Millimetre Wave / Microwave Power Combining from Arrays of Resonant Tunnelling

Diodes Defined by Zn-situ Focused Ion Beam Lithography

P. See, C. E. Collins, D. D. Amone, D. P. Stemson, E. H. Linfield, P. D. Rose, D. A. Ritchie and G. A. C. Jones

Abstract4ingle and planar arrays of embedded high current density resonant tunnelling diodes (RTDs) with a common emitter and collector electrical connection have been successfully fabricated. This was achieved through a novel technique combining in-situ Ga' focused ion beam (FIB) lithography with molecular beam epitaxy (MBE) regrowth. In these devices, the tunnel current path is defined by and confiied to regions intentionally left undamaged by the implantation. The ability to fabricate and contact an array of RTDs has a potential impact in realising millimetre wave / microwave power combining applications at terahertz frequencies. Room temperature DC and preliminary s- parameter results are presented here for individual and arrays of RTDs produced using this method.

Zndex Tenns-resonant tunnelling diodes, arrays, in-situ fabrication, focused ion beam lithography, power combining

I. INTRODUCTION Double barrier resonant tunnelling diodes (RTDs) have

been demonstrated to be the fastest solid state device, operating as a fundamental mode oscillator at 712 GHz, but only emitting modest amounts of millimetre wave / microwave power (- pW) in the terahertz frequency range [l]. These oscillators are usually very small (sub-lo pm in diameter), non-planar etched pillars (- 1 pm high) so as to minimise capacitance and heating effects. Larger power output (up to 5 mW at 1.18 GHz) has been achieved by combining the power from microscopic arrays of individual devices operating in a single circuit 121. However, the fabrication of such small area (< 100 pm') high current density RTDs and electrical connections to each individual pillar-shaped etched mesa is not easy using conventional optical lithographic methods. For example, firstly, the empty regions between each isolated, free-standing structure must be filled with an insulating material. The

P. See, E. H. Linfield, P. D. Rose, D. A. Ritchie and G. A. C. Jones are with the Semiconductor Physics Group, Cavendish Laboratmy, University of Cambridge, Madingley Road, Cambridge CB3 OHE, UK.

C. E. Collins and D. P. Steenson are with the Institute of Microwaves and Photonics, Department of Electrical and Electronic Engineering, University of Leeds, Leeds LS2 9JT, UK

D. D. Amone is with the Toshiba Cambridge Research Centre Ltd., 260 Cambridge Science Park, Milton Road, Cambridge CB4 4WE, UK.

0-7803-4903-2/98/$10.00 @ 1998 IEEE 219

deposited insulator is then planarised, ideally levelled with the top surface of the mesa. Often, this is achieved by an 'etch-back' technique, leaving the top ohmic contact exposed to allow electrical connections, thus avoiding any need for accurate alignment in the subsequent lithographic stages. The reliability and performance in such an array (as well as the yield) may be very poor because the exposed double barrier tunnel region and the highly n" doped grown ohmic layer are easily degraded during fabrication.

All these technical difficulties can be minimised, and even avoided, through a combination of in-situ focused ion beam (FIB) lithography and molecular beam epitaxy OMBE) regrowth when producing the actual semiconductor wafer itself. The post-growth device fabrication is relatively straightforward and the integrity of the heterojunctions preserved, being embedded and surrounded by highly insulating lattice-matched regions. A significant advantage then occurs since the characteristic resonant tunnelling current is only depended on areas intentionally left undamaged, independent of the large etched mesa, that also provides lower resistance than obtained conventionally. Moreover, this approach allows a common (emitter and collector) bias connection to an array of embedded RTDs with potentially better heat sinking, reliable circuit matching and accurate device modelling. Thus, improved heat dissipation for structures with peak current densities greater than 50 kA/cm* can be easily realised by using these microscopic arrays. This is crucial for operation in the terahertz frequency range. Furthermore, this technique offers the possibility of producing planar emission devices that do not rely on separate connections to individual RTDs for coupling radiation out of it. Another advantage is its compatibility with current integrated circuit design rules and geometry.

