Arrays of Resonant Tunnelling Diodes Defined by In-situ Focused Ion Beam Lithography: Potential Millimetre Wave / Microwave Power Sources?

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

Semiconductor Physics Group, Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 Om, United Kingdom.

+ Institute of Microwaves and Photonics, Department of Electronic an'd Electrical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom.

' Toshiba Research Europe Ltd., Cambridge Research Laboratory, :260 Cambridge Science Park, Milton Road, Cambridge CB4 4WE, United Kingdom.

Abstract

The ability to fabricate and electrically contact an array of small, high current density resonant tunnelling diodes (RTDs) with a common bias connection has potential impact in realising solid state millimetre wave / microwave power sources. In this paper, a novel technique is proposed and demonstrated to form these arrays using a combination of in- situ focused ion beam lithography (FIB) with molecular beam epitaxy (MBE) regrowth. Room temperature DC I-V characteristics of the resulting planar devices are presented, together with preliminary s-parameter measurements and power emission spectra of individual FIB RTDs.

Introduction

High current density resonant tunnelling diodes (RTDs) have been proven to be the fastest room temperature solid state devices, with fundamental mode oscillations of 712 GHz [ 11. These two-terminal oscillators consist of microscopic etched pillar-like structures (diameter < 10 pm, height - 1 pm) so as to minimise capacitance and improve thermal dissipation. A particular feature of this quantum transport device is that it exhibits negative differential resistance (NDR) in its current- voltage ( I -V) characteristics [2], thus implying that power may be extracted when embedded into a suitable circuit environment. However, being small in size, such devices are intrinsically weak, emitting limited amount of power (- pW) at high frequencies. Larger useful power output can be realised (as demonstrated up to 5 mW at 1.18 GHz) by combining the emission from an array of individual RTDs operating as a single circuit [3]. Enhanced heat sinking for such structures with peak current densities > 50 kA/cm2 can also be easily achieved through these microscopic arrays. Nevertheless, the fabrication (alignment) and electrical connections to each miniature, free-standing mesa (contact area < 100 pm2), not to mention an array of devices, is very difficult by conventional optical lithographic methods. A common approach involves re-filling and planarising the etched regions with insulating material so as to aid the delicate biasing using sharp whisker contact [ l , 31. Often, the

reliability of this bias method and the device integrity in such an array (a:s well as the yield) is quite poor.

Most of thlese technical difficulties can be avoided through a novel technique, involving a combination of in-situ focused

,ion beam (FIB) lithography and molecular beam epitaxy (MBE) regrowth for producing the actual semiconductor wafer itself [4]. Highly resistive regions formed by the ion implants are exploited for confining the resonant tunnelling current to areas deliberately left non-irradiated. Single and planar arrays of embedded RTDs can be easily realised, with the insulating lattice sites serving as heat sinks. As a result, the post-gowth device fabrication and bias connections are relatively straightforward. Lower resistance is also possible since the emitter contact is much larger than the active tunnel area. Furtlher advantages include better circuit matching and more accurate device modelling. In addition, this planar geometry facilitates radiation coupling out of these devices into wave.guides, and is also compatible with current integrated (circuit design rules.

In this paper, the overall feasibility of this unique fabrication scheme is iinvestigated and demonstrated to be viable. Room temperature DC measurements were performed for a single and planar array of FIB RTDs with peak current density > 20 kA/crn2. Preliminary high frequency s-parameter results (up to 30 GHz) and power emission spectra from single embedded RTDs are also reported.

