Clendenin, J E

Mamaev, Yu A

Maruyama, T

Mulhollan, G A

Subashiev, A V

Yashin, Yu P

CERN Document Server

SUPERLATTICE PHOTOCATHODES FOR ACCELERATOR-BASEDPOLARIZED ELECTRON SOURCE APPLICATIONS*J. E. Clendenin   #, T. Maruyama, G. A. Mulhollan, SLAC, Stanford, CA Yu. A. Mamaev, A. V. Subashiev, Yu. P. Yashin, SPTU, St. Petersburg, Russia                                                *Work supported by the U.S. Department of Energy contract DE-AC03-76SF00515, the U.S. Civilian Research and Development FoundationAward No. RP1-351, the Russian State Program Project 2.6.99, and the Russian State Programs “Physics of Solid State Nanostructures” under grant97-1091.#Email: clen@slac.stanford.eduAbstractA major improvement in the performance of the SLC wasachieved with the introduction of thin strained-layersemiconductor crystals. After some optimization,polarizations of 75-85% became standard with lifetimesthat were equal to or better than that of thick unstrainedcrystals. Other accelerators of polarized electrons,generally operating with a much higher duty factor, havenow successfully utilized similar photocathodes. Forfuture colliders, the principal remaining problem is thelimit on the total charge that can be extracted in a timescale of 10 to 100 ns. In addition, higher polarization iscritical for exploring new physics, especiallysupersymmetry. However, it appears that strained-layercrystals have reached the limit of their optimization.Today strained superlattice crystals are the most promisingcandidates for better performance. The individual layers ofthe superlattice can be designed to be below the criticalthickness for strain relaxation, thus in principleimproving the polarization. Thin layers also promote highelectron conduction to the surface. In addition the potentialbarriers at the surface for both emission of conduction-band electrons to vacuum and for tunneling of valence-band holes to the surface can be significantly less than forsingle strained-layer crystals, thus enhancing both theyield at any intensity and also decreasing the limitationson the total charge. The inviting properties of the recentlydeveloped AlInGaAs/GaAs strained superlattice withminimal barriers in the conduction band are discussed indetail.1  INTRODUCTIONThe polarization of electrons extracted from III-Vsemiconductor crystals has a theoretical upper limit of50%. In 1991 it was first demonstrated that growing athin crystalline layer on a substrate having a slightlydifferent lattice constant could raise this limit [1]. Thelattice mismatch introduces a strain in the epilayer thatremoves the degeneracy of the heavy-hole (hh) and light-hole (lh) valence bands. Polarizations on the order of 80%are routinely achieved from these strained-layer cathodes,which simultaneously exhibit quantum yields, Y , of>0.1%. Such a cathode, consisting of 100-nm of GaAsgrown on a GaAsP substrate, became the standard for theSLC polarized electron source, eventually accumulating~20,000 hours of operating time for the SLC program,making possible the single-most precise measurement ofthe electro-weak mixing angle, sin2 q Weff.Future colliders will require both higher polarization andmore charge. Higher polarization will not only increasethe effective luminosity of a collider, but if ‡ 95% willalso significantly increase the physics reach, especially inthe exploration of supersymmetry [2]. For JLC/NLC, therequired charge per 100-ns macropulse is ~300 nC at thegun, which is an order of magnitude greater than producedfor SLC. The problem is that the maximum steady-statecurrent in a long macropulse that can be produced from anSLC-type cathode is on the order of 0.2 A cm-2, whichwould require an impractical cathode diameter of 4 cm tomeet the collider charge requirement. The maximumcurrent of an SLC-type cathode can probably be increasedby an order of magnitude simply by increasing the dopantdensity from the mid 1018 cm-3 range to the low 1019 cm-3range. However, both experimental and theoretical effortsto date point to a limitation of the polarization of thestrained-layer cathode at the 85-90% level [2].A major source of depolarization for the strained-layercathodes is the relaxation of the strain as the distance fromthe heterojunction increases beyond the critical thickness,hc, for maintaining perfect strain, thus decreasing theaverage hh-lh separation. Typically hc is ~10 nm, whereasthe optimum epilayer thickness to maintain both highpolarization and high charge is ~100 nm. This problem isovercome in principle using the superlattice (SL)structure, in which the hh-lh degeneracy is removed by thelocalization of holes in quantum wells. Since the overallcritical thickness, Hc, for a short-period SL can beconsiderably larger than hc, the total thickness of the SLcan be on the order of 100 nm without relaxation of thestrain.SL structures were shown to produce >50% polarizationin 1991 soon after the first strained-layer results, but