Solid-State Electronics 64 (2011) 67–72

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Solid-State Electronics

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Design, fabrication and characterization of In0.23Ga0.77As-channel planar Gunndiodes for millimeter wave applications

Chong Li a,⇑, Ata Khalid a, Sonia H. Paluchowski Caldwell b, Martin C. Holland a, Geoff M. Dunn c,Iain G. Thayne a, David R.S. Cumming a

a School of Engineering, University of Glasgow, Glasgow G12 8LT, UKb Cambridge Silicon Radio Ltd., Cambridge CB4 0WZ, UKc School of Engineering and Physical Sciences, University of Aberdeen, Aberdeen AB24 SFX, UK

a r t i c l e i n f o

Article history:Received 9 May 2011Received in revised form 6 July 2011Accepted 8 July 2011Available online 30 July 2011

The review of this paper was arrangedby Prof. A. Zaslavsky

Keywords:Gunn devicesSemiconductor heterojunctionsIndium compoundsMillimeter wave sources

0038-1101/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.sse.2011.07.008

⇑ Corresponding author. Tel.: +44 141 3306690; faxE-mail address: c.li.2@research.gla.ac.uk (C. Li).

a b s t r a c t

We present detailed design, fabrication and characterization of In0.23Ga0.77As-based planar Gunn diodesin this paper. The devices have AlGaAs/InGaAs/AlGaAs heterojunctions that were grown on a semi-insu-lating GaAs wafer using molecular beam epitaxy technology. Electron beam lithography was used todefine anode and cathode terminal patterns. Devices with various anode–cathode separations (e.g. 4–1.4 lm) were fabricated on the same chip. Spectrum measurements showed oscillation frequenciesbetween 36 GHz and 118 GHz in the fundamental transit-time mode of operation. These devices showgreat potential as millimeter wave and sub-millimeter wave signal sources for their small size, MMICcompatibility and lithographically controlled oscillation frequencies.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Development of millimeter wave, sub-millimeter wave andterahertz sources has attracted a lot of attention recently due tothe growing demand in applications such as communications, ra-dar, imaging, spectroscopy and security screening [1–3]. Gunnoscillators have shown great potential to meet several require-ments including small size and low phase noise [4–6]. GaAs/Al-GaAs planar Gunn diodes operating at over 100 GHz in thefundamental transit-time mode of operation have also been dem-onstrated [7–9]. Unlike traditional vertical Gunn diodes [4–6],the oscillation frequency of a planar Gunn diode can be controlledby selecting the lithographic dimension, therefore multiple fre-quency sources can be achieved on a single chip. In addition, byreducing the anode and cathode separation (Lac), sub-terahertz oreven terahertz oscillation might be achieved [10]. Furthermore,simple two-terminal planar structures do not need complicatedgate process as transistors [11,12] but still have the capability tobe integrated with other devices on the same chip using monolithicmicrowave integrated circuit (MMIC) techniques.

InxGa1�xAs is a promising material with superior electricalproperties to GaAs that exhibits the Gunn Effect [13–19]; it can

ll rights reserved.

: +44 141 330 4907.

be grown on an InP substrate for lattice-matched structures withx = 0.53 or on a GaAs substrate to make lattice-strained layers forany other mole combination of indium and gallium. In the pastthere have been a number of experimental investigations into lat-tice-matched InxGa1�xAs for Gunn oscillations [13–15]. However,oscillation frequencies of only a few gigahertzes were achieved.Although slightly higher oscillation frequencies (approximately20 GHz) were observed in three-terminal pseudomorphic highelectron mobility transistor (pHEMT) structures using lattice-strained In0.15Ga0.85As, it was believed that the oscillations resultedfrom a real space transfer effect rather than a k-space transfer ef-fect (i.e. the Gunn or transferred electron effect) [16].

Early theoretical investigations showed that the high energyrelaxation time of In0.53Ga0.47As might limit Gunn oscillation tono more than 50 GHz [17]. More recently, however, theoreticalstudies on two-terminal, carefully shaped, planar In0.53Ga0.47As-based self-switching diodes showed that operation towards themillimeter wave frequency range was feasible [18]. In addition,Monte-Carlo simulation showed that ultrafast quasi-ballistic elec-trons in the C-valley would, under the influence of a high electricfield caused by an etched recess, achieve a velocity of up to 108 cm/s. Consequently, lattice-strained devices (In0.7Ga0.3As) with submi-cron dimensions could generate Gunn-like oscillations in the tera-hertz frequency range [10,19].

