A. F. Ioffe Physico-Technical Institute, Academy of Sciences of the USSR, Leningrad (b). Change of Electrical Properties in Electron Irradiated CdGeAsz Crystals.
V. X. R R u U x Y i et al.: Electrical Properties of Irradiated CdGeAs,
761
phys. stat. sol. (a) 49, 761 (1978) Subject classification: 11 and 14.3; 13.4; 22.8
Siberiwn V . D.Kuznetzov Physico-Technical Institute, ?'Omsk State University (a) and A . F. Ioffe Physico-Technical Institute, Academy of Sciences of the U S S R , Leningrad ( b )
Change of Electrical Properties in Electron Irradiated CdGeAsz Crystals BY
V. pr'. BRUDNYI (a), 31. A . K R ~ V O(a), V A. 1. Y o ~ a ~ o \( 7a ) , 1. K. POLUSHINA (b), V. I). PKOCHUKHAN (b), and Yu. V. R U D(b) The effect of 2 MeV electron bombardment a t 300 I< on the eiectricai (X,0)properties of 11and p-CdGoAs, are studied. Specimens of n- and p-CdGeAs, are closely-compensated after sufficiently heavy electron bombardment, and have n-type conductivity with a resulting free electron density = 3 x 1017 a t 300 K. lsochronal annealing experiments indicate two regions of restoring of R and u, a t about 300 t o 450 K acceptor defect and at about 470 to 600 K donor defect annealing.
Mccnenosatro BJIMFIHBe 3JIeKTPOHIlOrO o6nywHaH 2 MeV, 300 K Ha 3JIeKTpH9eCIine CBOfiCTBa (R,U) KpBCTaJlJlOB n- M p-CdGeAs,. OijHapyrneao, 9 T O He3aBMCBMO OT TbiIIa lIcXOnIIOfi npOBOAMMOcTB CMJIbHOO6Jly9eHHbIe 06pa3IJbI CdGeAs? I.iMeK)T n-THII npOB0ABMOCTB c n NN 3 x 1017 ~ r n n - ~p 300 ~ K 11 c ~ e n e ~KoMneHcaumi ~,~) 6naasoir K 100%. HaiineHbI m e cTanm ~ a o x p o w ~ ooTmnra ro npn TeMnepaTypax: 300 JO 450 K - o T m m aKqeIITOpHbiX ile@eKTOB II 470 R O 600 K - O T H m ne$eKTOB ZOHOPHOfi I7pMpOzLI.
1. Introductiori The structural defects in II-IV-Vz ternary semiconductors have attracted iiiuch attention because the properties of the specimens and fabricated devices are closely linked to the presence of structural defect's in these niat'erials. By heating in the various components substantial changes occur in the materials, indicating t'hat intrinsic lattice defects, i.e. vacancies (Y),interstitials atonis (IA), and antistructural defect's (ASD) are easily fornied and play a n important role in deterliking their properties. For example, for CdGeAs, a high presslire of elements (Cd, As) over the melt can lead to niodifications of the chemical composition, formation of intrinsic lattice defects in the Cd or As sublattices. Moreover, the ratios of the covalent radii for Cd (1.56 A), Qe (1.22 A) and As (1.17 A) are favourable for the'forniation of ASD such as [Geed] or [GeaJ [2]. So the n-type conductivity of CdGeAs, crystals is connected with the presence of [GecdJ defects or wjt.h vacancies in the As sublattice 11, 21. It is supposed froni the effect'ive charge and valence of ions in CdGeAs, that' [\r-4s], [Geed] should give donors and [V,,], [ V C ~ ]and , [Geas] acceptor levels in the energy gap. Some results of high-temperature annealing experiments are in good accordance with this assuiiiption [3]. Radiation defects (RD) in t'ernary seiiiiconductors have a special int'erest in two aspects: first', 1LD are produced in a cold lattice and highly mobile defect's, noriiially not, ret,ained after high-temperature experiments, can be frozen in for study and second, the radiation wit'h high-energy particles has technological importance for producing devices. I n t'he last few years some results are published about' lattice struct.ura1 defects which are formed in ZnSnAs, [4] and CdSnAs, [5] upon electron irradiation. Here we discuss the experimental results of the electrical properties of 2 MeV elect'ron-irradiated n- and p-CdGeAs,.
