Defect states and impurity states in amorphous As2Te3

Journal of Non-Crystalline Solids 35 & 36 (1980) 889-894 © North-Holland Publishing Company

DEFECT

STATES AND IMPURITY STATES IN AMORPHOUS As2Te 3

J. J. Hauser and R. S. Hutton Bell Laboratories Murray Hill, New Jersey 07974 U.S.A.

Amorphous films of (As~Te~)1_ Y M where M = Mn, Cu and Ge were deposited at 77K." H6p~i~g tonductivity is observed in all the as-deposited films. Similarly to undoped As2Te3, the localized paramagnetic defect states are also established in the as-deposited samples by an ESR signal with a 16G linewidth at g = 2. After annealing the defect states disappear, but in the case of Mn the hopping conductivity persists as a result of the impurity states revealed by wide ESR lines at g = 2 and g = 4.3. INTRODUCTION Variable-range hopping conductivity associated with localized gap states has been frequently reported in amorphous tetrahedrally coordinated semiconductors. These localized gap states have been related to singly charged defects (Do) such as dangling bonds by electron-spin-resonance (ESR) experlments [1,2] and by susceptibility measurements [3,4] which revealed the existence of a Curie paramagnetism at low temperatures. However, as stated by Street and Mott [5], the situation in pure amorphous chalcogenide glasses was different: Usually there was no ESR signal [6] or Curie paramagnetism at low temperatures [7], and no variable range hopping. On the other hand metastable states revealed by an ESR signal have been induced in chalcogenide glasses optically [8,9] and by electron bombardment [I0]. Furthermore, metastable states leading to both variable range hopping and an ESR signal were observed in As~Te. films sputtered at 77K [11-13]. These metastable states were llnked to the excess dlsorder caused by the low temperature deposition since they disappear upon annealing. Mort and Street [14] suggested two different explanations for the effect. First, that owing to the disorder, the energy gained by the relaxation of the center may in some cases be small enough to give a positive U so that some centers are singly occupied. Second, following Kastner et al. [15] the metastable states might be close pairs of doubly charged defects (D + or D-). The density of close pairs is expected to be higher in a sample deposited at low temperature and the broadening of such a level could give states at E F which would show exp T-I/4 conductivity and an ESR signal. A large randomness may also exist as a result of the nature of the alloy without cold deposition and may result in an ESR signal as reported in Ge-S glasses [16]. From another point of view one may also introduce states in the gap by doping the chalcogenide glasses with such impurities as Cu and Mn. The large increase in conductivity in Te-As-Ge-Si glasses caused by Mn [17-19] was attributed to the presence of Mn 2+ ions which was deduced from ESR experiments. This effect was explained by Mott [20]

889

890

J.J. Hauser, R.S. Hutton / Defect States and Impurity States

by suggesting that elements such as Cu and Mn can be fourfold coordinated and negatively charged and the compensating positive charge is provided by D + centers. Since the law of mass action ensures that the concentration of D- centers is small, the Fermi energy is no longer pinned and the activation energy for conduction decreases. Furthermore, Nott and Street [14] expected Ge to be fourfold coordinated and therefore neutral and should have no effect on the relative concentrations of D + and D-. EXPERIMENTAL RBSULTS AND DISCUSSION The electrical properties of a-As2Te 3 films doped with Mn and Cu are summarized in Table I: The first 2 6olumns give the values of the exponent and the prefactor for the as-deposited film Table I. SAMPLE

Properties T O (K)

of (As2Te3) l_ x M x films PRT(gcm)

1.1×104

2.5×i09a 2.1x109a

0 05 Mn

8.2xi08

0 i0 Mn

3.5xi08

Po[~Cm) -24 1 ixl0 -15 6 7x10 2 3x10 -I0

0 I0 Mn

5x108

8 5xlO -12

1.2x104

0 i0 Mn

4.8xi08

-12

0 005 Mn

5x109

(M=Mn, Cu) AE(eV)

or To(K )

1.6x104

0.40

8.4xi03

0.25

l.lxlO 4

2xlO 9a

005 Cu

5×109

i 3x10 - 2 4

1.8xlO 4

0.40

05 Cu

5×109

8 5xlO

-25

i0 Cu

3x109

4 2x10

-22

8.7x103 3 3xlO

0.36

15 Cu

4x109

4x10 -24

4xlO 3

0.36

-

values of T

3 9x10

0.43

o

while the last column pertains to films annealed at 300K. Variable range hopping is observed in all as-deposited films since the low temperature resistivity can be fitted by the relation: P = Po exp(To/T) i/4

(i)

where T o = 16~3/k N(EF) where ~ -i is • the width of the localized state functions and N(EF) is the density of localized states. It is clear from Table I that Mn increases N(EF) (T o decreases) while Cu has essentially no effect (T O remains fixed at the value observed in a-As2Te 3 [12]. After annealing the activation energy (AE) decreases with increasing Mn and Cu content, but in the case of Mn, variable range hopping persists for the high concentrations (x = 0.i). The fact that AE is unaffected by 0.005 Mn while the same amount yielded an appreciable decrease in Te-As-Ge-Si glasses [19] is consistent with Mott's suggestion [20] that the absence of D- states will be less pronounced in sputtered films. These electrical properties are in good agreement with the ESR results. The ESR spectrum shown in Fig. 1 obtained on the as-deposited sample (16G wide line at g = 2) is identical to that observed on undoped as-deposited As2Te ~ [12] but the total density of spins is 8x1018 cm -3 as compared to 2×~018 cm -3 in pure As2Te 3. Consequently, this resonance line corresponds again to defects states which are present in greater number because of the greater disorder caused by Mn. While after annealing, the g = 2 line disappeared completely for pure As2Te 3 [12], here one is left with