thepath to simultaneously high polarization and high Y  hastaken longer. Recently however, polarizations approaching90% with Y  within an order of magnitude of that of theSLC cathodes have appeared [3,4].2  SUPERLATTICE WITH ZEROCONDUCTION BAND OFFSETIn previously used AlxGa1-xAs/GaAs, InxGa1-xAs/GaAs andAlxGa1-xAs/InyGa1-yAs SL, the hole miniband splittingwas accompanied by the building of high barriers forelectrons as a result of the high value of the ratio of theconduction-band offset D Ec to the valence-band offset D Ev( D Ec/ D Ev » 2) at the heterointerface. The necessity for theelectrons to tunnel through the SL barriers results in alower electron diffusion rate along the SL axis0-7803-5573-3/99/$10.00@1999 IEEE. 1988Proceedings of the 1999 Particle Accelerator Conference, New York, 1999accompanied by growth of the spin relaxation rate.Therefore the spin diffusion length restricts the thicknessof the active layer for these structures.The main advantage of the AlxInyGa1-x-yAs/GaAs SLproposed here comes from the band line-up between thesemiconductor layers of the SL. The Al contentdetermines the formation of a barrier in the conductionband, while adding In leads to conduction band lowering,so the conduction band offset can be completelycompensated by appropriate choice of x and y, whilebarriers for the holes remain uncompensated. Therefore theuse of superlattices with the optimized quaternary alloycomposition can provide a high vertical electron mobilityand simultaneously a small spin relaxation rate while alsomaintaining a sufficiently large valence-band splitting.We have studied an Al0.20In0.18Ga0.62As/GaAs SLconsisting of 17 pairs of GaAs (4 nm) and AlInGaAs (4nm) Be doped with density 4x1017 cm-3 except the toplayer, GaAs, with 4x1018 cm-3. The dopant density wasmaintained low in the bulk to minimize depolarization buthigh at the surface to increase the negative affinity. Thesample was grown by molecular beam epitaxy (MBE) bythe Ioffe Physico-Technical Institute in St. Petersburg [4].A protective As coating was added as the final step.Evl2Evh2Ec2Ev1Ec1e1lh1hh1Al0.2In0.18G a0.62AsGaA sG 8G 6Fig. 1. Energy band diagram of AlxInyGa1-x-yAs/GaAs SL.The minibands (thin lines) are identified by notation e1and hh1, lh1 for electrons and holes respectively.The miniband spectrum of the SL is determined by theband offsets at the heterointerfaces. In the case of aheterointerface with lattice matched ternary solid solution(e.g., AlxGa1-xAs/GaAs) the conduction-band offset ratio,Qc1, is defined as Q E x E xc c g1 1= ( ) ( )D D , where D Eg1 isthe difference in the band gaps of the contacting crystalsso that the valence band offset ratio, Qv, is Qv=1-Qc. Foran InyGa1-yAs/GaAs interface the offset is modified by thestrain distribution in the contacting layers. For thestructure with a thin InyGa1-yAs layer grown on a thickGaAs substrate all the strain is assumed to accumulate inthe InGaAs layer. For this case, D Ec  is given by:D DE y Q E y E yc c def g gdef( ) = · ( ) + ( )( )2 2 2, d ,where Qc def2,  is the conduction-band offset ratio for thestrained InyGa1-yAs layer, while d E ygdef2 ( ) is the change ofthe band gap induced by strain that can be expressedthrough interpolated deformation potentials, elasticconstants, and the lattice constant mismatch. For the caseof the AlxInyGa1-x-yAs/GaAs SL, a linear interpolationbetween the values for AlxGa1-xAs/GaAs and InyGa1-yAs/GaAs interfaces should be valid for small x and y. Wehave found that interpolation starting with the unstrainedvalue of the band offset for the InyGa1-yAs/GaAsheterointerface gives band-gap values that are far from theexperimental data obtained from emission spectra. Usingthe band offset value for the strained InyGa1-yAs/GaAsinterface, we get:D D DE x y Q E x Q E y E yc c g c def g gdef,,( ) = · ( ) + · ( ) + ( )( )1 1 2 2 2d .The schematic of the position of the band edges forx=0.20, y=0.18 is shown in Fig.1. Taking Qc1=0.66 andQc2,def =0.7, we have found that the conduction band offsetappears to be minimized for x=1.1y.The calculated positions of the band edges of the layersin the strained SL sample are given in Table 1. It is seenthat the conduction band offset does not exceed 20 meV.For the thermalized electrons at room temperature theinfluence of the resulting periodical potential should benegligible. Besides, as a result of the conduction-band lineup, the 4-nm barriers for the electrons in the SL aretransparent. Thus the changes of electron mobility andspin relaxation rate should be small compared to pureGaAs.It is seen from Fig. 1 and the data of Table 1 that thestrain of the AlxInyGa1-x-yAs layers produces barriers forboth heavy and light holes, the barrier for the light holesbeing 75 meV higher, which leads to additional hole-miniband splitting favorable for the electron opticalorientation.Table 1. The positions (in eV) of the band edges at roomtemperature in an Al0.20In0.18Ga0.62As/GaAs SL