Zn0.24Cd0.18Mg0.58Se quantum cascade (QC) structures are reported. The samples were composed of 30 repeats of a three-well active region. Cladding layers were inserted to isolate the core of the EL structure from the heavily doped contact region and to obtain optical confinement. Electroluminescence was observed in the 4–5μm range. The observed narrowing of the electroluminescence linewidth was tentatively attributed, in part, to the incorporation of the ZnCdMgSe waveguide layers. A test sample consisting of multiple asymmetric coupled quantum well active regions separated by quaternary barrier layers was also investigated. The Fourier transform infrared (FTIR) absorption spectroscopy measurements suggest that QC structures with EL emission in the 3–4μm range can be achieved with these materials.

Amorphous In–Ga–Zn–O coplanar homojunction thin-film transistor

A fabrication process of coplanar homojunction thin-film transistors (TFTs) is proposed for amorphous In–Ga–Zn–O (a-IGZO), which employs highly doped contact regions naturally formed by deposition of upper protection layers made of hydrogenated silicon nitride (SiNX:H). The direct deposition of SiNX:H reduced the resistivity of the semiconductive a-IGZO layer down to 6.2×10−3 Ω cm and formed a nearly ideal Ohmic contact with a low parasitic source-to-drain resistance of 34 Ω cm. Simple evaluation of field-effect mobilities (μsat) overestimated their values especially for short-channel TFTs, while the channel resistance method proved that μsat was almost constant at 9.5 cm2 V−1 s−1.

Thin active layer a-Si:H thin-film transistors

We show that hydrogenated amorphous silicon thin-film transistors (TFT's) with active layer thickness less than 50 nm have improved performance for display applications. Using two-dimensional (2-D) modeling, we find previously observed degradation for thin active layers is due to electric field effects in the contact regions of staggered inverted devices and affects only the saturation characteristics; linear region performance actually improves with decreasing thickness. We have fabricated devices with extremely thin active layer (10 nm), and indeed find excellent linear region characteristics. In addition, direct tunneling across the undoped regions at device contacts reduces electric field effects, resulting in excellent saturation region characteristics, and gate-induced channel accumulation reduces the Schottky barrier width at direct metal contacts so that even devices without doped contact regions (i.e., tunneling contacts) are possible.

Negative luminescence from In 1− xAl xSb and Cd xHg 1− xTe diodes

Indium aluminium antimonide (In 1− x Al x Sb) and cadmium mercury telluride (Cd x Hg 1− x Te) heterostructure diodes, which comprise a near intrinsic active region bounded by more highly doped contact regions, exhibit positive or negative luminescence at medium to long infrared wavelengths when forward or reverse biased respectively at room temperature. In reverse bias, the carrier densities in the near intrinsic region are reduced below their equilibrium values by the effects of exclusion and extraction. In consequence, the radiative recombination is reduced and the devices emit less infrared radiation than the thermal equilibrium value. The observed intensity of the negative luminescence is in general agreement with expected values.

Near-contact diffusion and compensation in extrinsic photoconductors

Carrier diffusion from heavily doped contact regions has been experimentally monitored with resistivity measurements of variable thickness Si: P extrinsic photoconductors (n +−n−n +). Modeling of free carrier diffusion at the interface of heavily doped contacts and highly resistive bulk at low temperatures predicts extended regions (5–30 μm) of excess carriers in high purity materials so that, in thin device structures, free carrier diffusion profiles from each contact will overlap and determine the resistivity of the device. In this work, a decrease in resistivity of four orders of magnitude was observed in a 5 μm thick structure compared to a 10 μm thick device. The resistivity of an ohmic structure in the thin limit is strongly dependent on the bulk and near-contact compensation, and resistivity measurements can be used as a sensitive measure of compensation at interfaces or in the tail of implanted layers.

Modelling the effect of an impurity band on the I-V characteristics of resonant tunnelling diodes

Resonant tunnelling diodes for device applications often have to contain very heavily doped contact regions. When the doping is so high, there is a substantial degree of auto-compensation of the dopant, and the impurity band merges with the conduction band, giving a large band tail in the density of states at the bottom of the conduction band. The effect of this band tail on the current-voltage characteristics is investigated, using the GaAs/AlGaAs double-barrier diode as an example system. The bias at peak current is found to increase, and the peak itself to broaden. A small increase in valley current is also found. Also, experiments are suggested in which double-barrier diodes with deliberately heavily doped, highly compensated contact regions are used to study the extent of the impurity band.

Magneto-resonant tunneling from a lightly doped contact region interacting with quasi-two-dimensional states in an accumulation layer

We have studied the magnetic field dependence of the tunneling current in GaAs/AlAs double barrier structures. We report new results, periodic with the inverse of the magnetic field, on a weak feature on the rising side of the main resonant tunneling peak. This feature is attributed to resonant tunneling from a lightly doped contact region interacting with the quasi-two-dimensional state in the accumulation layer. A qualitative interpretation is given.

A two-dimensional electron-gas as a thermocouple phonon detector

A thermocouple consisting of a two-dimensional electron gas (2DEG) and the doped contact regions was found to be a sensitive detector for ballistic phonons.

