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LBNL-38912 UC-404 - 1 @od1=-%DqG287- ERNEST ORLANDO LAWRENCE I o o AL LAB B LEY N E R KE AT N RATO RY Neutron Transmutation Doped (NTD) Germanium Thermistors for SublMM Bolometer Applications E.E. Haller, K.M. Itoh, and J.W. Beeman Engineering Division September 1996 To be presented at the 30tb ESLAB Symposium, “Submiilimetre and Far-Infrared Space Instrumentation,” Noorili%ik, .. The Netherlands, . i ,- . : * k

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LBNL-389 12 uc-404 Neutron Transmutation Doped (Ntd) Germanium Thermistors for Sub-Mm Bolometer Applications E.E. Hallerl.2 K.M. Itoh,3 and J.W. Beeman2 luniversity of California Berkeley, California 94720 2Engineering Division Emest Orlando Lawrence Berkeley National Laboratory University of California Berkeley, California 94720 3Department of Instrumental Engineering Keio University Yokohama, 223 Japan September 1996 This work was supported by U . S . NASA under Contract Nos. W14606,16404,16164, and 17605 through interagency agreements with the U.S. Department of Energy under Contract No. DE-AC03-76SF00098.

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LBNL~38912 30th ESLAB Symposium, "Submillimetre and Far-Infmed Space Instrumentation," N C K N ~ W ~ J ~ The Netherlands, Sept 24-26,1996 NEUTRON TRANSMUTATION DOPED (NTD) GERMANIUM THERMISTORS FOR SUB-MM BOLOMETER APPLICATIONS E.E. HalIer1.2, KM, Itoh3 and J.W. Beeman2 University of C a l i f o r n i a at Berkeley, Berkeley, CA 94720 USA, Lawrence Berkeley National Laboratory, USA and Dept of Instr. Eng., Keio Univ. , Yokohama, 223 JAPAN Berkeley, CA 94720 EEHALLER@LBL.GOV, Phone (510) 486-5294, FAX (510) 486-5530 the relevant papers in these proceedings ( Gear et al., E. ABSTRACT W . Kreysa et al.). We w i l l not discuss preamplifier issues which are complex and require most careful attention. We report on recent advances in the development of One can state that in optimized systems the preamplifiers Neutron Transmutation Doped (NTD) semiconductor should not limit the performance of single element thermistors fabricated from Germanium of natural and bolometers or of anays. controlled isotopic composition. The near ideal doping uniformity which can be achieved with the NTD process, the device simplicity of NTD Ge thermistors and the high 2 THE NTD PROCESS AND THERMISTOR performance of cooled junction field effect transistor FABRICATION (FET) preamplifiers have led to the widespread acceptance of these thermal sensors in many The low operating temperatures required for high radiotelescopes operating on the ground, on high altitude performance bolometers make it necessary to use rather aircraft and on spaceborne satellites. These features also highly doped semiconductors for the sensing element, the have made possible the development of efficient thermistor. At typical operating temperatures of below bolometer arrays which are beginning to produce exciting lK, thermal activation of donor (acceptor) bound electrons results. (holes) no longer occurs at a rate sufficient to produce carrier concentrations which lead to resistivities of a few M IR cm. Such resistivities are required for the fabrication 1. INTRODUCTION of optimized thermistors. Rather, the semiconductor becomes an excellent insulator when lightly doped. A new the mm and near Detection of electromagnetic radiation conduction regime is observed when doping in Ge reaches mm wavelength range continues to pose a challenge, 1015 ~ m - ~ . especially for high ,performance applications in low concentrations of 2 In this mode, bound photon backgrounds. The invention of the semiconductor carriers hop from one dopant atom to a nearby empty neighbor dopant atom. The probability for hopping to thermistor [l] and subsequently of broadband composite occur depends exponentially on the inter-dopant distance bolometers by Coron ( 2 1 , which were optimized by since we deal with a tunneling process. Nishioka et al. [3], has led to widespread use of these sensors in recent years on many radiotelescopes. This distance shows a normal distribution in a truly randomly doped semiconductor. Any fluctuations in the The introduction of Neutron Transmutation Doped (NTD) average local dopant concentration (often referred to as Germanium thermistors [4] has put thermistor research on a f m "dopant striations" in melt doped crystals [5] will lead to and development scientific and solid engineering ) a change in the distribution of the inter-dopant distance This has stimulated the development of footing. bolometer arrays reaching pixel numbers of close to 100. which in turn will lead to fluctuations in the local hopping There are no fundamental limitations to further probability. Resistivity fluctuations over many orders of magnitude can easily result f + t h v doping striations improvement of bolometer performance. Lower operating r o m as temperatures, a reduction of the bolometer heat capacity, has been shown in experiments with Si thermistors [SI. improved bolometer array mounting schemes and efficient What is required is a doping process which leads to a huly coupling of the radiation to the bolometer are among the homogenous random distribution of donors and acceptors. major ingredients for higher performance devices. ' Neither melt doping, doping during epitaxial growth, all commonly used dopant diffusion nor ion implantation, 'In the following we will review the NTD process 'and semiconductor doping processes, lead to the required point out its advantages over conventional doping uniformity. techniques. The availability of isotopically controlled Germanium offers new opportunities for thermistor The only process which guarantees this homogeneity is based on a process in which individual Ge atoms in a very design. The major aspects of array developments will be mentioned, but for further details the reader is referred to pure and structurally perfect crystal are transmuted into