High-performance Hall sensors based on III–V heterostructures

4.50

Sensors and Actuators A, 41-42 (1994) 450-454

High-performance

Hall sensors based on III-V heterostructures

V. Mosser, S. Aboulhouda and J. Dems SC-~

Monbwge Recherche, BP 620-05, F-92542 Monhtnge Cedex (France)

S. Gmtreras, Ph Lorenzmi, F. Kobbl and J.L Robert GE S, URA-CNRS 357, Vnrvet~~ de Mon@elher II, F-34095 Monpellw Cedtx 05 (France)

Abstract Hall sensors mth a high senahvlty, a low thermal dnft and a low offset voltage based on AMXAs/InGrAs/GaAs heterostructures have been developed The physnzal phenomena responsible for the thermal dnft of the Hall senntwlty are rmewed and mvestlgated usmg a set of test devices Hnth well-controlled structure parameters These results have been used to optmuze the design of sensors v&h a Hall factor m the 1000 V/AfT range m order to reduce the temperature sensltmty of the channel electron denstty down to a few 100 ppmiT The dependence of the Hall factor and of Its thermal drift on the bias current has been mvestlgated We show that the bias-current level can be tuned to achieve a very low thermal dnft together mth a high absolute sennhwty m the range 0 4-O 5 V/T

1. Introdllction Magnetic mrcrosensors usmg the transverse Hall effect m a sermconductor material represent an attractrve solutron for a number of apphcatlons, because of their high senslt&y However, their use for metrologlcal apphcations 1s hampered by the thermal dnft of the magnetic sensltlvlty This reqmres the Implementation of some external or Integrated means to compensate the output slgnal for temperature and of&et effects, which increases the fabncatlon and cahbratlon costs III-V heterostructures Hnth a active layer consNmg of a 2D electron gas (ZDEG) m a quantum well appear to be a very pronusmg solution for deslgmng Hall devices with a high sensltlvlty, a reduced temperature sensltlvlty and a low offset voltage Hall sensors usmg A&Gal _&/GaAs [l] or @AlI _Js/I$Ga, _,As/InP [2] heterostructures have been proposed m the literature The Hall coe5iaent of the latter devux had a thermal dnft amountmg to 350 ppm/‘C, which is better than for any avadable SI or GaAs Hall sensors Another type of III-V heterostructure Hall sensor wth a low thermal drift has recently been proposed, whzh uses as the actwe layer a 3D low-doped In,.Ga,_,.As layer embedded between two confinmg In& _& layers on an InP substrate [3] We focused on the use of pseudomorphx Al,Ga, _&/ IqGa,_+/GaAs heterostructures for the fabncatlon of Hall sensors ~rlth a high senslhvlty, a low thermal dnft and a low offset voltage The mam Interest of this

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Sequota Au rlgbts reserved

technology with respect to other III-V heterostructure systems hes m ltsmatunty Indeed, It has been mtensrvely developed for the reabzation of field-effect microwave transistors Hrlth improved performance and IS already at an mdustrlal stage It 1s also supenor to the older AlGaAs/GaAs technology because of the more effectlve confinement of the electrons m the channel, due to the larger AlGaAs/InGaAs conduction-band offset Test devices Hrlthwell-controlled structure parameters were deslgned m order to mvestlgate the physical phenomena responsible for the thermal dnft of the channel electron density [4] These results were used to optlmlze the design of sensors with a Hall factor m the 1000 V/AR range 111order to reduce the temperature sensltnnty of the channel electron density down to a few 100 ppmPC [5] We then mvestlgated the dependence of the Hall factor and of Its thermal dnft on the bias current, and we show that the bias-current level can be tuned to achieve a very low thermal dnft together mth a high absolute sensltlvlty m the range 04-05 vrr

2. Test devices

Several senes of AlGaAs/InGaAs/GaAs heterostructures were fahncated by molecular beam epltaxy (MBE) on a semi-msulatmg GaAs substrate Hall bars with various lengths L, wdths W and aspect ratios Ll W were then patterned usmg a mesa techmque All

451

the devuzes have the same general structure (Fig l), the only varymg parameters being the spacer and caplayer thicknesses and the integrated sheet concentration of the dopmg atoms The Al and In contents are equal to 0 3 and 0 15, respectwely, and the quantum-well thickness amounts to 130 A A band diagram 1s shown m Fig 2 Dependmg on the fabncatlon parameters, the channel electron density n, as measured by the Hall effect, is between 5 X 101’ and 15 X 10” cmm2 The good control of the fabncatlon parameters IS clear m Fig 3, which shows n,, as a function of the mtegrated dopm density for gwen spacer (40 A) and cap-layer (600 1 ) thlck-