The overall feasibility of this novel fabrication scheme is investigated here and demonstrated to be successful. Room temperature DC measurements were performed on a single and an array of FIB RTDs with peak current density of - 5 Wcm'. High frequency s-parameter characterisation (up to 30 GHz) was carried out on a more suitable higher current density (- 20 kA/cm2) wafer for single embedded RTDs and the preliminary results are reported here.

High resolution focused ion beam

Ga’ implants Resonant tu” Outline of wet (Higbly resistive) current path etched mesa

n*GaAs emitter (500 n’ GaAs spacer (50nm)

uantum well (5nm)

Bottom GaAs spacer

(8) (b) Fig. 1. Schematic cross-section of the FT3 RTD (a) during Ga.’ ion implantation (after the MBE growth intempt) and @) after MBE regrowth, including an outline of the large etched mesa, with emitter and collector ohmic contacts. The bottom spacer consists of 100 nm n+ GaAs followed by 5 nm undoped GaAs, whilst the top is 5 nm undoped GaAs, then 50 nm n’ GaAs.

D. GROWTH, FABRICATION AND CHARACTERISATION High current density double barrier AlAs/GaAs/AlAs

RTD semiconductor wafers were produced by MBE growth on a semi-insulating (100) GaAs substrate (Figure l(a)) [31. This undoped substrate (as opposed to n”) provided reduced parasitic capacitance, better thermal radiation properties and suited planar device layout (compatible with other integrated circuits). Epitaxial growth commenced with a thick (1 pm) and highly n* Si doped (nominally - 2 ~ 1 0 ~ ’ GaAs buffer-cum-collector, providing a smooth interface transition t?om the substrate to the subsequent layers. Following this was a ‘two-layered spacer’ - 100 nm lightly doped (- 2 ~ 1 0 ’ ~ ~ m - ~ ) n’ GaAs, then a very thin 5 nm undoped GaAs layer. The purpose was to improve the characteristics of the device [4]. Next, double barrier resonant tunnelling junctions were formed and the MBE growth interrupted after 50 nm of the lightly doped n+ GaAs spacer above it. The wafer was then transferred to a FIB lithography unit via ultra high vacuum interconnected tubes. All chambers in the combined MBE / FIB facility had base pressures < 104mbar, thus minimising any possible contamination. Selected areas on the wafer (Figure l(a)) were targeted with high energy (30 keV) finely focused (resolution - 0.2 pm diameter) Ga’ ions (dose - 5x10” ionslcm’) at normal incidence to the wafer. This room temperature implantation and lateral patterning induced lattice disruption within the crystat, thus rendering the ion irradiated regions highly insulating [SI. These ions penetrate well past the NAs/G&s/NAs double barrier 161, consequently leaving the non-exposed areas to form the intended tunnel junctions in the RTD. After implantation, the wafer was returned to the MBE chamber and another 50nm n+ GaAs spacer followed by a low resistance 500 nm n* GaAs emitter layer regrown over the top of the structure (Figure l(b)).

Post-growth, standard optical lithography was used to concurrently produce both EIB and control RTDs in five

processing stages. First, the maximum vertical tunnel path was defined by wet chemical etching a mesa structure part- way into the n* collector. Next layered Ni/AuGe/Ni/Au ohmic metals were evaporated and annealed lightly into the doped emitter and collector. Each device was then further isolated electrically from the others with a deep etch into the undoped GaAs substrate. Finally, insulated areas were formed to prevent any stray interactions between the extemal contacts leading up to the emitter and collector ohmic metals. These large probe pads were formed by NiCr/Au merailisation. For all the FIB defined RTDs and the ordinary wet etched control devices 2 100 pm2, the emitter contact was sufficiently large to allow four-terminal DC measurements. Two- and four-terminal DC current- voltage (Z-V) characteristics were measured at room temperature on a pulsed curved tracer by direct probing with sharp gold alloy micro-needles. Preliminary one port (S11) high frequency (45 MHz to 30 GHz) s-parameter characterisations were pexformed using a network analyser via co-planar microwave probes on the ‘on-chip’ ground- signal-ground transmission lines leading up to the device. The collector in each device was shorted to the ground lines, providing 50 Q impedence-matched termination.

