We demonstrate fabrication of an integrated

micro cooler with the planar Gunn diode and characterise

its performance. First experimental results have shown a

small cooling at the surface of the micro cooler. This is first

demonstration of an integrated micro-cooler with a planar

Gunn diode

Cumming, D.R.S

Dunn, G.

Glover, J.

Hopper, R.

Khalid, Ata-ul-Habib

Kuball, M.

Montes, M.

Oxley, C.

Papageorgiou, V.

Stephen, A.

English

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     Khalid, Ata-ul-Habib, Glover, J., Hopper, R., Papageorgiou, V., Montes, M., Kuball, M., Dunn, G., Stephen, A., Oxley, C., and Cumming, D.R.S (2013) Fabrication of integrated planar gunn diode and micro-cooler on GaAs substrate. In: 40th International Symposium on Compound Semiconductors, 19-23 May 2013, Kobe, Japan.    Copyright © 2013 The Authors   A copy can be downloaded for personal non-commercial research or study, without prior permission or charge  Content must not be changed in any way or reproduced in any format or medium without the formal permission of the copyright holder(s)   When referring to this work, full bibliographic details must be given    http://eprints.gla.ac.uk/83932    Deposited on: 02 August 2013                   Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk  Fabrication of Integrated Planar Gunn Diode and Micro-cooler on GaAs Substrate Ata Khalid1, James Glover2, Richard Hopper2,	  Vasileios Papageorgiou1, Miguel Montes3, Martin Kuball3, Geoff Dunn4, Alex Stephen4, Chris Oxley2 and David R. S. Cumming1  1School of Engineering, University of Glasgow, UK 2De Montfort University 3University of Bristol 4University of Aberdeen ata.khalid@glasgow.ac.uk  Abstract—We demonstrate fabrication of an integrated micro cooler with the planar Gunn diode and characterise its performance. First experimental results have shown a small cooling at the surface of the micro cooler. This is first demonstration of an integrated micro-cooler with a planar Gunn diode. Keywords—micro cooler; Gunn diode; thermionic cooler; I.  INTRODUCTION PLANAR Gunn diodes with GaAs/AlGaAs heterojunctions have successfully demonstrated oscillation in the transit-time fundamental mode at 108 GHz [1]. Numerical simulation based on a self-consistent Monte Carlo method has shown the mechanics of Gunn domain formation and transport in such devices [2]. These novel devices have great potential as integrated millimeter-wave or terahertz signal sources. This is because the lithographically determine device anode and cathode spacing Lac and can be reduced to sub-micrometer dimensions to yield higher frequency of transit-time mode oscillation [3]. The reduction is Lac has to scale with higher doping level to achieve smaller domain size. This directly results in  an increase in electron current density in these devices,  significantly increasing the heat generated in the channel  leading to increased operating junction temperature of the device. High junction temperature will contribute to increased scattering in the channel and reduced electron velocity, increased noise as well as lower impact ionisation threshold. The early onset of impact ionisation would result in early device failure. Also, GaAs is known to have poor thermal conductivity compared to silicon (Si), which means passive heat removal would not be enough for stable Gunn diode operation. The presence of a micro-cooler only few micron away from the active channel would improve the heat removal and as a result planar Gunn diodes would be operating in a thermally stable environment.  Recently similar approach has been implemented is some of the compound semiconductor devices [4]. However, there is no previous report on integrated planar Gunn diodes with a micro-cooler. Hence this work would be a significant step to address the stable operation for short Lac Gunn devices. We are presenting for the first time an integrated thermionic micro-cooler integrated with a planar Gunn diode. We will describe the layer structure of the planar Gunn diode with integrated thermionic micro cooler followed by the device fabrication and finally first experimental measurement of the micro cooler. II. EXPERIMENTAL A. Planar Gunn Diode and MicroCooler Design In conventional vertical Gunn diode the cathode contact is  at the bottom and also acts as the heat-sink. The substrate is removed in that case and device placed in a metal cavity. However, planar Gunn devices are designed to be integrated with on chip electronic components  on a substrate. In this case, placement of  heat sinking will always be a long distance away from the active channel of the devices. An integrated micro-cooler is one possible solution to improve the heat removal  in a planar structure.  Figure 1 shows schematically the integration of a micro cooler for planar Gunn diode on a semi-insulating substrate. The planar Gunn diode and micro cooler are electrically isolated from each other by using a 5 µm thick un-doped GaAs separation layer. Monte Carlo simulations were carried out to estimate the smallest possible separation layer to achieve electrical isolation between two devices.  