Cap layer

Spacer Channel Buffer

S I GaAs substrate Rg 1 Schematic representatton of the AlGaAsiInGaAs/GaAs 2D heterostructures as grown by MBE

. Lap

L&h

16

5

14

3. Electrical characteristics In the mvestlgated devices the Hall factor ranges from 400 to 1200 V/AR The excellent hneanty of the Hall voltage as a function of the apphed magnebc mdution was already shown m ref. 5 Also mvestlgated was the of&et voltage that appears between the Hall electrodes 111the absence of any apphed magnetic mductlon, due to the imperfections of the device The magnitude of this effect can be expressed as an o&t resistance, defined as the offset voltage dlvlded by the bias current, or equivalently by the resultmg offset m the determmatlon of the magnetic mductlon Bo~~t=R,,,&KH Although this offset may anse from mhomogeneltles m both doping level or thuzknesses, it can also be represented by an eqmvalent nusahgnment AL = Q&tit W All these quantities are shown m Table 1 for two representative test devices with dtierent fabrication parameters Very low values (B,&, < 04 mT) are achieved, thanks to the very high electron mob@ m such devices, and foremost to the excellent homogeneity of MBE-grown devices If necessary, this offset could be further reduced by at least one order of magnitude using a swltchmg electrode technique [6]

Lbuff

Rg 2 Band diagram of the AlGaAs/InGaAs/GaAs structures used to make the Hall devxes

,ie

nesses A model was developed 111order to calculate and predict the electron density, and thus the Hall factor, of a gven device as a function of the fabncatton parameters of the heterostructure This model, which does not involve any fittmg parameters, performs the self-consistent resolution of the Schriidmger equation, Poisson equation and Ferm-Duac statlstlcs m the Hartree approxunation, using the Fermi-level pmmng at the surface as the boundary condition Modelhng results are also shown 111Fig 3

2D hetero-

.“““““.“““‘I.“’ /

4. Thermal behaviour of the 2D electron density Under constant-current biasing comhtions, which are generally used m order to achieve good metrologcal properties, a factor of ment of a Hall sensor 1s the TABLE 1 Example of row d c o&t censtancc, correspondmg o&et mduction and eqmvalent mlsahgnment measured on some test devuxs with large dunenslons No swtchmg electrode techmque has been appbed Electron density n*, (cm-‘)

Integrateddopmg dens@ (LO” cm ‘) Fig 3 Hall electron dens@ vs mtegrated sheet doPmg density for several dcvlces mth a 600 8, thick cap layer Dots, expenmental pomts, dashed curve, slmulatlon

Hall bar wrdtb W (w) Aspect ratlo L/W Kn Wlvr) Ro(flct(a) Bom~( (mT) M (W)

14x10’2 150 10 445 0 18 04 0045

7 x 10” 150 10 900 03 033 004

AIGsASAIlGaAS/GaAs

/ l/n8 dns/dT = 270 ppm/=‘C -100

-50 0 50 Temperature (“C)

loo

Fu 4 Equhbnum electron denuty no vs temperatare for an AlGaAs/hGaAs/GaAs Hall dewe wtb a Hall factor density I&-400 v/Ail-

Hall factor, defmed as K,=r,/(en,) [6] In III-V 2D heterostructures, the Hall scattenng factor r, remams equal to umty, because the 2DEG 1s degenerate due to the low 2D dennty of states m the quantum well Thus, the thermal sensltlvlty of the Hall factor IS that of the electron density m the actnre layer The condltlons reqmred m order to achieve an electron density as constant as possible have been detaded m refs 4 and 5 The locahzed states introduced by the doping atoms (so-called DX centres) must be located high enough m energy m order to avoid any thermally dependent electron trappmg Thus, one has to design a sharp potential curvature m the large band-gap layer (Fig 2), which can best be achieved using a delta dopmg profile and a spacer wrth not too large a tlnckness These condltlons, together with an AI,Ga,_&/I$. Ga,& conduction-band offset as large as possible, also prevent the occurrence of the parasltlc parallel conduchon m the large band-gap AlGaAs layer, whch may be observed m AlGaAs/GaAs field-effect transistors However, the mfluence of the Fernu-level pmmng on the mid-gap states at the surface leads to a thermal dnft of the channel electron density that cannot be completely suppressed The resultmg thermal sensltlWy of tbe channel electron density measured at low bias current, &,,= (l/n~)(dn,/dZ”), mamly depends on both II~ and the thickness of the confinmg layer The temperature dependence of no 1s shown m Fig 4 for a device ~th KH= 400 VIA/r and a 600 A thick wniimng layer In devices designed to have a Hall factor equal to 1000 V/m but Hnth various dopmg concentrations and wnfinmg layer thicknesses, S, decrease from about 400 down to 100 ppm.PC if the thickness of the ConfinIng layer 1s mcreases from 600 to 2500 A