m. RESULTS AND DISCUSSIONS Typical Z-V characteristics of an RTD are shown in

Figure 2. As the applied bias on the collector becomes more positive (with the emitter earthed), the energy of incident electrons in the doped emitter gradually matches the quasi- bound state within the GaAs quantum well. At this threshold point, a resonant tunnelling current flows and grows with the applied potential until it reaches a maximum value (resonance condition). The current then ceases once this quasi-confined level comes out of alignment with the emitter conduction band (off-resonance state) [71. Further increase in current beyond this valley is due to non-resonant components (inelastic scattering, thermionic emission, leakage, etc.). Oscillations in the negative differential

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Applied bias VI V Fig.2. Room temperature I-V characteristics of a four-terminal 100 pm2 control RTD (tunnel mesa defined by etching; continuous linle) compared to FIB patterned devices (dotted and dashed lines) wtth different tunnel areas, embedded inside a 400 p2 etched mesa. (Implant dose - 5~10'~ Ga' iondcm?. Devices biased with respect to the collector while the emitter remained grounded. The tunnel area for FIB devices is also expressed as a fraction of A (= 100 w*). Inset shows the leakage current (in PA) through a totally FIB damaged 400 pm* mesa within the device bias range.

conductance (NDC) regimes can be attributed to intrinsic bistability and extemal bias circuit instability [SI. The slight inflexion around 0.65 V arose from resonant tunnelling of electrons from the quantised two-dimensional levels formed in the accumulation (light and undoped) layer [9].

Conventionally defined (by wet etching the tunnel mesa) control RTDs showed symmetrical I-V characteristics around the origin (Figure 2, continuous line) and high peak current density J p (Jp=4.5 Wan2 at an applied potential V = 1.05 V). There was also a good peak-to-valley current ratio (PVCR=4) over a wide NDC voltage range AV,, (AV" = 0.75 V). AI1 these observation indicated that the material quality was not affected by the MBE growth interrupt and transfers for the in-situ FIB lithography stage.

Figure 2 also includes the I-V characteristics (dotted and dashed lines) for FIB pattemed RTDs of different tunnel areas (nominally 9 to 100pm~ formed in large etched 400pm2 mesas). The irradiated regions in these devices were - 5x10'' Ga+ ions/cm2. The peak / valley bias positions (1.05 / 1.8OV, respectively) of the FIB RTDs aligned consistently and agree with the control device. Note also the near superposition of traces for the FIB defined and

control RTD with equal tunnelling area (100 pm2). These observations indicated that the ion implants do not adversely affect the energy band profile or the wafer structure.

Leakage current (Figure 2, inset) through a totally implanted 400pn2 mesa (no intended tunnel path) was negligible (< 20 pA, highly resistive) in the device operation range, without any signs of isolation breakdown. Since there is no conduction through the FIB implanted sites, this implied that the tunnel current must flow through the regions intentionally left undamaged. By accounting for the FIB defined tunnel area, the peak current density J p of, for instance, the 100 pm2 FIB RTD ( J ~ = 4.4 kA/cm2) was in excellent agreement with the control ( J ~ = 4.5 kA/cm2). It can also be noted that the peak tunnel current in FIB pattemed devices scaled proportionally and consistently as a fraction of the area A = 100 pm2 (Figure 2). All these characteristics were further observed to be independent of the physical wet etched mesa size (from 400 to 3600 pm2).