J=Jo Hauser, RoS. Hutton / Defect States and Impurity States

2300

2550 I

2800 [

3050 I

3300 I A

J 3200

I 3250

3550 I

3800 I

4050 I

4300

.~-As

DEPOSITED

IV

LANNEALED AT293K

I

,J I I 3300 3350 MAGNETIC FIELD (GAUSS)

891

I 3400

Figure 1 ESR spectra obtained at 9.26 GHz and /0K on a-(As2Te.)0 ~ Mn 0 .. The vertical scale is -8 times ~arger for the annealed sample. The lower abscissa scale refers to the asdeposited, the upper to the annealed. two resonance lines which correspond to impurity states: a 600G wide line with hyperfine interaction at g = 2 and a 320G wide line at g = 4.3. In analogy with similar lines reported for Mn in Te-As-Ge-Si glasses [17-19] one can conclude that the g = 2 line corresponds to Mn-Te clusters, while the g = 4,3 line corresponds to Mn atoms in the lattice. The lack of hyperfine structure in the g = 4.3 line as compared to that seen in the Te-As-Ge-Si glasses could be due to the much larger Mn concentration used here. Indeed, as shown in Fig. 2 for a lower Mn concentration, the g = 4.3 line is quite similar to that reported for the glasses. The narrow line shown in Fig. 2 at g = 2 corresponds to a few remnant defect states after annealing (7x1015 cm-3), The impurity states corresponding to the two broad lines at g = 2 and g = 4.3 explain why variable range hopping does not disappear in annealed (As2Te3) O 9Mn0 1 (Table I). The ESR spectrum obtained on as-deposited (As Te)0 8;Cu0 15 (Fig. 3) is again similar to that of undoped As2Te 3 except for'a slightly higher density of spins (4x1018 cm-3) which may result just as with Mn from the greater disorder caused by Cu atoms. However, as shown in Fig. 3 annealing does not lead to defect states. This is consistent with the fact that variable range hopping disappears upon annealing (Table I) and that the activation energy decreases only slightly. The different behaviours of Cu and Mn were not predicted in Mott's model [20] and are not understood presently. On the other hand, alloying with Ge is in complete agreement with theoretical predictions [14]. The ESR spectra for as-deposited and annealed As2Te3Ge films are identical to those shown in Fig. 3 for the case of Cu. The fact that one expects Ge to be fourfold coordinated and therefore neutral [14] is consistent with the fact that the total density of spins in as-deposited As2Te3Ge is 9×1017 cm -3 and with the absence of defect states in the ESR spectrum of the annealed sample. Furthermore, as shown in Table If, T and AE remain essentially con. O stant as the Ge content is xncreased to 0.7. The somewhat lower

892

JoJo Hauser, R.S. Hutton / Defect States and Impurity States

c >

, g =43

I

g =2

c3

I

,

I

k

1

,

900

I

,

[

,

I

,

I

,

1

,

I

1700 2500 3300 MAGNETIC FIELD(GAUSS)

,

I

,

4100

Figure 2 ESR spectrum obtained at 9.26 GHz and 5K on annealed a-(As2Te3) 0.995 Mn0.005

~

E ~ , ~ . j ~

<{ u.

3205

I

I

3255

AS DEPOSITED /

]

/

I

ANNEALED AT 293K

[

I

3305 5355 MAGNETIC FIELD (GAUSS)

I

5405

Figure 3 ESR Spectra obtained at 9.26 GHz and 5K on a-(As2Te3 )0.85 Cu0 15- The vertical scale is 32 times larger "£6r the annealed sample

JoJ.

893

Hauser, R.S. Hutton / Defect States and Impurity States

density of spins of As2Te3Ge as compared to As2Te 3 and the fact that AE is maximum at the-As2Te3Ge are consistent-with the fact that Ge, as a result of its fourfold-coordination, improves the glass properties of As2Te 3. Table II.

Properties of (As2Te3)l_ x Ge x Films

As2Te 3 No. 7

To(K ) 4.1×109

0o(~Cm) 1.6×10 -22

0RT(~cm) 4.1×104

AE(eV) or To(K ) 0.40

(As2Te3)0.9Ge0. 1

7.9×109

2.2×10 -28

2.3×104

0.41

(As2Te3)O.83Ge0.17

4.4x109

4x10 -23

5x104

0.40

(As2Te3)0.7Ge0. 3

4.4xi09

3.4xi0 -23

7.5x104

0.43

2.2xi05

0.43

SAMPLE

(As2Te3Ge No. 2 5.1x109

2x10 -24

l.lxl05

0.45

(As2Te3)0.3Ge0. 7 No.l 4.3x109

1.2×10 -22

4.8xi05

0.40

(As2Te3)0.3Ge0. 7 N~.2 4.3×109

l.lxl0 -22

6.1x105

0.42

(As2Te3)0.1Ge0. 9 No.l 2.7xi09

2×10 -22

3.5xi04

5.8x109a

As2Te3Ge No. 3

a Value of T

o

ACKNOWLEDGMENTS We would like to thank S. Greenberg Kosinski and V. G. Lambrecht, Jr. for their technical assistance.

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J.J. Hauser, R.S. Hutton / Defect States and Impurity States

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