The importance of contact injection in undoped AlGaAs/GaAs heterostructures

A two-dimensional finite-difference analysis is used to simulate the current flow and charge distribution in undoped AlGaAs/GaAs MODFET structures called heterostructure insulated-gate field-effect transistors (HIGFETs). How an electron channel can be induced at the AlGaAs/GaAs interface when there is no doped supply layer is studied. The origins of the charge in this channel are studied by comparing its behavior in equilibrium with that when a current flows. By considering the different geometries of a Schottky gate and implanted source and drain contacts, the interrelationship between gate control and electron injection from the doped contact regions is shown and a graphic view of the origins of the interfacial charge in a HIGFET is given.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Correlation Between the Interfacial Nonuniformity and the Specific Contact Resistance of Ohmic Contacts to Gaas

ABSTRACTConsidering the effect of the simultaneous presence and interaction of the different phases at the contact, a modification of the model presented by Wu and coworkers (Solid-St. Electron. 29 (1986) 489] for explanation of ohmic contact resistance of n-GaAs was developed. The modified model combines the existence of the mixed phase structure of AuGeNi/n-GaAs contact with assumptions proposed by Wu et al. that the specific contact resistance Rc contains two parts Rcl and Rc2, where Rc1 is the specific contact resistance of the alloyed and underlaying doped contact region, and Rc2, is that of the high-low junction between the heavily doped contact region and the bulk semiconductor. The Rc1 depends strongly on the apparent barrier height and the effective impurity concentration formed by doping from the contact alloys during annealing. In the present paper a new theoretical model for Rc1 is proposed and compared with the experimental results.

An improved model to explain ohmic contact resistance of f-GaAs and other semiconductors

A model is proposed which explains the inverse proportionality between specific contact resistance ϱ c and the carrier concentration ( N D ) of n-GaAs ohmic contact obtained experimentally very well and can be extended to the ohmic contact of other semiconductors. This model assumes that ϱ c is composed of two parts ϱ c 1 and ϱ c 2. The contact resistivity ϱ c 1 is due to the contact between the alloy and the underlying heavily doped contact region ( N DC for n-type contact, N AC for p-type contact) formed by doping from the contact alloys during annealing. The contact resistivity ϱ c 2 is caused by the barrier height (Φ 2) of the high-low junction between the heavily doped contact region and the bulk material. There are two aspects: (1) If bulk material is degenerated, i.e. N D > N C ( N C : effective density of states in conduction band) or N A > N V ( N V : effective density of states in valence band), the barrier height Φ 2 vanishes and ifϱ (c) is mainly determined by ifϱ c 1 which depends solely on concentrations N DC and N AC . It is calculated theoretically. (2) When the bulk material is lightly doped, i.e. N D < N C or N A < N V , then Φ 2 appears and increases with the decreasing of N D ( N A ). If N DC or N AC are high enough, the field emission is the main mechanism to control the carrier transport between the contact alloy and the underlying layer. In this case ϱ c is predominantly determined by ϱ c 2 and an inverse proportionality between ϱ c and N D or N A can be found.

Amorphous silicon based alloy solar cell modeling with new diffusion length interpretation

We have developed a computer model of amorphous silicon based alloy solar cells which includes the complete set of transport equations, and the use of heavily doped contact regions to realistically set up the boundary conditions. We have used this model to reveal the physics of amorphous silicon based alloy devices and to deduce material parameters from experimental data. For p-i-n cells illuminated through the n + layer our results show that boron doping in the intrinsic layer can increase the short circuit current by enhancing the electric field near to the n +−1 interface. We show that the open circuit voltage in practical devices is determined by the recombination current, rather than by the built-in potential, and that provided the n + layer is heavily doped, carrier back diffusion does not significantly affect the short-circuit current of p-i-n devices. Our analytical and computer calculations demonstrate that the zero field minority carrier diffusion length is intensity dependent. We relate this dependence to the density of localized states near to the valence band edge. This provides us with a novel experimental technique to measure the slope of the density of states approximately 1.2 to 1.3 eV below the conduction band edge which our experimental results show to be approximately 2800 K.

Focused ion beams in microfabrication

Microfabrication techniques are being investigated in which a focused high-energy ion beam bombards a surface to create high-resolution patterns of implantation doping, damage in crystal structures, or other chemical change such as molecular bond breaking. The objective is to demonstrate unique microfabrication techniques such as maskless implantation doping and the formation of self-aligned structures. Preliminary results are presented of doped regions written in silicon by a focused boron beam. Electrical measurements on heavily doped contact regions and on a lightly doped resistive region are in good agreement with what would be expected from conventional ion implantation employing masks. The exposure of electron beam resist by a focused beam of 60-keV helium ions is also reported. The resist sensitivity is ∼100 times greater for these ions than for electrons. The focused ion beam used in these studies was generated in a focusing arm which includes apertures, deflection plates, and an einzel lens that has been added to an existing ion implantation system. The focusing system performance is discussed and compared with the properties of a published electrostatic lens. Experimental results indicate a minimum beam spot diameter of ∼3.5 μm with a 150-μm-diam object aperture and a beam energy of ∼60 keV. Aperture diameters are calculated to produce a focused spot with submicrometer diameter.