The effects of different Ga+ ion doses on the RTD were also investigated [3]. I-V characteristics for devices implanted with low or insufficient number of ions (c 5x10" ions/cm2) were found to be dominated by additional leakage current (especially beyond the valley) through the large etched mesa that contained the embedded RTD. In this case, the tunnel current paths were not totally isolated or properly defined. However, a reduced peak current was observed in samples irradiated with very high dose (> lx1013 ions/cm2). This was attributed to significant lateral ion straggling and depletion effects from the implantation. The induced lateral constriction, which increased with ion dose, effectively reduced the tunnelling area and led to current cut off at a finite geometrical size. For instance, the depletion width for an ion dose of - I x ~ O ' ~ ions/cm2, was estimated to be 0.43 pm. The optimum Ga' dose for implantation, as used here (with negligible leakage and reliable lateral resolution), was - 5 ~ 1 0 ~ ' ions/cm2. Nevertheless, this does depend strongly upon the incident ion energy and position of the growth interrupt.

Preliminary s-parameter characterisation on devices from a different and more suitable, higher current density wafer (Jp - 20 kA/cm2) indicated that the maximum 'cut- off oscillation frequency for 25 pm2 tunnel area FTl3 RTDs (- 18 GHz, Figure 3(a)) rivalled the conventional wet etched control sample (- 20 GHz, Figure 3(b)). Therefore, by implication, the extra shunt capacitance contributed by the large overlaid contact (on the 400 pm' mesa with the 25 pm2 FIB RTD embedded inside) did not appear to reduce the measured 'cut-off frequency in this case. More detailed measurements are currently being carried out.

(Here, the maximum 'cut off frequency is defined as the point where the s-parameter trace for the device intercepted the unity gain circle on the Smith chart after initial de-embedding, but inclusive of a short transition co- planar waveguide length leading up to the device. Consequently, the actual 'cut-off frequency is likely to be higher.)

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ACKNOWLEDGEMENT he authors would like to acknowledge partial funding

from the Engineering and Physical Sciences Research Council (UK). E. H. Linfield and D. A. Ritchie acknowledge the support from the Isaac Newton Trust and the Toshiba Cambridge Research Centre respectively.

(a) (b) Fig. 3. One port s-parameter (S11) charactensation from 45 MHz to 26 GHz of (a) FIB defined RTD (25 pm2 tunnel area embedded inside a 4OOpm’ mesa) and (b) conventional wet etched tunnel mesa RTD (25 pm2). Maximum ‘cut-off kequency marker MI (a) - 18 GHz and (b) - 20 GHz. (Device biased in the NDC regime, with the collector shorted to ground line.)

Independent and coherent resonant tunnelling were observed (in DC measurements) through each device (separated by the highly resistive irradiated areas) for arrays of RTDs fabricated using this technique [3]. The total current flowing in an array of FIB RTDs equalled the sum through each individual device. It was also noted that these characteristics were independent of the device configuration and separation in the array, without any detrimental effects from the ion implanted regions. Moreover, the insulating lattice sites should improve thermal dissipation in high current density microscopic arrays, crucial for devices operating at terahertz frequencies. Current summation supports the feasibility of power combining when the circuit is under the correct matched conditions. Further investigations on the interaction between individual RTDs and the optimum device arrangement for maximum power coupling to the extemal circuit are necessary.

N. CONCLUSIONS Single and planar arrays of small tunnel area RTDs

(down to < 10 pm’) with a common emitter and collector connection have been successfully fabricated using a combined in-situ Ga+ FIB lithography and MBE regrowth technique. Such structures are often very difficult to achieve with standard optical lithographic methods. The characteristic resonant tunnelling features observed were not affected by the surrounding resistive lattice regions induced by the ion implants, Furthermore, the current was determined to flow solely through the areas intentionally left undamaged, independent of the large wet etched mesa that held the embedded tunnel junctions. Both the DC and AC characteristics of these FIB RTDs were comparable to conventional devices produced by wet etching the tunnel mesa. There were also no excessive parasitic effects from this large mesa at high frequencies. All these observations suggest that the FIB patterned array of RTDs can be adopted to demonstrate power combining applications in the terahertz frequency range. Furthermore, this CO-planar fabrication method is suitable and compatible with current microwave and millimetre wave integrated circuits.

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