68 C. Li et al. / Solid-State Electronics 64 (2011) 67–72

On the contrary, there has been little work on lattice-strainedInxGa1�xAs for Gunn oscillations [20,21]. The first report appearedin [20] where the Monte-Carlo simulation confirmed that the‘‘Kink’’ effect in HEMT devices resulted from Gunn domains. Thenwe reported some initial results on In0.23Ga0.77As-based planarGunn diodes that showed an oscillation frequency of 116 GHz froma 1.45 lm device for the fundamental mode of operation [21]. Inthis paper we demonstrate in detail that design, fabrication andcharacterization of such two-terminal In0.23Ga0.77As-based planarGunn diodes that generate fundamental mode of oscillations in awide frequency range on a single chip. These results are of signif-icant importance since they establish InxGa1�xAs as a suitablematerial for high frequency planar Gunn oscillators on a semi-insu-lating GaAs substrate. Using the technology we present here, it willbecome possible to exploit the aforementioned advantages of pla-nar two-terminal Gunn devices.

15 nm

20 nm

12 nm

20 nm

GaAs/InGaAsContact layers

GaAs

Al0.23Ga0.77As

In0.23Ga0.77As

Al0.23Ga0.77As

S. I. GaAs

LacAnode Cathode

dopingδ-

Fig. 1. Schematic view of the epitaxial device layers and the arrangement of thecontacts and channel recess. The d-doping layer has a sheet electron density of8 � 1011 cm�2.

2. Device design and modeling

As with GaAs and InP, several intrinsic properties of the ternarycompound InxGa1�xAs make it a suitable candidate for Gunn oscil-lators. It has a direct bandgap for all values of x and negative differ-ential mobility when a high electric field is applied. InxGa1�xAs islattice-matched to InP for only x = 0.53 [22]. Early work onIn0.53Ga0.47As planar Gunn diodes could only investigate simplebar or ‘‘H’’ shaped planar devices, without heterostructures, fabri-cated on InP substrates [13,14]. Lattice-matched heterojunctionscan be made with In0.52Al0.48As and the quaternary compoundsemiconductor InGaAsP and InAlGaAs for appropriate alloy mixes[22]. However, as technology has progressed, it has become possi-ble to grow pseudomorphic InxGa1�xAs onto GaAs and AlGaAs.Using this method strained AlGaAs/InGaAs/GaAs or AlGaAs/InGaAs/AlGaAs heterostructures could be realised [23,24]. PHEMTsbased on these structures have already demonstrated better per-formance than conventional GaAs-based HEMTS such as low noiseand high peak electron drift velocity because these heterojunctionsresult in a larger conduction band discontinuity that ensures great-er electron confinement and density. Furthermore, the latticestrain and large conduction band discontinuity enhance the effi-ciency of any modulation doping. The reduction in ionized donorscattering in the channel leads to improved electron mobility.Therefore, it is expected Gunn diodes made using In0.23Ga0.77As/AlGaAs heterojunctions may exhibit better performance.

Fig. 1 shows the layer structure and device architecture that hasbeen investigated. The 12 nm undoped In0.23Ga0.77As channel issandwiched between two double d-doped Al0.23Ga0.77As layers.The channel thickness was so as not to exceed the critical valuedcr (nm) � �3.6 + 3.66x in order to obtain a defect-free channel[25]. The mole fraction of aluminum in the AlGaAs layers was cho-sen so as to avoid possible DX centers, and to maximize the con-duction band discontinuity [26]. Each d-doping layer has a sheetdensity of 8 � 1011 cm�2. Double d-doping has been demonstratedto increase the carrier concentration in a two-dimensional electrongas [27]. This technology has also previously been used in planarAlGaAs/GaAs Gunn devices to increase the carrier density by

Table 1Semiconductor material parameters used in the simulation.

