J. Sexton

Thickness control of Molecular Beam Epitaxy grown layers at the 0.01-0.1 monolayer level

Electron tunnelling through semiconductor tunnel barriers is exponentially sensitive to the thickness of the barrier layer, and in the most common system, the AlAs tunnel barrier in GaAs, a one monolayer variation in thickness results in a 300% variation in the tunnelling current for a fixed bias voltage. We use this degree of sensitivity to demonstrate that the level of control at 0.06 monolayer can be achieved in the growth by molecular beam epitaxy, and the geometrical variation of layer thickness across a wafer at the 0.01 monolayer level can be detected.

Fabrication of Submicrometer InGaAs/AlAs Resonant Tunneling Diode Using a Trilayer Soft Reflow Technique With Excellent Scalability

A trilayer soft reflow fabrication method using solvent vapor that resulted in a submicrometer resonant tunneling diode (RTD) is reported in detail. The processing steps are simple, time efficient and are all based on conventional i-line photolithography. The trilayer soft reflow technique is able to shrink the emitter lateral width from 1 down to 0.35 μm (65% reduction) using a solvent at a very low temperature (<;50 °C). Studies of devices peak current density (JP) suggest that excellent scalability is achieved as the emitter area reduces from ~29 down to ~0.5 μm2 with no significant increase in peak voltage (VP) due to high series resistance normally associated with submicrometer dimensions. The valley current (IV ), however, increases due to side-wall damage introduced by the reactive ion etching (RIE) process. As a result, the peak-to-valley-current ratio decreases from 5.0 (6.3) to 3.8 (4.1) in forward (reverse) direction as the emitter area decreases. We therefore successfully demonstrated the fabrication of a submicrometer RTD using a trilayer soft reflow technique that has the benefit of excellent scalability, high throughput, repeatable, and a reliable low-cost process.

Extremely Uniform Tunnel Barriers for Low-Cost Device Manufacture

We report on the final steps needed to achieve the level of control over the properties of single tunnel barriers of AlAs needed to allow the manufacture of high-volume low-cost microwave and millimeter-waves detectors. We achieve a 1% standard deviation of the current-voltage characteristics across 2-in wafers and average currents from different wafers varying by 1%, when modeling shows that a monolayer error in the AlAs barrier layer thickness would result in a 270% change in the same electrical characteristics.

Using Quantum Confinement to Uniquely Identify Devices

Modern technology unintentionally provides resources that enable the trust of everyday interactions to be undermined. Some authentication schemes address this issue using devices that give a unique output in response to a challenge. These signatures are generated by hard-to-predict physical responses derived from structural characteristics, which lend themselves to two different architectures, known as unique objects (UNOs) and physically unclonable functions (PUFs). The classical design of UNOs and PUFs limits their size and, in some cases, their security. Here we show that quantum confinement lends itself to the provision of unique identities at the nanoscale, by using fluctuations in tunnelling measurements through quantum wells in resonant tunnelling diodes (RTDs). This provides an uncomplicated measurement of identity without conventional resource limitations whilst providing robust security. The confined energy levels are highly sensitive to the specific nanostructure within each RTD, resulting in a distinct tunnelling spectrum for every device, as they contain a unique and unpredictable structure that is presently impossible to clone. This new class of authentication device operates with minimal resources in simple electronic structures above room temperature.

Elimination of Current Blocking in Ternary InAlAs-InGaAs-InAlAs Double Heterojunction Bipolar Transistors

Molecular beam epitaxy-grown wafers are used to fabricate all ternary In_0.52Al_0.48As-In_0.53Ga_0.47As-In_0.52Al_0.48As double heterojunction bipolar transistors (DHBTs) with knee voltages of less than 1 V, showing no current blocking characteristic even at current densities of 200 kA/cm². A set of wafers with a judicious combination of doping interface dipoles and composite collector designs were grown, and devices with a wide range of emitter areas from 20 × 20 down to 1 × 5 μm² were fabricated to investigate the effects of the different epitaxial and geometrical design tradeoffs that culminated in an optimum design that is able to achieve high breakdown and high current gain without introducing current blocking. Despite the use of a heavy dipole doping of 4 × 10¹⁸ cm^-3, a breakdown voltage BV_CEO of 5.8 V at 0.2 kA/cm² is achieved at room temperature. We believe this to be the first demonstration of an all-ternary large band gap InAlAs-InGaAs-InAlAs DHBTs with no current blocking up to a high current density of 200 kA/cm². These new DHBTs that use only ternary alloys may lead to simplified device growth and fabrication options and give deeper understanding of the design tradeoffs in these structures.

