IEEE TRANSACTIONS ON NUCLEAR SCIENCE

CORRELATION BETWEEN LITHIUM DRIFT MOBILITYAND MINORITY-CARRIER DRIFT MOBILITY

IN GERMANIUM

Guy A. Armantrout

University of California, Lawrence Radiation Laboratory

Livermore, California

SUMMARYThe minority-carrier drift mobility of drift-

able and nondriftable germanium was measuredin an effort to determine the presence of an im-purity which reduced the lithium drift mobilitybut did not affect the resistivity and lifetime ofsome of the material used to make lithium-driftdetectors. The measurements were made at77'K where the mobility is limited by impurityscattering rather than lattice scattering.

The mobility was measured as a function oftemperature for five samples of Ge which hadvarying lithium drift mobilities. Good correla-tion was found between the driftability of the ma-terial and the minority-carrier drift mobility at77°K. Estimated impurity levels in the range of1014-1015/cm3 apparently reduce the effectivelithium mobility, while impurity concentrationsgreater than 5 X 1015 make the Ge unsuitablefor making lithium-drift detectors.

A mass-spectrometer analysis indicated thatthe impurity apparently has a mass number lessthan Mg; oxygen is a likely possibility due to itsknown behavior with Li in silicon.

1. INTRODUCTIONThe fabrication of Ge lithium drift detectors

involves compensation of the material by themovement of the Li ions under an applied field.Any reduction in the Li drift mobility will reducethe thickness of the depletion region that can beformed. Some of the material used for makingGe lithium-drift detectors has shown a markedreduction in the Li mobility that may be as muchas a factor of 30 below the optimum value.1

The mechanism responsible for the reductionin lithium mobility is probably the formation ofion-pairs or lithium-ion complexes with someimpurity present in the Ge. The behavior of theLi is similar to lithium-ion pair formation asreported elsewhere2 and it would require a sig-nificant concentration of the impurity to cause thereduction in Li mobility that was observed.

There is no significant difference in the re-sistivity, lifetime, or etch-pit density for thedriftable and nondriftable material. This indi-cates that the impurity is electrically inactiveand does not introduce appreciable trapping cen-ters in the band gap of the Ge. In this paper, theminority-carrier drift mobility is used to deter-mine the presence and quantity of the impurity or

impurities in the Ge not detected by the previousmeasurements.

11. MINORITY CARRIER DRIFT MOBILITYAt higher temperatures (> 200°K) the drift

mobility of the minority carriers is limited bythe lattice scattering in lightly doped material.The mobility increases with decreasina tempera-ture for electrons according to the expressionAe = 4.9 X 107 T-1*66 cm2/V-sec, where T is indegrees Kelvin. However, at lower temperaturesthe drift mobility is limited by scattering fromneutral and ionized impurities and by carrierinteractions. Scattering by ionized impurities isgreatest when the carrier velocity is the lowest,so the greatest mobility reduction occurs at verylow temperatures. Scattering caused by neutralimpurities is less pronounced and less tempera-ture-dependent than that due to ionized impuri-ties, but will be significant at lower temperaturesif many impurity centers are present.

The electron-hole interaction is related tothe fact that the chiarge carriers are traveling inopposite directions. Thus the minority carriersencounter a certain amount of drag which signifi-cantly reduces the minority-carrier drift velocityat 770K.3 Fortunately, the level of the interactiondepends on the doping level of the crystal andshould be relatively constant for any given dopinglevel. In the crystals used for these mobilitymeasurements, the doping is sufficiently uniformso that the effect of electron-hole drag can beconsidered as a constant.

A measurement of the drift mobility shouldthen yield a curve which closely follows the curvefor lattice scattering at higher temperatures anddeviates from this curve at lower temperaturesbecause of the scattering of impurities and elec-tron-hole interactions. The amount of the reduc-tion below the level determined by the electron-hole interactions would be an indication of the im-purity concentration in the germanium.

III. EXPERIMENTAL APPROACHThe method used to measure the minority-

carrier drift mobility was essentially that usedby Prince.4 In this method, a sweeping field isapplied to a bar of Ge as shown in Fig. 1. Apulse of minority carriers is then injected at oneend and swept down the bar by the field. Thearrival of the minority carriers at the other end

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1966 ARMANTROUT: LI AND MINORIT

of the bar is measured by a reverse-biased junc-tion. It is a simple matter to directly calculatethe drift mobility from the sweeping field, thedistance traveled by the carriers, and the transittime.

Our measurements were made with a bar ofdimensions 0.5 X 0.5 X 2 cm. The large cross-sectional area minimized surface effects. Theohmic end contacts were electroless-nickelplated over a lapped surface, and the junctionsused for the carrier injection and detection wereformed by alloying 10-mil Au-Sb spheres into thebar 2 mm from the Ni contacts. The sweepingfield was measured both across the sample andacross the two junctions. A consistent field read-ing indicated a negligible drop across the ohmiccontacts. The contact structure was designedsuch that readings were possible over a widerange of temperature.

The finished sample was mounted in theholder shown in Fig. 2. The temperature-con-trolled plate had an internal heater and a heatleak to an LN cold finger so that the temperaturecould be continuously varied over the range 770to 300°K. The sample was insulated from thetemperature-controlled plate by a 1/2-mil Mylarsheet and a thermal-conducting grease. All read-ings were taken after the system had time to sta-bilize at a given temperature.

A check on the reliability of the readings wasmade by varying the value of the sweeping field.A consistent value of mobility indicated that therewere no significant nonlinear effects, and thatsample heating was not a problem. Occasionally,low mobility readings were obtained for highsweeping fields, indicating that sample heatingwas occurring. Low field readings appeared togive the best results.

