ELECTRICAL AND OPTICAL PROPERTIES OF UHV SUBLIMED a-As

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ELECTRICAL AND OPTICAL PROPERTIES OF UHV SUBLIMED a-As

R. Phillips, A. Mackintosh, A. Yoffe

To cite this version:

R. Phillips, A. Mackintosh, A. Yoffe. ELECTRICAL AND OPTICAL PROPERTIES OF UHV SUBLIMED a-As. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-869-C4-872.

�10.1051/jphyscol:19814190�. �jpa-00220817�

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JOURNAL DE PHYSIQUE

CoZZoque C4, suppZ6ment au nOIO, Tome 42, octobre 1981 page C4-869

ELECTRICAL AND OPTICAL PROPERTIES

O F

U H V SUBLIMED

a-As

R.T. Phillips, A.J. Mackintoshand A.D. Yoffe

Cavendish Laboratory, MadingZey Road, Cambridge, CB3 OHE, U.K.

Abstract.- Thin films of amorphous arsenic have been prepared by sublimation on to substrates held at 1.100 K in ultra high vacuum. Annealed films show an optical bandgap of %1.0 eV. Low temperature ion bombardment greatly en- hances the dc conductivity, which after bombardment follows the T-$ law.

Interpretation in terms of variable range hopping leads to a Fermi level density of states after bombardment of 1.5 x 1017 ev-1 ~ m - ~ , which falls to

2 1017 ev-1 cm-3 after annealing at 295 K. Comparison with similar experiments on amorphous r.f. sputtered phosphorus lends support to those models of defect states which suggest that charged defects are more stable in a-P than in a-As.

Introduction.- Ion bombardment at low temperatures is known to introduce majorstru- ctural rearran ement in already amorphous semiconductors, and has been shown to lead to enhanced T-r

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conduction in a-Si and a-Ge. Amorphous arsenic in bulk and thin film furys gives approximately activated conductivity near room temperature, but shows T - 4 conduction after low temperature preparation (I) or squeezing to about 35 kbar (2). Furthermore, free spins are created by low temperature light irradia- tion or bombardment with 2 MeV electrons (3). We reuort here on conductivity indwed in a-As by ion bombardment, which ~rovides some further evidence for the stability of defects in this material.

Film Preparation.- RF sputtered a-As has been investigated extensively (4), and some measurements have been reported on sublimed a-As films(5). For the present stu- dy, sublimation in ultra high vacuum on to single crystal sapphire substrates has been used to produce high quality films for optical and electrical measurements.

The bulk material for sublimation was obtained from MCP Electronics Ltd. and is prepared by chemical transport in hydrogen, some of which is undoubtedly incorporated into the bulk material (6). Quoted purity with respect to heavy elements is 99.9999%. Sublimation from the tungsten basket often occurs explosively and conse- quently the deposition rate is very high - typically >I00 81s. The pressure in the UHV system before deposition was 5 x 10-10 to 10-9 torr, the arsenic and basket were degassed before a run and pressure during sublimation was kept in the 10-9 torr range.

Film Properties.- Inmediately after deposition onto substrates at 100 K the a-As film was transparent in the visible region of the spectrum, while material deposited on the glass walls of the chamber first appeared yellow and quickly became opaque and black. In situ optical measurements have not been made, but visual observation suggests that the bandgap of the quench condensed arsenic must be over *2.5 eV. It is known that the major constituent of As vapour is the As4 molecule (7) and O'Reilly (8) has calculated energy levels of an isolated As/+ molecule and suggests that a molecular solid of As4 may well show a gap of this order if the solid properties strongly reflect those of the molecular species. Whether there is any difference between this form of arsenic and the yellow deposit is not clear, though some early experiments indicate that the yellow material may itself be a molecular As4 solid, possibly crystalline since it regrows from CS2 solution as rhombic dodeca- hedra (9).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814190

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C4-870 JOURNAL DE PHYSIQUE

Figure 1: resistivity of a-As: (1) as Figure 2: ~2 and h v G 2 - versus energy at deposited; annealed (2) 1 day, (3) 1 295K.

week, (4) 3 weeks.

The in situ measurement of the dc conductivity showed that the as-deposited material had a high resistance until well above the preparation temperature, and line 1 of Figure 1 shows the behaviour of p as the film warmed after formation.

Line 2 gives the resistivity after annealing the film overnight at room temperature.

The low temperature region is approximately linear versus T-4, and the resistivity in that region increases as the 295 K anneal time increases. Line 3 shows the behaviour after I week of annealing in situ, and immediately after transfer to a heavy ion accelerator. Further annealing continued the increase in resistivity in this region. No changes in resistivity were observed during transfer in air between the two systems. Resistivity was measured over a wider range of T in the accelerator, the temperature being monitored by a doubly calibrated Lake Shore Cryotronics DT500-KL Si diode. Two weeks further annealing, after transfer,led to the resistivity shown in line 4. Figure 2 shows results of a determination of the optical re- flectance and transmittance of a film after transfer. The dielectric function has been extracted from R and T numerically, and €2 and h v G a r e plotted against h v . At 295 K the hv6intercept is ~ 1 . 0 eV indicating an optical gap smaller by about 0.1 eV than that reported previously (4,5).