Figure 1.  A schematical layout of integrated planar Gunn diode and micro cooler on GaAs substrate. B. Device Layer Structur & Fabrication Molecular beam epitaxy was used to grow layers on a 620 µm thick semi-insulating GaAs substrate. A 0.5 µm GaAs This work was supported by UK EPSRC and e2v Technologies (UK) Ltd.    buffer layer was grown first on the substrate followed by a 300nm n-GaAs lower contact layer doped at 4×1018 cm-3, 50nm graded aluminium gallium arsenide (Al0-0.10Ga0-0.90As), 100 periods of a Al0.10Ga0.90As-Al0.20Ga0.80As super-lattice (totaling 2µm and maintain doping in supper-lattice and graded layer at 2×1018 cm-3), 50nm graded (Al0.10-0Ga0.90-0As), and 300nm n-GaAs top contact layer. The super-lattice reduces phonon transport, providing a thermal barrier between the hot bottom contact layer and the cold top contact layer.  A further 5 µm un-doped GaAs separation layer was grown on the top contact layer of micro-cooler layers. It is followed by the active part of the Gunn device consisted of the channel made of 50 nm un-doped GaAs t sandwiched by a 20 nm double δ-doped Al0.23Ga0.77As layers. 15 nm of highly doped GaAs was grown on top of the Al0.23Ga0.77As barrier layer to avoid oxidation. Finally, a 100 nm GaAs cap layer doped at 4×1018 cm-3 was grown for Gunn diode contacts. Figure 2 shows the epitaxial layer structure of the integrated Gunn and micro-cooler. The fabrication of the integrated devices starts with the planar Gunn device and the fabrication process is already described elsewhere [1]. The micro-cooler structure is defined by mesa etching, which enables a large number of different cooler geometries to be fabricated, as well as TLM test cells for measuring the contact specific resistance. The measured specific resistance of the ohmic contacts was 5x10-6Ωcm2. The measured total resistance of a cooler, with an area of 20,000µm2, was approximately 1Ω Figure 2.  Layer structure of the integrated Gunn and micro-cooler. III. RESULTS AND DISCUSSION The fabricated coolers were measured on wafer by applying DC bias using micro-probes. To minimise the effect of probe heating at the top (cooled) contact, it was wire bonded to a metallised probe pad. The temperature difference between the hot and cold contacts is expected to be small therefore accurate temperature measurements are required. The contacts were monitored respectively, over a range of bias conditions, using high emissivity micro-particles imaged by infra-red (IR) thermal microscopy [5]. This enabled more accurate temperature measurements to be made than is possible using conventional IR measurements. The micro-cooler can be represented by the equation:     RIQRTth2−=Δ where ∆T= Tc-Th the difference in temperature between the hot (Th) and the cold (Tc) contacts, I2R is the ohmic dissipation in the cooler (R is the resistance of the top contact and half of the bulk internal resistances of the cooler), Rth is the thermal impedance of the cooler and Q is the cooling power. Q=ITcSn, where Sn is equivalent to the Seebeck coefficient. The equation can be re-arranged as  Figure 3 shows the measured cooling of ~0.5°C at an ambient of 125°C and internal heating (I2R) depending upon the current direction in the micro-cooler. Figure 3.  Measured results of micro cooler on GaAs substrate IV. CONCLUSIONS The paper presents the the design of an integrated micro-cooler with planar Gunn diode. The fabrication of an integrated micro-cooler with the planar Gunn diode is described and the characterization of the micro-cooler.Initial experimental results have shown 0.5Cofcooling at the top contact surface of the micro cooler.  REFERENCES [1] A. Khalid, N. J. Pilgrim, G. M. Dunn, M. C. Holland, C. R. Stanley, I. G. Thayne, and D. R. S. Cumming, “A planar Gunn diode operating above 100 GHz,” IEEE EDL., vol. 28, no. 7, pp. 849-851, Oct. 2007. [2] N. J. Pilgrim, A. Khalid, G. M. Dunn, and D. R. S. Cumming, “Gunn oscillations in planar heterostructure diodes,” Semicond. Sci. Technol., vol. 23, no. 7, 075013, Jul. 2008. [3] S. Perez, T. Gonzalez, D. Pardo, and J. Mateos, “Terahertz Gunn-like oscillations in InGaAs/InAlAs planar diodes,” J. Appl. Phys., vol. 103, no. 9, 094516, May 2008. [4] Jizhi Zhang, Neal G. Anderson, and Kei May Lau, AlGaAs superlattice microcoolers, Appl. Phys. Lett. 83, 374 (2003). [5] C. Oxley, R. Hopper, D. Prime, M. Leaper, G. Evans, and A. Levick, “Probe propels IR thermal microscopy to a new level,” Compound Semiconductor, vol. 17, no. February, pp. 33-36, 2011.  

Fabrication of integrated planar gunn diode and

micro-cooler on GaAs substrate

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Fabrication of integrated planar gunn diode and micro-cooler on GaAs substrate

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