Highly Sensitive Nanotesla Quantum-Well Hall-Effect Integrated Circuit Using GaAs–InGaAs–AlGaAs 2DEG

This paper reports on the first low power (10.4 mW) and ultrasensitive linear Hall-effect integrated circuits (LHEICs) using GaAs-InGaAs-AlGaAs 2D electron gas technology. These LHEICs have a state-of-the-art sensitivity of 533 μV/μT and are capable of detecting magnetic fields as low as 177 nT (in a 10-Hz bandwidth), at frequencies from 500 Hz to 200 kHz. This provides at least an order of magnitude improvement in sensitivity and a factor of four improvements in detectability of small fields, compared with commercial Si linear Hall ICs.

Temperature dependence characteristics of In0.53Ga0.47As/AlAs asymmetric spacer-layer tunnel (ASPAT) diode detectors

A novel single-barrier In0.53Ga0.47As-AlAs tunnel diodes with asymmetric spacer layers for millimetre and THz detection were designed and fabricated. These devices meet the requirements of detectors with a much weaker temperature dependence than Schottky diodes. The I-V measurements over the temperature range 77K to 400K revealed temperature independency of the diodes characteristics within this wide range. The DC properties and the temperature dependence characteristics of this In0.53Ga0.47As/AlAs ASPAT diodes are presented for the first time and the temperature dependency of the new ASPAT diodes are found to be superior to those of conventional GaAs/AlAs ASPAT diodes.

GaAs/AlAs tunnelling structure: Temperature dependence of ASPAT detectors

A GaAs/AlAs Asymmetric spacer layer Tunnel diode (ASPAT) with very thin (10ML) layer of AlAs have been successfully grown by solid source molecular beam epitaxy (SSMBE). The Current-voltage (IV) characteristics of these ASPAT diodes were measured for both different emitter geometries and over the temperature range of 77 to 398K. A comparison was made between an in-house fabricated Schottky barrier diode (SBD) and the ASPAT in term of temperature dependencies. A close agreement is achieved for the IV characteristics of the ASPAT diode between simulated and measured data. A calculated value of cut off frequency of 200GHz can be achieved for diodes with an emitter size of 6×6 μm2 used in this study.

Extracting random numbers from quantum tunnelling through a single diode

Random number generation is crucial in many aspects of everyday life, as online security and privacy depend ultimately on the quality of random numbers. Many current implementations are based on pseudo-random number generators, but information security requires true random numbers for sensitive applications like key generation in banking, defence or even social media. True random number generators are systems whose outputs cannot be determined, even if their internal structure and response history are known. Sources of quantum noise are thus ideal for this application due to their intrinsic uncertainty. In this work, we propose using resonant tunnelling diodes as practical true random number generators based on a quantum mechanical effect. The output of the proposed devices can be directly used as a random stream of bits or can be further distilled using randomness extraction algorithms, depending on the application.

RF performance of a novel all ternary InAlAs\InGaAs DHBT

All ternary In0.52Al0.48As–In0.53Ga0.47As–In0.52Al0.48As double-heterojunction bipolar transistors (DHBTs) have been grown using solid-source molecular-beam epitaxy. The development of these DHBTs required epitaxial design trade-offs which culminated in an optimum structure achieving high-breakdown (5.5 V), and RF performance demonstrating unity cutoff frequency (ft) and maximum oscillation frequency (fmax) of 140 GHz and 95 GHz at relatively relaxed emitter dimensions of ∼1×5 µm2. This is the highest reported cutoff frequency for an all ternary In0.52Al0.48As–In0.53Ga0.47As–In0.52Al0.48As DHBT. The demonstration of the frequency performance at relaxed emitter dimensions shows potential for this material type.