762
. ' 1 N. BRUIJNYI et al.
2. Experimental Procedure Single crystals of n- and p-CdCeAs, are grown from the stoichonietric melt by t h e Bridgman method. All crystals are specially iindoped except the specimen S 5 , which is produced by high-temperature diffusion of Cu in the n-CdGeAs, crystal. The electron irradiation ( 2 MeV) is performed with the \-an de Graaff accelerator at about 300 K. Pre- and postirradiation iiieasurenients include the electrical conductivity a and the Hall coefficient R. After irradiation the recovery of these parameters is monitored during 10 niin isochronal annealing up to 700 K. The electrical properties ( K , a) of the specimens before and after electron irradiation are shown in Table 1. Table 1 Electrical properties (R, a) of n- and p-CdGeAs, crystals before and after electron irradiation ( T = 300 K)
N
1 2 3 4 5
Ra
Go
(em3A-l s-l )
(!2l
26 (n) 1.41 (n) 196 (P) 5.24 (p) 1.7 (P)
1.05 x 10' 5 x lo2 0.29 9.68 33
cm-l)
qs
G@
(electrons/cm2)(cm3A-' s-1)
(W1 em-l)
2 x 1.1 x 2 X 1.2 x 2 x
lo1*
39.7 19.7 (n) 4.3 x 102 1.64 (n) 65 (n) 13 1.43 8.5 (p) 1.45 x lo4 (n) 7 x
l0ls 1018 10'8 10l8
3. Experimental Results The effect of 2 MeV electron irradiation on the electrical properties of n- and pCdGeAs, crystals is shown in Fig. 1. For an initially n-type CdGeAs, saniple ( X l ) R and 0 change negligibly even after electron irradiation up t o doses of @ = 2 x l0ls cm-2. For the p-type sample (N3) the carrier density and conductivity decrease with electron fluence and at about @ x ( 2 t o 4) x lo1' cni-2 p-n conductivity conversion occurs. As it is shown in Fig. 1 cmln(exp)= 1.2 x 10-1 Q-1 c n - l a t 300 K for @ = ( 3 t o 4) x lo1' just as the calculated value omin(calc)= 2epP (bK x 1.8 x !kcni-I l for ,up= lo2cni2 Y-l s-l,
j
, , , , , , , , , ,
-06
'
100 ''
zoo
' " '
300
100
500
L Fig. 2. Isochronal (10 min) annealing of lorn electron irradiated CdGeAs, samples. (2) nCdGeAs, (N2), (1)p-CdGeAs, (K4), T = 300 K Fig. 1. R (circles) and u (squares) versus electron fluence (2 MeV, 300 K ) for CdGeAs, samples N1 ( 0 , m), 3 3 (0, G ) , T = 300 K
Change of Electrical Properties in E
Fig. 3. Isochronal annealing R ( 0 ) and a (A) for high electron irradiated n-CdGeAs, (NI) sample, T = 300 K
I1 I
100
,
,
,
,
f
, , ( , I
200
~
,
/
,
,
300
LOG
,
1
6 i%)
----+
Fig. 4. Isochronal annealing R (0) and u (.)-of high electron irradiated initial p-CdGeAs, (N3) which converted t o n-type conductivity after irradiation, T = 300 K
V. X. BRUUNYI e t al.
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Then a t temperatures higher than 420 K the donor defects are annealed and the n-p conversion occurs up to 700 K.