ever, m steady-state operation the potentml remams flat withm the substrate Thus, the voltage drop across the undoped buffer layer, 1e , the perpendicular electnc field m tbe buffer layer, depends on the location m the channel This excess electnc field cannot be created by the depletion of free electrons or accumulation of free holes m the substrate Indeed, the concentration of free carnem m the GaAs substrate 1s extremely low, m the W-10’ cmv3 range, because of the deep traps that are Introduced on purpose Hnth a concentration m the 1016cmm3 range to render the substrate semlmsulatmg [7] The Fernu level 1s pinned on the energy levels of these traps, located at mtd-gap Nevertheless, these traps m the part of the substrate just below the undoped buffer layer are able to exchange electrons ~th the channel, with a tnne constant m the m&second range, m order to restore the local electnc eqmhbnum after a dram-source current has been swltched on [8] This effect gwes nse to a channel-substrate capacitance C,, which m first approxnnabon ISmversely proportional to the thickness of the buffer layer C, = z/L, Because of the analogy with the gate of a field-effect transistor, this effect IS referred to as the backgatmg effect As a consequence, the wmbmabon of the channelsubstrate capacitance and the channel resistance acts hke a lumped RC network, which modulates the electron density along the channel Accordmg to this model, If the device 1sbiased mtb a current I, the channel electron density vanes wth the distance y from the source acwrdmg to

(1) Here n, denotes the eqmhbrmm channel electron dens@ m the absence of any dram-source blasmg For Hall contacts located m the middle of the Hall bar, the electron density as measured by the Hall voltage IS nH =n,&,), with y,= L/2 Obviously, as seen from eqn (l), the expenmental Hall electron density nH, and thus the Hall factor K&) = l/(en,(z)), wdl shghtly depend on the bias current Such a behavlour 1s shown m Fig 5 for a device wltb a Hall factor K,,=404 mV/ mA,T and an mput resistance R,,=3500 fl under low bias current The sensltnQ versus current of the Hall factor induced by the backgatmg effect, S,=dZ&/dr, amounts to 25 mV/mA’m These values for KH and S, are smnlar to those obtamed with the best state-ofthe art vertical slhwn Hall-effect sensors [9]

5. Backgatlug effects

6. Thenaal behaviour of the Hall factor

In a biased device the channel potenttal drops wntmuously between the source and dram contacts How-

As the channel resistance &=(ekn,)-‘(LIP’) depends on the temperature, mamly through the mob&y

453 800 a Baa0 B e 4

400

1

200 ~~,,,,' \

8 0 -500

Blascurrent

Iti

@A)

Btas

Fig 5 Hall factor vs btas current for an AlGaAsiInGaMGaAs Hall device with a Hall electron dens@ n,= 15 X 1Ol’ cmW2

16.

. . . . . . ..I.‘.

I.... .

14-

3 +- 12 “5

1

zi _

OS 06

I,,_:,:

150

200 250 300 Temperature (K)

350

Fig 6 Inverse mob&y vs temperature for an AlGaAs/InGaAs/ GaAs Hall devxe with a Hall factor Km=-810 V/&T, showmg the hnear relatlonshlp /L.-‘-T

p,,, the magnitude of the backgatmg effect will be temperature dependent If we now consider a Hall device where the thermal dnft of the free electron density m the channel has already been reduced to a few hundred ppm/?Z, eqn (1) can be differentiated to yield 1 dnH

1 dn* dT

--=---nH dT

n,

c, --1 eras0 enso

L

d(~L,-l)

2W

dT

(2)

The thermal behavlour of the moblhty appeanng m eqn (2) can be determined expenmentally As shown m Fig 6, m such 2D devxes the inverse moblhty has a hnear temperature dependence CL,,-’=aT 111the temperature range of interest This 1s not surpnsing since the moblhty 1s hmlted mainly by acoustic phonon scattermg Thus, the relatnre thermal drift of the Hall factor reads

c&,.Z$p-’

s,=s,[

1

1

250

(3)

As expected from eqn (3), a plot of the experunental thermal dnft of the Hall factor S, shows a hnear dependence on the bias current, as shown in Fig 7

0

250

500

current @A)

Fig 7 Thermal sensltlvlty of the Hall factor as a function of the bias current for the same device as III Fig 6

