M J Kelly

Tunnel devices are not yet manufacturable

In spite of many remarkable prototype device performances reported using semiconductor multilayers that incorporate thin tunnel barriers, it has never been established that the devices are capable of routine manufacture. With reference to the simplest possible tunnel device structure, a single thick AlAs barrier within an asymmetric doping environment in GaAs (the so-called ASPAT microwave detector diode), we show here that is not yet possible to design, grow or qualify the semiconductor multilayers with sufficient accuracy, precision, uniformity or reproducibility that would allow reverse engineering, a prerequisite for manufacture, to be undertaken with confidence. We describe the improvements in design, growth and qualification that will be required to achieve manufacturability, and comment on the feasibility of attaining this goal. Our conclusions for thin tunnel barrier device concepts apply a fortiori to any device ideas that seek to exploit mesoscopic phenomena in semiconductors.

2D Monte Carlo simulation of proton implantation of superconducting YBa2Cu3O7−δ thin films through high aspect ratio Nb masks

Abstract Perturbation of proton beam damage profile due to sidewall interactions in very high aspect ratio implant masks has been studied using Monte Carlo simulations. The model structure is composed of amorphous Nb metal mask, crystalline high temperature superconducting YBa 2 Cu 3 O 7− δ (YBCO) thin film, and amorphous LaAlO 3 substrate. The simulation results reveal the existence of enhanced proton beam penetration in target materials due to sidewall interactions.

Manufacturability of heterojunction tunnel devices: further progress

A second attempt to establish the manufacturability of a simple heterojunction tunnel diode has made some significant progress towards the goal, but has failed to demonstrate a reverse engineering capability, leaving major challenges in design, materials growth and materials qualification.

Ex situ re-calibration method for low-cost precision epitaxial growth of heterostructure devices

A simple method for high-precision epitaxial semiconductor growth, using ex situ materials analysis for frequent re-calibration, has been developed. The method is shown to allow reproducible growth of material for use in low-cost commercial heterostructure devices. Even tunnel devices, with their extreme sensitivity to growth parameters, show little variation in the electrical characteristics from wafers grown several months apart.

Irradiation damage technology for manufacturable Josephson junctions

Abstract We have shown in a series of studies that irradiation of YBa 2 Cu 3 O 7− δ (YBCO) with ions of energy in the range of 30–350 keV through a suitable mask can be used to create highly localized damage regions in the films. This technique has been successfully employed to create high quality Josephson junctions in YBCO with an ion beam implanter capable of in situ low temperature electrical measurement during implantation and focussed ion beam nanolithography. The fabricated devices show a clear dc and ac Josephson effects. This technique is very promising in terms of simplicity and flexibility of fabrication and has potential for high density integration.

The free-space oscillation of heterojunction GaAs/AlGaAs Gunn diodes as a design guide

We demonstrate that free-space oscillations can be used to estimate the optimum frequency of operation of a heterojunction GaAs/AlGaAs Gunn diode, in terms of efficiency, when the device is placed in a cavity. The variation of the natural frequency of oscillation with transit region length and potential bias has been measured and found to be in good agreement with results from Monte Carlo simulations.

BULK UNIPOLAR DIODES FORMED IN GAAS BY ION IMPLANTATION

Abstract In an attempt to emulate epitaxially manufactured semiconductor multilayers for microwave device applications, we have produced a camel diode structure in GaAs for the first time, using the tail of a Mg + implant into a molecular beam epitaxially grown n + –n – –n + structure. Using a range of ion energies and doses, samples are observed to exhibit bulk unipolar diode characteristics. With low dose and energy, a diode with barrier height of ∼0.8 V and ideality factor ∼1.25 is achieved. ‘Punch through’ diode characteristics are obtained at high ion dose and energy, some with knee voltages in excess of 7 V.

Novel GaAs/AlAs tunnel structures as microwave detectors

We have designed (using a specially developed simulation package) some novel GaAs/AlAs tunnel structures with highly asymmetric current-voltage (I-V) characteristics for use as microwave detectors. The asymmetry arises from having unequal spacer regions either side of a single AlAs tunnel barrier. Recent designs have a microwave performance at 9.4 GHz which matches a zero-bias Schottky diode in terms of voltage sensitivity and dynamic range, but with a much better (equals weaker) temperature dependence. The new diodes also out-perform existing germanium back diodes in their voltage sensitivity and dynamic range, although the variation of sensitivity with temperature is not quite as small; it is expected however that this gap can be closed. The main competition for the new diodes is expected to come from the recently developed planar-doped-barrier detector (PDB) diodes, which combine the high sensitivity and dynamic range of the Schottky diode with a somewhat weaker variation with temperature. However, our diodes are still expected to have a weaker temperature dependence, and they would seem to be more easily and cheaply manufactured due to the problems associated with control over the p+ doping spike in a PDB diode: we have successfully made structures by both MBE and MOCVD. In this paper, we describe the design of our diodes and demonstrate the above points with detailed graphs of d.c. and microwave performance for one MBE-grown structure and one MOCVD-grown structure.

Acceptor profile control in GaAs using co-implantation of Zn and P

Abstract While co-implantation is well known as a means of dopant control in GaAs, we have examined the effect of displacing the two implant profiles with Zn as the acceptor and P as the co-implant. We have established that lower resistivities, comparable sheet resistances and higher mobilities can be achieved when Zn is implanted into the surface tail of a deep P implant, as compared with Zn being implanted to overlap a shallower P implant.

The electronic transport through ion-implanted double-barrier diodes

The d.c. characteristics of AlGaAs/GaAs double-barrier diodes have been investigated before and after implantation of low doses of long-range ions. This was carried out to investigate the electrical characteristics after incorporating a low density of defects within the mesas: a low density of defects usually remains after high-temperature annealing of GaAs implanted with a high dose of ions. We are investigating this because we are attempting to integrate double-barrier diodes with other devices on a circuit by implanting high doses of ions through the double-barrier diodes, which are then annealed to create a high-resistivity layer beneath them. Our results show that the post-implantation d.c. performance of the double-barrier diodes is limited by the resistivity of the spacer layers between the contacts and the double-barrier structure. The current at high temperatures was limited by Poole - Frenkel emission of electrons from the defect states, and field-enhanced ionization below 25 K. The performance of our ion-implanted diodes was degraded more severely with increasing thickness of the spacer layers. Annealing the diodes at only C recovered the as-grown d.c. performance almost completely. Surprisingly, the post-anneal peak-to-valley current ratio at 77 K was in many cases more than twice the as-grown value. We attribute this to the decrease of electrons which do not thermalize into the accumulation layer before tunnelling; such ballistic electrons are scattered easily by the presence of a few defect states.