4. Discussion and Conclusion Eroni the change of the electrical properties of CdGeAs, crystals a t electron irradiation and subsequent isochronal annealing the model of the defect levels illustrated in Fig. 5 can be proposed. For this model where the exact position of defect levels is unknown, we assume that the donor levels N , are near E , and most of them are ionized ( N , ) ;= N ; ) a t 300 R for the carrier densities involved in this experiment ( n = 1017 cnl-3). lye also assume that there are acceptor levels (Ar,) in the forbidden gap. On the base of this model changes of the the electrical properties of CdGeAs, during irradiation N,) can be understood. If the irradiation produces more donors than acceptors, ( N , the hole concentration decreases in p-CdGeAs, and the electron concentration increases in n-CdQeAs, with fluence, so that the Ferrni level moves up t o N,. For high fluences when the Ferrrii level is close to N,, some of them are neutral and the quantity of N , increases more rapidly than N: and the saturation range on the A!(@) curve appears (Fig. I). Then the concentration of N , and N , in electron irradiated CdQeAs, can be estimated. For this model we can write for high irradiated sample (Fig. 4) that (no)llm= (3; - N , - Np),where N , is the initial hole concentration in the sample. A s it is shown by the annealing experiments, N , are annealed up t o 420 K, therefore we can write n(7', x 420 K ) = ( N & - N P ) = 7 x 1017 (Fig. 4) and N i = N , = z ci x 1017 This corresponds to the additional donor and acceptor rates a t about 0.35 c1ii-l and 0.3 cm-l, respectively. The analogous computation for n-CdGeAs, (Fig. 3 ) results in dN,ld@ = 0.5 cn1r3 and dN,ld@ = 0.45 cn1-l. Thus, the value of additional donor ( z0.4 to 0.5 cm-I) and acceptor (0.3 to 0.45 cm-l) rates are typical for simple defect formation in semiconductors a t 2 MeV electron irradiation. The experimental value of rate of hole removal (or electron additional rate) is equal to ( N D- 3,) z 0.1 cm-l and depends on the depth of the Fernii level in the initial samples. I n CdGeAs, as in the earlier investigated ternary arsenides ZnSnAs, [4] and CdSnAs, [5] the electrical properties change readily a t electron irradiation as their binary analogues InAs and GaAs. CdGeAs, crystal upon electron irradiation acquire electrical properties intermediate between n-type (noli, = 2 x 10l8~ 1 1 1 for ~ ~ )InAs [GI and i-type conductivity for GaAs 171. For CdGeAs,, as it was earlier supposed for ZnSnAs, [4] and CdSnAs, [5], it can be assumed that the n-type conductivity of high electron irradiated samples is connected with vacancies in the As-sublattice [V,,], while the acceptor levels are connected with vacancies of Cd- arid Ge-sublattices [V,,], [V,,]. It was supposed a s earlier [4, 51 that the I A are mobile a t = 300 K.
>
Np
Fig. 5. The radiation induced defect level model for electron irradiated p-CdGeAs,. a) After high electron fluence, b) after partial post-irradiated annealing up to = 450 K. A'= and N A are radiation induced donor and acceptor densities, X, is the intial acceptor density
Change of Electrical Properties in Electron Irradiated CdGe&
765
Therefore, donor and acceptor levels are introduced in n- and p-type conductivity CdGeAs, crystals upon electron irradiation a t about 300 K and the donor defects have a higher temperature stability than the acceptor defects.
References [I] A. S. BORSREVSKII e t al.. Semiconducting AIIBIVCT Compounds, Ed. N. A. GORYUNOVA and Yu. A. VALOV,Soviet Radio, AIoscow 1974. [2] V. D. PROCHUKRAN, Materials of VI. Winter School on Phys. of Semieond., A. F. Ioffe Physico-Technical Institute, Academy of Sciences of the USSR, Leningrad 1974 (p. 280). [3] CH. DAVLETIIIURATOV, Yu. V. RUD, A. ALLANAZAROV, M. SERGINOV, and E. 0. O s ~ ~ h - o v , Izv. $cad. Xauk Turlrm.SSR, Ser. fiz.-tekh., khimichesk., and geolog. Nauk 6, 18 (1975). [4] V. S. BRUDNYI,D. L. BUDNITSKII, &I.A. KRIVOV,and V. G. MELEV,phys. stat. sol. (a) 35, 425 (1976). [ti] V. S. BRUDNYI,0. V. VOEVODINA, and 31. A. KRIVOV, Fiz. Tekh. Poluprov. 10, 1311 (1976). [6] B. I. BOLTAKS and E. P. SAITIN, Radiation Physics of Non-Metallic Crystals, Nauka,Minsk 1970 (p. 116). [7] V. X. BRUDXYI,E. P u . BRAILOVSKII, RI. A. KRIVOV,and V. P. REDKO,Izv. vuzov (USSR), Ser. fiz. 10, 118 (1974). (Received April 10,1978)