The effect of various experimental errorssuch as occur in sample dimension determinationwas minimized by adjusting all of the mobilityreadings to a standard room-temperature valueand then correcting the low-temperature readingsby the same factor. This was justified becausethe lattice scattering limited the mobility of theminority carriers to a constant value at roomtemperature for lightly doped material. Themaximum correction in all cases was less than10 percent.

Figure 3 shows the results of the drift mo-bility measurements as a function of temperaturefor five samples. Curve 1 was obtained for drift-able material. Curve 2 is for very slowly driftingmaterial, while Curve 3 is for essentially non-driftable material. Curves 4 and 5 are for non-driftable material from a different source. Thereis good correlation between the driftability of thematerial and the value of the minority-carrierdrift mobility measured at 770K.

A good indication of the validity of the mea-surements is the fit of the measured mobilitycurves with the curve for lattice scattering ef-fects at the higher temperatures. Nonlinearitieswhich might cause a significant error at low tem-peratures would tend to appreciably change theshape or the slope of the curve at higher tempera-tures. The only significant deviations of the

'Y-CARRIER DRIFT MOBILITY IN GE

curves are at low temperatures, where impurityscattering and electron-hole interactions pre-dominate. The experimental approach is as-sumed to be accurate enough for the determina-tions made here.

IV. DISCUSSIONThe value of electron mobility at 770K was

23,000 cm2/V-sec for the good material. Thisagrees reasonably well with the data obtained byPaige for material of this doping level when theeffects of electron-hole drag are included.5 Themobility readings that are below this level at77°K indicate increasing quantities of impuritiesin the various samples.

From the resistivity measurements it wouldappear that the impurity is neutral. Erginsoy6has derived an expression for minority-carrierscattering due to neutral impurities by assuminga hydrogen-atom-like model and no temperaturedependence of the scattering. The impurity con-centration present in the nondriftable Ge, asestimated from this model with the observedmobility reduction, would be on the order of1017/cm3. It is suggested that this model gives apessimistic answer because of the assumptionswhich are made; it is likely that the actual impu-rity concentration is below this value.

A lower limit can be set for the probable im-purity concentration by assuming that the scat-tering is caused by ionized impurities. Again,using the data of Paige, an impurity concentrationgreater than 7 X 1014/cm3 can be inferred forsamples 2 and 3, and greater than 3 X 1015/cm3for samples 4 and 5. The lowest probable im-purity concentration that has a significant effecton the Li drift mobility is in the range of 1014-1015/cm3. Germanium with an impurity concen-tration in excess of approximately 5 X 1015 ap-pears to be nondriftable.

A mass-spectrometer analysis was made onsome of the nondriftable Ge. The mass spectro-meter was useful only for the detection of elementmasses heavier than Mg and also was contami-nated in the mass range between 50 and 62. Thesensitivity for the heavier elements was 10 ppb,which should be sufficient to detect any impuritiespresent in the nondriftable material. No signifi-cant traces of impurities were found in the usefulrange of the spectrometer indicating that theprobable impurity is lighter than Mg. The tran-sition elements were ruled out because of theirelectrical activity in Ge, as were some of theother electrically active elements lighter thanMg.

Of the remaining lighter elements, oxygen isone of the more likely impurities. Oxygen is elec-trically inactive in Ge under certain conditionsand presumably can reduce the mobility of Li inGe in a manner similar to the mobility reductionof Li in Si by oxygen as reported by Pell.7 Theimpurity concentration determined here is wellwithin the solubility range of oxygen in Ge.Further work will be necessary, however, tobetter determine the impurity that is causing thereduction in Li mobility.

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IEEE TRANSACTIONS ON NUCLEAR SCIENCE

V. CONCLUSIONSThe measurement of the minority-carrier

drift mobility has indicated the presence of an im-purity in nondrifting Ge which has not been de-tected by resistivity or lifetime measurements.As such, this method may be useful in controllingthe quality and further studying the properties ofGe intended for use in making lithium-drift detec-tors.

REFERENCESIt,

Work performed under the auspices of the U. S.Atomic Energy Commission.

1G. A. Armantrout, "Ambient Storage Effectsand Mounting Problems of Very Large VolumeGe LID Detectors," IEEE Transactions on NuclearScience NS-13, No. 1, Feb. 1966, p. 84.

-OPERATING SCHEMATIC

Fig. 1-Operating schematic for measuring the driftmobility of injected minority carriers.

2G. Norgate and R. J. McIntyre, "Stabilizationof Lithium-Drifted Radiation Detectors,," IEEETransactions on Nuclear Science NS-11, No. 3,June 1964, P. 291.

3T. P. McLean and E. G. S. Paige, "A Theoryof the Effects of Carrier-Carrier Scattering onMobility in Semiconductors," J. Phys. Chem.Solids 16, 220 (1960).

4M. B. Prince, "Drift Mobilities in Semicon-ductors. I. Germanium," Phys. Rev. 92, No. 3,681 (1953).

5E. G. S. Paige, "The Drift Mobility of Elec-trons and Holes in Germanium at Low Tempera-tures," J. Phys. Chem. Solids 16, 207 (1960).

6C, Erginsoy, Phys. Rev. 79, 1013 (1950).

7E. M. Pell, "Study of Li-O Interaction inSi by Ion Drift," J. Appl. Phys. 32, No. 6, 1048(1961).

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Fig. 3-Electron drift mobility as a function of tem-perature for driftable and nondriftable sam-ples of P-type germanium.

Fig. 2-Apparatus for measuring the minority-carrierdrift mobility vs temperature.

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