Parameter (at 300 K) In0.23Ga0

Permittivity 13.9Bandgap (eV) 1.1Affinity (eV) 4.26Effective conduction band density of states (cm�3) 2.9 � 10Low field mobility (cm2/(V s)) 8000Electron saturation velocity (cm/s) 2 � 107

120% when compared to single d-doped structures, and to enhanceGunn domain formation [8,28].

Detailed simulations for the device were performed using atwo-dimensional drift–diffusion modeling tool (Medici). Thismethodology has been found to be useful for device design despiteit not providing a detailed model for the quantisation that occurs inthe accumulation layers [28–30]. In this work the highly doped15 nm GaAs cap layer was assumed to be partially depleted byits surface potential. Therefore, the simulated cap layer heightwas 5 nm to give a good agreement with experimental results.The annealed anode and cathode Ohmic contact regions were as-sumed to reach just below the In0.23Ga0.77As channel. This is rea-sonably true when an annealed process is applied onto metalalloy contacts [28]. The contact resistance was based on the mea-sured value of 4 � 10�6 O cm2. The semi-insulating GaAs substratehad 5 � 1015 cm�3 p-type doping. A GaAs-like mobility model wasused in all materials. In this model, as the electric field increases,the electron drift velocity reaches a peak and then begins to de-crease at high fields [31]. Other important material parametersare listed in Table 1 [22,32–34].

Fig. 2 shows the anticipated conduction band edge discontinu-ity (approximately 0.43 eV) between the In0.23Ga0.77As well andAl0.23Ga0.77As barriers. The Fermi energy inside the In0.23Ga0.77Aswell is above the conduction band edge at zero bias indicating ahigh concentration of free electrons that is far more obvious thanGaAs-channel planar Gunn diodes [28]. The calculated electronconcentration is also plotted in Fig. 2 and the high electron concen-tration (approximately 1018 cm�3) in the In0.23Ga0.77As well that isthe channel of planar Gunn device is demonstrated. Since the chan-nel is electron rich and no parasitic parallel conduction paths areevident, it is expected that the majority of the current will flowin the In0.23Ga0.77As well. The simulated dc current contours forthe 1.45 lm device with an anode–cathode bias voltage (Vac) of2.5 V are shown in Fig. 3. The current–voltage characteristics fromsimulation for several Lac (1.45 lm, 3 lm and 4 lm) are plottedagainst the experimental results in Fig. 4. One can see from Fig. 4

.77As GaAs Al0.23Ga0.77As

12.9 12.21.424 1.714.07 3.82

17 4.7 � 1017 5.9 � 1017

8500 40001 � 107 0.8 � 107

Fig. 2. Simulated conduction band structure of the device with Lac = 1.45 lm andelectron concentration in each layer at zero bias. The buffer is partially shown andthe semi-insulating substrate is not shown due to the large size compared to theactive layers.

Fig. 3. Simulated current flow in the 1.45 lm device. The contours show that themajority of the current is in the In0.23Ga0.77As channel. The entire device wasmodeled, but only a small region is shown for clarity.

C. Li et al. / Solid-State Electronics 64 (2011) 67–72 69

that (a) unlike conventional Gunn diodes the I–V curves of planarGunn diodes do not show prominent negative differential resis-tance (NDR) as the bias voltage exceeds the threshold values butsaturate for long devices (3 lm and 4 lm) and increase for theshort device (1.45 lm); (b) the current level of shorter devices ishigher than that of longer devices. Such I–V characteristics may re-sult from injecting electrons from the cathode, especially for shortdevices [35,36].

Fig. 4. Simulated and measured current–voltage characteristics of devices withLac = 1.45 lm, Lac = 3 lm and Lac = 4 lm.

3. Device fabrication

The epitaxial layers were grown by molecular beam epitaxy(MBE) as schematically shown in Fig. 5. A 0.5 lm GaAs buffer layerwas first grown on a 620 lm semi-insulating GaAs substrate fol-lowed by 20 periods of GaAs/AlGaAs superlattices. The active chan-nel was made of un-doped In0.23Ga0.77As that was sandwiched bytwo double d-doped Al0.23Ga0.77As layers. The d-doping layer hada sheet density of 8 � 1011 cm�2. 15 nm of highly doped GaAswas grown on top of the upper Al0.23Ga0.77As barrier layer to serveas a cap layer. This was followed by a 5 nm Al0.8Ga0.2As etch stoplayer doped at 4 � 1018 cm�3. The top of the wafer was then fin-ished with multiple graded layers of GaAs/InGaAs to facilitate goodOhmic contact formation [7].