Dievice Fabrication and Characterisation

The high current density RTD semiconductor wafer (AlAs barriers / GaAs quantum well) was grown by MBE on a semi-insulating (100) GaAs substrate (Fig. l(a)) [4]. This chosen epilayer (instead of n+) possessed lower parasitic capacitanci:, improved heat dissipation properties and suited the planar ,geometry layout. MBE growth started with a thick (1 pm), highly nfc Si doped (nominally - 2 ~ 1 0 ' ~ ~ m - ~ ) GaAs buffer-cum!-collector, followed by a 'bi-layer spacer' - 80 nm lightly doped (- 2 ~ 1 0 ' ~ ~ m - ~ ) n" GaAs and 5 nm undoped GaAs. Such a sequence was adopted to improve the overall

16.2.1 0-7803-4774-9/98/$10.00 0 1998 IEEE IEDM 98-433

High resolution focused ion b&m (30 key Ga'ions)

Ga+ implants Resonant tunnel Outline of wet (Highly resistive) current path

\ I n* GaAs emitter (500 nm

ni GaAs spacer (30 nm

p GaAs spacer

s barrier (1.7 nm) uantum well (5.0 nm) s barrier (1.7 nm)

Bottom GaAs spacer

(a) (b) Fig. 1: Schematic cross-section of the FIB RTD (a) during Ga' ion implantation (after the MBE growth intempt) and (b) after MBE regrowth, including an outline of the large etched mesa, with emitter and collector ohmic contacts.

characteristics of the device [5]. Next, undoped AlAs barrier (1.7 nm) / GaAs well (5.0 nm) / AlAs barrier (1.7 nm) tunnel junctions were formed and the growth interrupted after 50 nm of the 2 ~ 1 0 ' ~ cm-3 n+ GaAs above it. The wafer was then transferred to an in-situ FIB lithography system via ultra high vacuum interconnected chambers, therefore avoiding any likelihood of contamination. There, selected areas on the wafer (Fig. l(a)) were irradiated with high energy (30 keV) finely focused (resolution - 0.2 pm) Ga' ions (dose - 0.5- lx1013 iondcm'). Significant lattice disruptions within the crystal were induced by the implants, rendering the exposed regions highly resistive [6]. As these ions penetrated past the hetero-junctions, the non-exposed areas were left to form the intended tunnel current path in the RTD. After implantation, the wafer was returned to the MBE chamber and the remaining n+ GaAs (30nm) spacer followed by a low resistance 500 nm n++ GaAs emitter layer were regrown (Fig. l(b)). For the tunnel junctions of the device, the growth temperature was decreased to minimise Si dopant diffusion between epilayers.

Post-growth, standard optical lithography was employed to produce concurrently both FIB and control RTDs (by wet etching the tunnel mesa) with co-planar ground-signal-ground configuration transmission lines. The collector contact for each device was shorted to the ground lines, thus providing 50 S2 impedence-matched termination and ensuring DC bias stability. Two terminal room temperature I-V characteristics were recorded on a pulsed curved tracer by direct probing. One port (SI 1) s-parameter measurements (0.45-30 GHz) were performed using a vector network analyser with the reference plane set to the ends of the co-planar microwave probes by adopting the line-reflect-measure (LRM) technique on standard calibration substrates. Power emissions spectra were analysed by connecting a double stub co-axial tuner as a load to the RTD.

Results and Discussions

Typical I-V characteristics for the RTDs are depicted in Fig. 2. Quasi-bound states are formed in the GaAs quantum well due to the closely spaced AlAs barriers. This particular band profile acts as an energy filter, permitting incident electrons with energy matching the lowest quasi-confined level to traverse the barriers. Therefore, peaks that occur in the transmission probability as the voltage is increased are in turn reflected in the I-V plots [2]. When the applied bias is increased further, up to and beyond the valley current, the quasi-bound state slowly moves out of alignment with the conduction band electrons in the emitter, hence the resonant tunnel current ceases with the observed NDR ensuing. Subsequent increase in current arises from various non- resonant and leakage components (e.g. inelastic scattering, thermionic emission, etc.). Random fluctuations in the NDR regime are due to intrinsic bistability and/or external bias circuit oscillations [7]. The inflexions seen around +0.75 V and -1.10 V originates from resonant tunnelling of electrons from the quantised two-dimensional levels formed in the top spacer layer [8]. Slight asymmetries in the traces were attributed to small deviations from the nominal epilayer thicknesses and the doping levels near the tunnel junctions during MBE growth.

