Doped Contact Layers Research Articles

Terahertz optical properties of thin doped contact layers in GaAs device structures

We investigate the transmittance of thin doped GaAs layers in the terahertz (THz) frequency range taking into account multiple reflections. Our experimental and theoretical study aims at providing a guideline for designing the top-side contact layers for THz emitters and receivers with direct, antenna-free coupling of the radiation. It is shown that the surface conductivity of the contact layer is the determining factor for the THz transmittance.

Tailoring the electronic properties of GaAs/AlAs superlattices by InAs layer insertions

We investigate the electrical and optical properties of GaAs/AlAs superlattices (SLs) in which a thin (⩽1.2 monolayers) InAs layer is inserted in the central plane of each GaAs quantum well. The InAs layer modifies the structure of the SL unit cell and provides an additional design parameter for tailoring the energy of the lowest miniband and the size of the minigap. We exploit this effect to enhance electron injection from a doped contact layer into the first miniband and to inhibit interminiband coupling.

Tunnel diode collector contact in InP based PNP heterojunction bipolar transistors

A tunnel diode collector contact to InP based PNP heterojunction bipolar transistors (HBTs) is suggested and demonstrated. The additional heavily doped n-type contact layer replaces the thick p-type contact layer required in conventional structures. The thermal and electrical properties of the collector contact layer thus become similar to those of NPN HBTs. A secondary ion mass spectroscopy study explores the maximum tin doping level that can be obtained in the base. Finally, the temperature dependence of the current gain is presented and interpreted.

Some Considerations on Optical Confinement and Free Carrier Confinement of Quantum Cascade Lasers

Calculating the refractive index dispersions considering the influence of doping concentration on refractive indices, we have illustrated the heavily doped contact layers, n = 1 × 1019 cm-3 for λ similar 9 μm, and n similar 1 × 1020 cm-3 for λ = 5 μm quantum cascade lasers, to suppress the refractive indices and get sufficient optical confinement. By calculating the transmission of energies through a graded gap superlattice, it is demonstrated that for energies E1 and E2 (the two lower states) the electrons can easily get through the graded gap superlattice. While for energy E3 the graded gap superlattice creates a minigap (the transmission of the energies E1 and E2 is 102 times more than that of E3). In this way, the electrical confinement can be achieved.

A four-color quantum well infrared photodetector

A four-color quantum well infrared photodetector (QWIP) has been demonstrated in this work. Four stacks of quantum well structures with four different detection wavelengths are sandwiched among three highly doped contact layers. The peak wavelengths of the four colors are centered at 4.7, 8.5, 9, and 12.3 μm. The 4.7 and 8.5 μm stacks are separated from the 9 and 12.3 μm stacks by a middle ohmic contact layer, and the change of peak detection wavelengths within the two-stack QWIPs is achieved by varying the bias voltage. Four different combinations of two-color simultaneous reading can be obtained. The detector could achieve simultaneous reading of four colors by adding two extra contact layers to the design with appropriate readout circuitry. By using a small number of quantum wells, we are able to use all four stacks for voltage-tunable detection with two terminals. In spite of using InGaAs/AlGaAs and GaAs/AlGaAs materials in the four stacks, the device shows excellent material quality and performance characteristics.

Investigation of a multistack voltage-tunable four-color quantum-well infrared photodetector for mid- and long-wavelength infrared detection

A four-color quantum-well infrared photodetector (QWIP) for mid- and long-wavelength infrared (MWIR and LWIR) detection has been demonstrated in this work. Four stacks of QW structures with four different detection wavelengths are sandwiched between three highly doped contact layers. Peak detection wavelengths of this device are centered at 4.7, 8.5, 9, and 12.3 /spl mu/m, respectively. The 4.7- and 8.5-/spl mu/m stacks are separated from the 9- and 12.3-/spl mu/m stacks by a middle contact layer, and the peak detection wavelength within these two double-stack QWIP's can be tuned by the applied bias. Four different combinations of two-color simultaneous readings can be achieved. By using a small number of QW's and balancing the impedance between the stacks, we are able to use all four stacks for voltage-tunable multicolor detection with two terminals. The bias and temperature dependence of the dark current and peak responsivity as well as the dynamic impedance in this four-color QWIP were systematically studied over a wide range of bias and temperature. In spite of using four different stacks of strained InGaAs-AlGaAs and GaAs-AlGaAs materials, the device shows excellent material quality and performance.