To make the devices, the anode and cathode contact regionswere defined by electron beam lithography using polymethyl-methacrylate (PMMA) resist. Pd/Ge/Au/Pd/Au metal was depositedby e-beam evaporation followed by lift-off [37]. The contacts werethen annealed at 400 �C for 60 s in a rapid thermal annealer. Amesa was etched using 1:1:10 H2O2:H2O:H2SO4 for 90 s. A 50 Xcoplanar waveguide (CPW) structure was then added using200 nm gold to form test probe pads for RF and dc measurements.The CPW had a ground-signal separation of 40 lm and a centralsignal track width of 60 lm. The unwanted multiple graded layersof GaAs/InGaAs were etched away by dipping in 3:1 citric acid:-H2O2 solution for 20 s. Finally, an extra layer of gold was evapo-rated on the anode side to form composite contacts in order toimprove the contact stability and avoid device early breakdown[38]. Devices with different anode–cathode separation (Lac) rangingfrom 1.0 lm up to 4.0 lm were fabricated on the same chip. Fig. 6shows a scanning electron micrograph (SEM) of a device with achannel width of 60 lm. The anode–cathode separation is1.45 lm. It is also clear to identify a short extension layer overthe mesa at the anode side of the device in the SEM image.

4. Experimental results and discussion

The current–voltage characteristics of the devices were mea-sured using an Agilent B1500A semiconductor device analyserand a pair of Kevin probes on a Cascade auto-prober station.Unfortunately, short devices with Lac < 1.3 lm died easily due tooverheating. Nevertheless, experimentally measured current–voltage characteristics for devices with Lac ranging from 1.45 lmto 4.0 lm are plotted in Fig. 4.

The RF output spectra of these devices were studied using a50 GHz spectrum analyser (Agilent 4448A), the operating rangeof which was extended using external mixers. Appropriateground-signal-ground (GSG) probes with either an external or anintegrated bias-T was used to apply a dc anode–cathode voltageto the devices and probe the resulting ac oscillation at differentfrequencies. For devices oscillating below 75 GHz, the spectrumanalyzer by itself, or in conjunction with a V-band (50–75 GHz)mixer (Farran Technology WHMP-15) was used to measure thespectrum. In order to make measurements in the W-band(75–110 GHz) the spectrum analyzer was fitted with a W-bandmixer (Farran Technology WHMP-10). The W-band mixer was fur-ther calibrated using a known source and a power meter (HP 8563)with a calibrated sensor (W8461 from Agilent Technologies).

The fabricated devices, with Lac in the range 1.4–4.0 lm, exhib-ited oscillation frequencies between 36 GHz and 118 GHz (Fig. 7a).Typical bias voltages for In0.23Ga0.77As-based planar Gunn diodeswere of the order of 3 V for the shortest devices, extending up to5.5–6 V for the devices with largest Lac. As expected, the deviceswith smaller Lac oscillated at higher frequency. The measuredpower was, on average, approximately �25 dBm and relatively

Fig. 5. Schematic view of the epitaxial wafer layers as grown.

Fig. 6. Scanning electron micrograph of a 1.45 lm device. Coplanar waveguidesignal (S) and ground (G) tracks are labeled. The inset shows a schematic view of afabricated device.

Fig. 7. (a) Variation of output power and frequency vs. anode–cathode distance forthe In0.23Ga0.77As planar Gunn diodes, and (b) linearly extrapolating the inversefrequency curve to determine the ‘‘dead’’ zone of the device.

70 C. Li et al. / Solid-State Electronics 64 (2011) 67–72

invariant as a function of Lac. These results indicate that In0.23-

Ga0.77As-based planar Gunn diodes may have slightly better per-formance than GaAs-based planar Gunn diodes [7,8,28].