The films were further disordered by ion bombardment at 10 K with >1014 7 5 ~ s + , 4 0 ~ r + or 4 ~ e + at;100 keV energy. This resulted in greatly enhanced low temperature conductivity, the logarithm of which is linear with T-4 (Figure ( 3 ) ) . Saturation ion doses have always been used, and the only detectable difference between the effects of the different ions is that subsurface gas bubble formation occurs upon annealing inert gas ion bombarded films. The use of different ions per- mits a wide range of penetration depths to be achieved, and by this means it has been shown that even under conditions such that no ions penetrate to the electrodes beneath the film, the same form of conductivity is obtained. Figure 4 gives the high field variation of resistivity after annealing at 50 K. It seems reasonable to take the low field behaviour of Figures 3 and 4 to signal variable range hopping;

the curves in Figure 4 indicate the high field predictions of the hopping model of Apsley and Hughes (10) for a flat band of states with localisation length

102.

The localisation length is the only adjustable parameter for these curvessince 24a3/Flkrr is fixed by the slope of the low field run, and the preexponential is the

same for all fields. As may be seen from Figure 3 the slope of the resistivity

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0.21 0 30 0.36 04.2 1TIK)-z Figure 3: resistivity of a-As, (1) pre bonbardment (2) annealed at 30K, (3) 50K,

(4) 100 K, (5) 295K.

1 1 1 1 t 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 r l l ~ l l l l , l l l l l l l l , , l l l ' l ' l l l l ~ 0 -38 042 0.L6 1TIK)-F

Figure 4: electric field dependence or resistivity of bombarded a-As after annealing at 50K.

variation increases as the anneal stage temperature is increased (see table), cor- responding to a decrease in the Fermi level density of states. This one may expect, given the observed annealing of the

T-t

tail immediately after deposition: the states assaciated w i ~ h either low ~elnperature preparation or excess disorder induced by ion bombardment are unstable at higher temperatures. This contrasts with the behaviour of states introduced by application of high pressure, but agrees with ob- servations of films sputtered at 80 K.

Discussion.- The low substrate temperature used in these sublimation experiments seems to have resulted in the formation of yet another form of a-As, composed of ran- domly arranged As4 molecules. Rearrangement of this form into a defective continuous random network occurs as the film warms, as attested by visual observation of the film transparency. Some preliminary results of X-ray diffraction on what is presum- ably this type of material have already been reported and corroborate our conclusions

High field conductivity measurements enable the independent extraction of Ila and N(EF) from data for a single film. Given that only one parameter is adjusted (]/a) good fits may be obtained, but are not consistent for a given Ila as the field is increased. This is shown in Figure 4 by the curves indicating the theoretical results for localisation length 108. Similar problems have been encountered in fitting this theory to data for a-Si and a-Ge (12). The general trend in the value of I/a is towards larger values at higher anneal stages, and in N(EF) towards lower values as the anneal temperature is increased. In the following table the Apsley-Hughes model is used with ]/a assumed to have a constant value of 102, in order to demonstrate the behaviour of N(EF).

ANNEAL T(K) To (K) L ~ ~ I O ( P / ~ C ~ ) N ( E ~ ) (ev-l crne3)

30 1.8~10 8 -1 1.6 5x10'~

5 0 2 . 8 ~ 1 0 ~ -13.2 3 x 1 0 ~ ~

7 0 3.2x108 -13.4 3 x 1 0 ~ ~

100 3 . 5 ~ 1 0 ~ -12.6 2 x 1 0 ' ~

295 8 x108 - 1 1 1 x 1 0 ' ~

I/a =

lox

is assumed in this table. Inclusion of the increase in l/a would accen- tuate the decrease in N(EF).

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JOURNAL DE PHYSIQUE

The presence of a tail in the conductivity of the as-deposited and briefly annealed films strongly suggests some

T-i

contribution to the conductivity, which in the high T region becomes approximately activated.The slope of the Arrhenius plot changes continuously as one passes from the

T-t

regime, and reaches up to 0.6 eV at 295 K in the present data. Brief annealing at 295 K after bombardment leads to more prominent

T-A

conduction which can be eliminated by annealing at higher temperatures or by prolongued annealing at 295 K.

The introduction of defects giving

T-f

behaviour in a-As em~hasizes the similarity of this material with the amorphous Group IV elements. which also show enhanced

T-t

conduction after low T bombardment. It is interesting to note that this behaviour seems to contrast with conduction in bombarded a-Se prepared by evap- oration in ultra high vacuum. In this material no enhancement of conductivity has been observed for ion doses 51015 cm-2 (R.T. Phillips

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unpublished work). In rf sputtered a-P enhanced conductivity is observed after He' ion bombardment at low temperature. and preliminary work on this material suggests that saturation of damage occurs at somewhat higher ion doses than in the case of a-As. Thus the response of a-As and a-P in this respect is in good agreement with present models of defects in these materials. In a-As the energy balance between the charged and neutral defects is suggested to be very fine (41, and increased disorder is now shown to introduce defects which lead to variable range hopping conduction. As in a-Si and a-Ge these supernumerary defects are removed upon annealing. If neutral defect creation in a-P is indeed less marked than in a-As, this may indicate that neutral defects are less likely in this material.

Acknowledgements.

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This research has been supported by the Science Research Council.

RTP also thanks Clare College, Cambridge for the award of a grant.

References.

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35

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(8) O'Reilly, E.P.

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private conununication.

(9) Mellor, J.W. A Comprehensive Treatise on Inorganic and Theoretical Chemistry (Longmans, London 1929), Vol. IX, pp. 17-20 and references therein.

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(12)Apsley, N., Ph.D. Thesis, University of Cambridge, (1977).