The value of the slope, 0 75 K-l A-l, 1s to be compared with the expected value of the term m brackets in eqn (3) Takmg into account the expenmental value of the Hall factor under low biasing conditions, I&,= 810 V/m, a device aspect ratio LIW=6, the channel-substrate capacitance C, = lo-’ F/cmm2, and the expenmental thermal denvatlve of the inverse mobility from Fig 6, the calculated slope amounts ot 09 K-’ A-’ In spite of the crudeness of the model, this value 1s m good agreement with the expenmental one, thus demonstrating the vahdlty of this approach Interestingly, eqn (3) predicts the existence of a constant bias current for w&h the thermal drift of the experunental Hall factor will remam very small Tins allows a large operatmg temperature range without external compensation This fact was also noticed by Kyburz ei al [3] m 3D InAlAs/InGaAs/InP Hall sensors For the devxes shown m Fig 7, the value of this lowthermal-d& bias current 1s about 05 mA, and the resultmg absolute sensmvlty amounts to 0 4-O 5 VfT The corresponding voltage drop m the device IS about 3 V, and the power dissipation remains low, m the mW range Usmg the same heterostructure, the value of the absolute magnetic sensltlvlty could be m pnnclple increased by 50% by reducmg the aspect ratio of the Hall bar down to L/W=4

7. conclusiolls Magnetic sensors Hnth supenor performance can be designed by makmg use of the band-gap engmeenng of III-V heterostructures The pseudomorphic AlGaAs/ InGaAs/GaAs system, whose technology is available from mdustnal foundnes, 1s of particular interest [4, 51 Hall-effect devices with a Hall factor amounting to at least 1000 V/A/T can be designed The mtnnslc temperature coefficient of the active layer electron

454

density can be reduced to a few 100 ppmPC Prehmmary work mdlcated values as low as 100 ppmPC for devices with a buned active channel The output sIgna of these sensors also shows a low offset and an excellent hneanty versus the magnetic mductlon The non-hneatrty of the Hall factor versus the bias current, orlgmatmg from backgatmg effects due to the semi-msulatmg GaAs substrate, is comparable to that reported for the best Sl Hall sensors [9] This backgatmg effect can be used to decrease the temperature coefficlent of the sensltlvlty down to 100 ppm?C, or maybe even lower, m magnetic sensors ~th an absolute sens1t1vlty of 0 4-o 5 v/T Further work 1s under way to nnprove further the performance of such devrces

Acknowledgements This work was supported m part by the French Minis&e de 1’Enselgnement Superleur et de la Recherche Thanks are due to Drs K Zekentes, from FORTH-IESL (Herakhon), J F Rochette, from PIcopga and A Marty, from LAAS-CNRS, for the fabncatlon of the devices used m this study

References 1 Y SuByama, H Soga and M Tacano, IQhly-senatlve Hall element with quantum well superlattlcc structure., J Cryst Gmwth, 9.5 (1989) 394-397 2 H Sugyama, Y Takeucbl and M Tacano, Highly-sensltnre 2DEG-Hall dence made of pseudomorphlc InGaAs heterostructure, Sensors and Actuators A, 34 (1992) 131-136 3 R Kyburz, J Schnud, R S Popowc and H Mel&or, Highly senslhve gated InGaAs/fnP Hall sensors wxtb low temperature coeffiuent of the sensrtwdy, Pmc ESSDERC 93 Conf, Grenoble, France, 1993, pp 655-658 4 V Mosser, S Contreras, S Aboulhouda, P Lorenzmt, F Kobbl, J I_+ Robert and K. Zekentes, Phyxs of AlGaAs/ InGaAs/GaAs heterostructures for high performance magnetic sensors, Proc ESSDERC 93 Conf, Grenoble, Fmnce, 1993, pp 659-662 5 V Mosser, S Contreras, S Aboulhouda, P Lorenzuu, F Kobbl, J L Robert and K Zekentes, High sennhvlty Hall sensors wth low thermal dnt? usmg AlGaAs/InGaAs/GaAs heterostructureq Sexwrs and ActuutorsA, 43 (19%) u1 press 6 R S Popovx, Hull E’zct Dewcq Adam Hdger, Bnstol, 1991 7 P F Lmdqmst and W M Ford, m J V DI Lorenzo and D D Khandelwal (eds ), Ga4s Fet Pnnctplesand TechrwlogyArtech House, Dedham, MA, USA, 1982, pp S-59 8 L E Larson, An unproved GaAs MEFET equwalent clrcmt model for analog Integrated cucmt apphcattons, IEEEJ SohdState C~rcrutsSC-22 (1987) 567-574 9 U Falk and RS Popover, Vertical Hall effect dcvlces with suppressed lunctlon field effects, Tech DI@, 7th Int Conf Sol&-StateSemo~ andActuators (Tmnsducn ‘93), Yokohama, Japan, June 7-10, 1993, pp 902-903