Capacitance Spectroscopy of Single-Barrier GaAs/AlAs/GaAS Structures Containing InAs Quantum Dots

In this paper we investigate the capacitance-voltage, 0(V), and magneto-capacitance, 0(B), characteristics of a series of single-barrier ni-n GaAs/AlAs/GaAs heterostructures incorporating a layer of self-assembled InAs quantum dots (QDs) in the AlAs barriex. Thicknesses of the layers of AlAs deposited before and after the InAs QD growth are varied. The C(V) and C(B) results are compared with those from a control sample that has no InAs and a sample in which the InAs thickness is such that a continuous wetting layer (WL) of InAs is grown. The samples presented in this study were grown on a (100) n+-GaAs substrate using MBE and all consist of n-type GaAs devices with an AlAs tunnel barrier region surrounded by 100 nm not intentionally doped spacer8 to minimise the effect of dopant diffusion into the central barrier region. These spacers are then enclosed by 100 nm of n-type GaAs doped to a density of 10 16 cm -3 lightly doped region, followed by 100 nm doped to 10 17 cm-3 , and then a heavily doped contact layer, at 10 18 cm -3 . The control sample consists of a 10 nm AlAs single barrier, while the WL sample consists of an AlAs double barrier structure enclosing 1.4 ML of InΑs. The QD samples consist of a InAs QD layer enclosed in AlAs layers grown previously and subsequently to the QD growth. The QDs are produced by deposition of 1.8 ML of InAs at a growth temperature of 520°C. The detailed layer make-up of the samples presented in this work are shown in Table. The band diagram of the samples at zero voltage and under applied bias is drawn schematically in Fig. 1. Electrical contacts were formed on the doped GaAs

Photofield interface impurity spectroscopy in blocked impurity band Si:B structures

The far infrared spectral response of photoconductive boron doped silicon blocked impurity band structures was measured at electric field values up to 7 kV/cm and sample temperatures in the range of 4–15 K. The structures are constructed of adjacent layers of pure (NA=5×1013 cm−3) and boron doped (NA=1018 cm−3) silicon inserted between highly doped ohmic contact layers. A broad-band photoresponse is observed which is attributed to the impurity band of the doped layer. In addition, several peaks of response are observed, even at the lowest sample temperatures, which are attributed to optical transitions from ground states to excited states of isolated impurities. It is shown that these transitions originate from a thin layer at the interface between the doped and the pure silicon regions. When the electric field is oriented towards the doped layer, the photocurrent peaks are attributed to the tunneling of photoexcited holes across excited states into near neighbor atoms within a hole depleted delta sheet, followed by relaxation to the ground state. Then hopping transport occurs across the doped layer to the ohmic contact. In this scheme, the hole depleted layer is generated by the effect of the applied electric field and involves only 109–1010 cm−2 impurities. This study provides evidence of a new mechanism for extrinsic photoconductivity at interfaces.

Fully self-aligned tri-layer a-Si:H thin-film transistors with deposited doped contact layer

We demonstrate a new self-aligned TFT process for hydrogenated amorphous silicon thin-film transistors (a-Si:H TFTs). Two backside exposure photolithography steps are used to fabricate fully self-aligned tri-layer TFTs with deposited n/sup +/ contacts. Since no critical data alignment is required, this simple process is well suited to fabrication of short channel TFTs. We have fabricated fully self-aligned tri-layer a-Si:H TFTs with excellent device performance, and contact overlaps 10/sup 7/.

Full wafer optical characterisation of resonant tunnelling structures using photoluminescence excitation spectroscopy

We report on a novel optical technique for accurately characterising unprocessed GaAs AlGaAs tunnelling structures having two or more thin barriers and thick, heavily doped, contact layers. The results from photoluminescence excitation spectroscopy measurements are presented in which the intensity of the photoluminescence from the heavily doped contact layers rather than photoluminescence from the quantum wells is monitored. This method gives essential information for growth and quality control of “as-grown” layers prior to device fabrication. The usefulness and precision of this technique is illustrated by the measurement of a 0.3% variation in layer thickness from the centre to the edge of an as-grown wafer, and a 1% wafer to wafer variation in layer thickness in a series of triple barrier resonant tunnelling structures that have been designed for use at THz frequencies.