A device with Lac = 1.45 lm that was biased at 2.96 V and took acurrent of 30.14 mA showed a peak power of �24 dBm at a fre-quency of 116 GHz [21]. It was further confirmed by using a vectornetwork analyzer (VNA) measurement technique that the 116 GHzoscillation was the fundamental oscillation rather than a harmonicof some lower mode [39]. The measured phase noise of this devicewas�71 dBc/Hz at 10 MHz offset. The effect of varying bias voltageon the frequency and power of this device has also been investi-gated as shown in Fig. 8. The power output rises slightly as the biasis increased towards 2.96 V, before decreasing again at higher volt-ages. On the other hand, the frequency decreases slightly(150 MHz/V) as the voltage increases. The latter phenomenon hasalso been observed in conventional GaAs-based vertical Gunndiodes [40] as well as MMIC-vertical Gunn diodes [41].

One of the explanations for frequency change with Vac is basedon the Gunn effect itself. The effect relies on electrons acceleratedin an electric field decelerating as the electric field in the channelincreases, scattering the fast electrons from the C-valley into thehigh effective mass L-valley. As a consequence, the slow electronsare caught up with by the fast electrons emerging from the cath-ode, leading to the formation of a Gunn domain and therefore areduction in current. Once a Gunn domain reaches the anode andis removed from the channel, the current returns to its originallevel; meanwhile another Gunn domain starts forming near the

cathode. This generates a complete cycle of oscillation and theoscillation frequency is inverse to the transit time of the domainbetween the cathode and the anode. In order for the oscillation fre-quency to decline it is necessary for the rate of domain formationand transport to be reduced. The first explanation, therefore, forthe behaviour we observe is that in our devices, an increase in biasvoltage increases the electric field in the channel so that electronsscatter and decelerate more rapidly, or earlier. This has the effect ofreducing the average velocity of electrons, hence domains, in thechannel, leading to a lower frequency of oscillation.

An alternative explanation is that the change in bias leadsto a modification of the device impedance [40,42]. This explana-tion is consistent with our high frequency observations (below

Fig. 8. Frequency shift and power variation as bias voltage was altered.

C. Li et al. / Solid-State Electronics 64 (2011) 67–72 71

resonance) as seen in inset of Fig. 4 in [21]. However the data offersno detailed physical explanation of the origin of the effect.

Finally, we suggest that there may be a small amount of chan-nel-length modulation occurring in the device as a function ofVac. For devices with large Lac the oscillation frequency, f, is approx-imately determined by Lac

�1. However, as can be clearly seen fromFig. 7a. for small Lac, this relationship fails. This is because there is asmall ‘‘dead’’ zone in the channel [7,40,42], so that we find

f ¼ vLac � Ldead

ð1Þ

This is entirely consistent with the aforementioned explanationfor frequency variation based on the alteration of the average elec-tron drift velocity, v. By linearly extrapolating the graph of f�1 vs.Lac to f�1 = 0 (Fig. 7b), we estimate a typical Ldead for the devicesof 0.25 lm that is in good agreement with earlier work for verticalGunn diodes [43]. If channel length modulation is the origin of thevariation in frequency with Vac, the results would be consistentwith a change in Ldead with bias. This behaviour is consistent witha movement in the domain nucleation point towards the cathodewith increasing bias voltage. This may occur if electrons in thechannel heat more rapidly, leading to earlier onset of domain for-mation. In effect, the dead zone length is decreased, giving rise to alower frequency of oscillation. The channel length modulation andearly scattering model are thus in fact the same.

It is important to mention that all aforementioned spectrumand power measurements were carried out directly onto the planarGunn diodes as shown in Fig. 6. No external on-chip circuits suchas resonators or matching networks were deployed. However, itis believed that by applying appropriate circuits the device perfor-mance will be enhanced in a significant way.

5. Conclusion

We have demonstrated that In0.23Ga0.77As-based planar Gunndiodes constructed in pseudomorphic structures can generateGunn oscillations in millimeter wave frequency range. Althoughonly slight better performance in terms of oscillation frequencyand output power was found in these devices when compared toGaAs-based diodes, this work establishes In0.23Ga0.77As as an alter-native semiconductor material for planar Gunn devices.

Acknowledgements

Chong Li would like to thank Dr. Karol Kalna for useful discus-sions on Medici simulation. This work was supported by UK EPSRCand e2v Technologies (UK) Ltd.

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