sample.aEvh2 Evl2 Ec1 Ec2 Eg1 Eg2-0.056 b -0.138 1.423 1.445 1.422 1.501a 1- GaAs, 2- AlxInyGa1-x-y As layer.b Zero energy is at valence-band edge of the GaAs layer.The choice of the layer thickness is dictated by the needto split the hole minibands. The splitting grows whenbarriers are broad enough and wells are narrow and deep.Still the thickness of the strained AlxInyGa1-x-yAs layershould be less than the critical thickness hc(y). The overallcritical thickness for the superlattice with alternatinglayers of equal thickness can be estimated as Hc=hc(y/2).The thickness of a single AlxInyGa1-x-yAs layer was takento be 4 nm (to be less than the calculated hc(y)), and inaccordance  with  X- ray data  the chosen thickness of theSL sample (0.136 m m ) exceeded  much less than in thecase of a cathode structure with one strained GaAs layer.A model-dependent calculation of the hole minibandenergies yields absolute values of Ehh1 13=  meV andElh1 54=  meV. For a small conduction-band offset theelectronic band can be taken as the average of theconduction-band energies of the contacting layers. Thusthe band gap, Eg , can be estimated as:HC1989Proceedings of the 1999 Particle Accelerator Conference, New York, 19993–998466A5Fig. 2. Electron spin polarization and quantum yield as a function of excitation energy at roomtemperature for the Al0.20In0.18Ga0.62As/GaAs SL sample (SPTU data). The band gap energy Eg and thelight hole excitation energy Elh are indicated by arrows.E E E E Eg g hh c c= + + -( )1 1 2 1 2 ,which is in line with experiment.3  EXPERIMENTAL RESULTSThe Mott analysers both at St. Petersburg TechnicalUniversity (SPTU) and at the Stanford LinearAccelerator Center (SLAC) were used to measure thespin polarization, P, of photoelectrons.In Fig. 2 the polarized emission and quantum yielddata measured at SPTU are shown as a function of theoptical excitation energy. The maximum polarizationobtained was 86% and the corresponding quantum yieldwas 7.5 · 10-3%. The yield for the excitation energy atthe polarization maximum is sensitive to the activationprocedure and the vacuum in the setup. At SLAC,P=82.7% with Y=9.4 · 10-2% was obtained for a crystalobtained from the same wafer. For another sample (notshown), in which the time lapse between the two setsof measurements was very short, the polarizationresults were found to be similar despite the somewhatdifferent techniques of sample preparation, activation,and vacuum. Due to better vacuum conditions in theSLAC apparatus, the values of Y  measured there wereregularly higher than those measured at SPTU.The maximum current density that can be extractedfrom the SL samples similar to the one described abovehave yet to be determined. Initial measurements atSLAC gave anomalously low values, perhaps becausethe surfaces were being heat-cleaned at 600 °C exactlyas for the SLC strained-layer cathodes. Other SLstructures–all of which were produced in Japan–thathave been studied for charge limit have shown normalemission after removal of the As cap at low (~400 °C)temperature [5,6].There is now strong evidence that the SCL can beovercome by using extremely high dopant density( ‡ 4 · 1019 cm-3) in at least the surface layer [6]. Thisgreatly reduces the barrier to holes tunneling to thesurface, which are responsible for neutralizing thesurface charge. It may also be necessary to increase thenegative affinity at the surface by using a large bandgap and also possibly a large offset of the conductionminiband. All of these parameters are easily controlledfor an MBE-grown SL structure with compensatedconduction band as described here.4  CONCLUSIONSElectron spin polarization as high as 86% has beenobtained at room temperature from strained AlxInyGa1-x-yAs/GaAs superlattice with minimal conduction bandoffset. Modulation doping of the SL provides highpolarization and high quantum yield. The position ofthe polarization maximum can be easily tuned to anexcitation wavelength by choice of the SLcomposition. Further improvement of the emitterparameters can be expected with additional optimizationof the SL structure parameters.5  REFERENCES[1] R.A. Alley et al., Nucl. Instrum. and Meth. A 365 (1995) 1, andreferences therein.[2] A.V. Subashiev et al., Phys. Low-Dim. Struct. 1/2 (1999) 1, alsoissued as preprint SLAC-PUB-8035 (1998).[3] T. Nakanishi et al., Proc. of Low Energy Polarized ElectronWorkshop, St. Petersburg (1998) 118; also Polarized GasTargets and Polarized Beams, AIP Conf. Proc. 421 (1998) 300.[4] A.V. Subashiev et al., Proc. of Low Energy Polarized ElectronWorkshop, St. Petersburg (1998) 55, also issued as preprintSLAC-PUB-7995 (1998).[5] Y. Kurihara et al., Jpn. J. Appl. Phys. 34 (1995) 355.[6] K. Togawa et al., Nucl. Instrum. and Meth. A 414 (1998) 431.1990Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

Superlattice photocathodes for accelerator based polarized electron source applications

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