Near-Intrinsic Microcrystalline Silicon for Use in Thin Film Transistors

ABSTRACTInverted-staggered thin film transistors (TFTs) incorporating hydrogenated microcrystalline silicon for both contact and channel regions have been fabricated by plasma enhanced chemical vapour deposition (PECVD) using the high hydrogen-dilution method. The deposition parameters for the channel region were chosen to yield near-intrinsic material with a dark conductivity activation energy of 0.7 eV and a Tauc gap of 1.98 eV, while the doped contact layer was optimised to produce a high dark conductivity of 10 S/cm.These devices exhibit a low off-current but the field effect mobility is found to be lower than that of similar devices incorporating an optimised amorphous silicon channel region. The mobility activation energy in these devices is similar to those incorporating an amorphous channel, but the mobility pre-factor is reduced. We propose that this is due to inhomogeneous conduction through a microcrystalline region with a smaller grain size at the dielectric/channel interface.

New Fabrication Technology for Integrating Field Effect Transistors and Diodes

We demonstrated a new integration technology for field effect transistors (FETs) and diodes. This technology uses selective-area metalorganic chemical vapor deposition (MOCVD) for FET and diode layers. FETs (called FECFET) fabricated by this technology have very short distances between the drain and source, so that the characteristics such as knee voltage and transconductance are superior to those of conventional FETs with the same gate length. Because this structure has an n+-doped contact layer under an n-doped active layer, it is easy to integrate with a high-quality diode having low series resistance. By this approach, diodes with low series resistance and low junction capacitance have been fabricated. To demonstrate the integration of the diode and FECFET, a 36 GHz single balanced diode mixer and an intermediate frequency (IF) amplifier have been fabricated on a GaAs wafer.

Hybrid Shubnikov–de Haas-photoluminescence analysis of two-dimensional electron density in strained quantum well structures with heavily doped contact layers

A hybrid analysis technique is presented to accurately extract the two-dimensional (2D) electron density of PHEMT structures in which multiple subbands are occupied and severe parallel conduction by heavily doped contact layers occurs. Complications due to shorted Hall voltages by the parallel contact layer, which precludes simple Hall analysis, and to multiple subband occupation, which requires high magnetic-field sweeps in Shubnikov–de Haas (SdH) measurements, are eliminated by this hybrid analysis that combines SdH measurements with photoluminescence measurements to extract the total 2D density. Comparisons with other methods demonstrate the high accuracy of this new technique.

Characterization of Si/SiGe Strained-Layer Superlattices Grown by an Ultrahigh Vacuum/Chemical Vapor Deposition Technique

High-resolution double-crystal X-ray diffraction and cross-sectional transmission electron microscopy were used to characterize the Si/SiGe strained-layer superlattices grown by the ultrahigh vacuum/chemical vapor deposition (UHV/CVD) system. A dynamical X-ray simulation was employed to analyze the experimental rocking curves. Good matches between the experimental rocking curves and the simulated ones demonstrate that the high quality Si/SiGe strained-layer superlattices with abrupt interface and excellent thickness and composition uniformity have been achieved. Thickness uniformity was further confirmed by the cross-sectional transmission electron microscopy. In addition, high-resolution double-crystal X-ray diffraction was proven to be a powerful technique for determining the doping level and the thickness of heavily doped contact layer in Si/SiGe device structures prior to further processing. Good wafer-to-wafer uniformity can be simultaneously achieved by the UHV/CVD technique; as a result, the UHV/CVD technique is readily applicable to manufacturing.

GaAs via-hole etching and MOMBE regrowth

The dry etching of features up to 40 mu m deep in GaAs using PCl3/Ar electron cyclotron resonance discharges and thin (<or=2000 AA) metal masks is described. Of the masking materials investigated, Ni was found to be most resistant to erosion during the plasma exposure and the resultant GaAs surface morphology was smooth and suitable for epitaxial regrowth of doped contact layers. Metalorganic molecular beam epitaxy is shown to provide conformal coverage of GaAs on the dry etched surface. Vertical sidewalls are easily obtained when using metal masks, and this process is completely compatible with fabrication of heterojunction bipolar transistors.