Consider the positive bias range of the RTD. For the 49 pm2 control RTD fabricated by conventional techniques (wet etching the tunnel mesa), the I-V characteristics were nearly symmetrical about the origin (Fig. 2, continuous line). A peak current Zp = 13 mA (equivalent peak current density J p = 26 kA/cm2) at an applied potential V = 1.60 V was observed together with good peak-to-valley current ratio (PVCR=2.9) across a wide NDR voltage range ( A V N ~ R = 0.60 V). All these features indicated that the MBE growth interrupt and transfers for the in-situ FIB lithography stage did not affect the material quality.

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Fig. 2: Room temperature I-V characteristics of a two-terminal 49 pmz control device (continuous line) compared to FIB defined RTDs (dotted and dashed lines) with different tunnel areas, each embedded in a 400pm’ mesa. Devices were biased with respect to the collector whilst the emitter remained grounded. The tunnel area for the FIB RTDs is also expressed as a fraction of A (= 49 pm’). Inset: Leakage current (in PA) through a totally FJB damaged 400 pm2 mesa as a function of the device bias range.

I-V characteristics (dotted and dashed lines) for FIB patterned RTDs of different tunnel areas (nominally 9 to 49 pm2 etched on 400 pm2 mesas) were compared (Fig. 2) . Peak / valley bias positions (1.45 / 2.10 V, respectively) for these RTDs aligned consistently and agreed with the control device. (Slight variations due to contact resistance that cannot be accounted for in these two-terminal measurements.) Notice the close superposition of curves for the FIB and control RTD with nominally equal tunnel area (49 pm’). Ip and J p for this FIB RTD (12.8 mA and 25.5 kA/cm2, respectively) were in excellent agreement with the control sample. Observe also that the peak currents scaled nearly in proportion to the fraction of the FIB defined tunnel area A = 49 pm’. Minor discrepancies (apparent in small tunnel areas) were attributed to significant lateral ion straggling and depletion effects (< 0.3 pm) from the FIB lithography [4]. Leakage (Fig. 2 inset) in a completely irradiated 400 pm’ mesa (no intended tunnel path) was negligible (< 5 pA, highly resistive) for the device operation range. Since there is no conduction across the ion implanted areas, this indicated that the tunnel current must flow through regions purposely left undamaged. These features were also determined to be independent of the etched mesa dimensions (400-3600 pm2).

Fig. 3: Two-terminal room temperature I-V characteristics for a triangular array of threi: 16 pm2 tunnel area FIB RTD (continuous line) compared to single 16 pm2 (dotted-dashed line) and 48 pm2 (dotted line) LlB RTDs, and three times the current of a single 16 pmz device (dashed line).

Measurements were then performed on arrays of FIB RTDs with common emitter and collector contacts. Independent and coherent resonant tunnelling were observed through each device (separated by the highly resistive ion implanted regions) for various arrangements of FIB RTDs in the arrays. Consider the two-terminal I- V characteristics for an equilateral triangular configuration of three 16 pm2 tunnel area FIB IRTDs (8 pm centre-to-centre device separation) inside a 4010 pm’ mesa (Fig. 3) . It was nbted that the total current flowing through the array of FIB RTDs equalled the sum in each individual device. For example, the trace for an array of three 16 pm2 devices (continuous line) was almost identical to three times the current of a single 16 pm’ (dashed lines) or an RTD with equivalent tunnel area (48 pm’, dotted lines). The characteristics were also found to be independent of the device arrangement and separation in the array. This current summation supports the feasibility of power combining when the circuit is properly matched to a suitable load. Further investigations on the interaction between individual RTDs and the optimum device configuration for maximum power coupling to the external circuit would, though, be necessary. It is also very likely that the implanted lattice sites will improve heat dissipation in these microscopic high current density arrays, which is crucial for successful operation.