Diffusive electrical conduction in high-speed p-i-n photodetectors

We report on time-resolved photocurrent measurements in InGaAs/GaAs-quantum well p-i-n photodetector structures in which the photocurrent is optically excited and electrically detected at different locations on the sample. As the electrical pulse propagates out from the point of excitation, a temporal broadening of the signal is induced by the in-plane resistance of the doped contact layers. This temporal broadening agrees quantitatively with analytical solutions obtained from the diffusion equation used to describe the lateral voltage propagation. We discuss the significance of this temporal behavior for the optimization of high-speed photodetectors. Finally, we point out the relationship between our optical-pump/electrical-probe experiments and the all-optical pump/probe experiments reported by Livescu et al.

Enhanced mobility top-gate amorphous silicon thin-film transistor with selectively deposited source/drain contacts

Amorphous silicon thin-film transistors (TFTs), in a top-gate staggered electrode structure have been prepared using selectively deposited doped silicon contact layers, formed in-situ by plasma-enhanced chemical vapor deposition (PECVD). Selective deposition reduces the number of processing steps and assures the formation of low-resistance contacts. Devices fabricated with two photomasks and one plasma deposition step show saturation and linear mobilities as high as 1.1 and 0.9 cm/sup 2//V-s, respectively, with threshold voltages between 3 and 6 V. On/off ratios are >10/sup 6/, with a subthreshold slope of 0.8 V/decade. The mobilities are at least a factor or 2 higher than previously reported for top-gate structures and are similar to values reported for bottom-gate (inverted staggered) TFTs.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Stacked Si-bipolar diodes with lateral polycrystalline inter-connections grown by two-step MBE

Individually addressed Si-bipolar diodes stacked monolithically in two planes were fabricated, applying a variety of novel technologies. A two-step differential Si-MBE process allows us to grow monocrystalline devices embedded in a poly-Si surrounding which provides lateral or vertical isolation and buried interconnections. The doping levels of the active devices ( n, p ≲ 10 17 cm −3) are concomitant with a high resistivity poly surrounding (10 4-10 5 Ω cm), while highly doped contact layers (> 10 19 cm −3), grown between the two device levels and above the second level are of low resistivity in the monocrystalline (10 −3-10 −1 Ω cm) and in the poly area (10 −2−5 × 10 −1 Ω cm). MBE-growth was interrupted after deposition of the intermediate contact layer to define interconnections by photo-lithography, which were subsequently overgrown by a second MBE process. Patterning of the highly conductive cap layer and etching of contact holes to the by now buried leads permits electrical access to both device levels. The measured device characteristics demonstrate the feasibility of the concept, which also appears very attractive for three-terminal devices. Routes towards optimized growth and doping sequences are discussed.

Influence of the boundary between monocrystalline and polycrystalline silicon on the electrical characteristics of diodes grown by differential MBE

The differential MBE technique is suggested as a tool for separation or interconnection of advanced devices on a common chip. Bipolar diodes with various diameters of 23 to 800 mu m were fabricated and their electrical behaviour measured. Implantation of etching methods were used to eliminate the influence of a highly doped contact layer on top of the epitaxial layer sequence. Leakage currents from 10-7 to 10-9 A are slightly higher than those of reference mesa diodes. It is estimated that the differential technique is applicable to many advanced devices for planar integration.

Carrier transport effects on the photocurrent spectra of an intrinsic GaAs/AlGaAs multiple quantum well with highly doped AlGaAs contact layers

In this study, we demonstrate how electroreflectance (ER) measurements as a function of bias, and of angle of incidence ( θ 0), together with bias dependent photocurrent (PC) measurements, can be used to provide understanding of the complex electric field profile and carrier transport effects in a GaAs/Al 0.3Ga 0.7As multiple quantum well (MQW), grown inside n + contact layers. The PC measurements exhibit split excitonic features, the components of which change in strength with the applied bias. The effect is explained by absorption in the front of the MQW stack, with the back of the stack acting as detector. We examine the θ 0-dependence of the ER lineshape, to determine the depth of the layers responsible for each feature. The ER and PC lineshapes and their bias dependence are explained by the unusual electric field profile across the stack. The field profile appears to be determined by tunnelling of the dark current.