16.2.3 IEDM 98-435

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c ~ ~ ~ . , . - . ~ ~ s c ~ I z s-i looMBl

(a) Fig. 4: Power emission spectrum of RID embedded in a 400 pm2 mesa, wet etched tunnel mesa RTD.

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(b) (a) a FIB defined 25 pmz tunnel area compared with (b) a control 25 pmz

Preliminary s-parameter measurements of single FIB RTDs biased in the NDR region indicated stable operation within the swept frequency range (0.45-30 GHz). For a 25 pm2 FIB defined tunnel area etched on a 400 pm2 mesa, the ‘cut-off frequency’ was - 10 GHz. This implies that the implanted lattice sites acted as a dielectric (instead of being a shunting resistor) at high frequencies. Power emission from the FIB RTD compared to the control sample supported this inference (Fig. 4). Sharp peaks were observed when the circuit was tuned about 3.30GHz across a 1OOMHz span, with the background signal about -70 dBM. Typical values were -10.14 dBm (9.7 pW) at 3.28 GHz for the FLB RTD (Fig. 4(a)), almost matching -8.84 dBM (13.1 pW) at 3.41 GHz for the control sample (Fig. 4(b)). Since the ability to electrically contact an array of such devices has been demonstrated above, these emission measurements support the possibility of extracting the combined power of individual RTDs in the array.

These high frequency results, though preliminary, are rather encouraging. However, it was noticeable that the control RTDs operated to a much higher ‘cut-off frequency (- 25 GHz). Therefore, this suggests that the capacitance of the 400pm2 FIk patterned mesa (with the 25 pm2 RTD embedded in it) may be significant and must be taken into account. Possible options for reducing this capacitance include optimising the wafer structure (e.g. increasing the depletion region in the spacer layer to reduce the capacitance) or changing the most appropriate ion implant dose (hence improving the dielectric properties of the damaged regions). It is also worth noting that in an array of FIB RTD, the ratio of the irradiated to non-irradiated areas will be low and the effects are likely to be less dominant. Whilst further studies are required, the technique nevertheless appeared to be a viable approach in fabricating millimetre wave / microwave power combined sources. Even if the maximum ‘cut-off frequency is limited, this still may be a useful alternative for achieving power combining at lower frequencies.

’ In this case, the maximum ‘cut off frequency is defined as the point where the trace of the s-parameter intercepted the unity gain circle on the Smith chart. However, this included a short CO-planar waveguide length leading up to the device. Consequently, the actual ‘cut-off frequency may be higher.

Conclusions

Single and planar arrays of small tunnel area RTDs (< 10 pm2) with a common electrical (emitter and collector) connection were successfully fabricated through a novel technique combining in-situ Ga+ FIB lithography and MBE regrowth. Such devices are often very difficult to produce using standard optical lithographic methods. The resonant tunnelling characteristics were not affected by the resistive lattice sites induced by the ion implants. Thus, resonant tunnelling was determined to occur solely through regions intentionally left unimplanted and also independent of the large etched mesa that held the embedded tunnel junctions. Moreover, the damaged lattice sites could improve heat dissipation. DC characteristics of the FIB RTDs were comparable to conventional ones fabricated by wet etching the tunnel mesa. Initial AC measurements were promising. These results suggest that arrays of FIB RTDs can possibly act as potential millimetre / microwave power generation. In addition, this co-planar geometry is suitable and compatible with current integrated circuit designs.

Acknowledgements

The authors would like to acknowledge partial funding from the Engineering and Physical Sciences Research Council of the United Kingdom. E. H. Linfield and D. A. Ritchie would also like to acknowledge the support from the Isaac Newton Trust and Toshiba Research Europe Ltd. respectively.

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