Short Notes K189

phys. stat. sol. (a) - 73, K189 (1982)

Subject classification: 16; 13.4; 22.2.1

Laboratorio de Ffsica del Estado Solido, Facultad de Ciencias Valladolid l a ) and Centred’ Etudes d’ Electronique d e s Solides, U. S. T. L. ,Montpellier (b)

On the Photoconductivity Response of GaAs

Doped with Chromium and Oxygen

1

BY J. JIMENEZ (a), M.A. GONZALEZ (a), L .F . SANZ (a), and J. BONNAFE (b)

The typical spectral response of photoconductivity for an insulating o r semi- insulating material containing impurities, shows several low energy thresholds

and maxima, which provide an indication of the location of imperfection levels in the forbidden band-gap of the material. In this short paper we report the ex-

trinsic photoconductivity spectra of semi-insulating GaAs, doped with chromium

and oxygen, in the photon energy range between 0 .7 and 1.5 eV. The photocon- ductivity of GaAs has been intensively investigated /1 to 31. There were, how-

ever, some misinterpretations in the results reported, because the thermal and

optical exposure history of the sample had not been taken into consideration

when measuring photoconductivity responses. The usual techniques for photoconductivity measurements can be character-

ized by comparing the excitation time constants and those of the solids. These techniques are: steady -state continuum illumination and chopped light illumina-

tion. In the former method, photoconductivity is measured when the steady-state

has been reached and equilibrium exists between trapping and photoionisation

processes in impurity levels, under incident monochromatic photon flux. In the

latter method, using the conventional lock-in techniques with chopped light illu- mination, only the fast part of the photocurrent response is measured, neglecting

the characteristically long time decay response. The photoconductivity spectrum is therefore very sensitive to both the frequency of chopping and the hvsweeping rate. In both cases the resolution and shape of the spectra can be modified by

adjusting the photon flux intensity and the t ime constants used in the measure-

ments. It is obvious that no reliable information about the properties of the level impurities can be obtained from photoconductivity in materials with very

long time decay constants. This is the case for the semi-insulating G a b , which

1 ) Valladolid, Spain.

M 90 physica status solidi (a) 73

Fig. 1. Extrinsic photoconductivity spectra at several temperatures for two different GaAs:(Cr, 0) samples; sample 1: A 86 K, V 115 K, o 170 K; sample 2:. 89 K

has very long time constants under extrinsic photon excitation (photomemory effect).

In this note we intend to show that a ~ ( ~ v ) - - - . valuable information on this compound can

be obtained by means of a procedure of elimination of its optical memory from thermal erasing. The steady-state photoconductivity for each frequency can then be measured in the absence of the background current due to the previous wavelength optical excitation.

The measurement procedure can be described a s follows: i ) The sample is heated to room temperature in order to erase its photo-

memory. ii) The sample is cooled in the dark down to liquid nitrogen temperature

and then is excited with monochromatic light.

iii) When steady-state is reached, the sample is heated at a slow heating rate, and its photoconductivity is recorded a s a function of the temperature.

iv) The cycle is repeated for another monochromatic light excitation. Obtaining a full photoconductivity spectrum is a tedious procedure. In order

to reduce the time required to measure it we have used a small cryostat, with a very small thermal inertia, so that thermal cycles can be performed in a few minutes only.

Excitation was carried out with light from a halogen lamp through a Bausch-Lomb grating monochromator. For each wavelength a fixed incident photon f lux of 1 013 cm indium soldered. The sample current was measured with a Keithley 610C electrometer. Polarization was carried out in the linear zone of the I-U characteristics (* 15 V cm ). The samples employed were semi-insulating

3 GaAs wafers ( 6 ~ 2 ~ 0 . 2 mm ) doped with chromium (2x1 0-4 weight proportion of Cr) and oxygen (some G?o3 was added to the bulk).

temperature side) and 0.35 eV (high temperature side). In all cases studied

-2s-1 was used. Electrical contacts were made with

-1

The dark current has two slopes with activation energies of 0.22 eV (low

Short Notes a91

Fig. 2. Thermally stimulated current (TSC) spectrum after 15 min excitation with 1.17 eV monochromatic light at liquid nitrogen temperature

the photoconductivity was significantly greater than JU” the dark current.

The extrinsic photoconductivity spectra of GaAs 100 150 200 250

T(K)- for several temperatures in the range between 80 and 300 K a r e shown in Fig. 1. The results have been obtained for the spectral range 0.7 to 1 . 5 eV by the procedure described above. We can observe in the photoconductivity spectra a sharp peak at 0.87 eV, width 30 meV at half-height, which does not broaden with increasing temperature. The existence of this

peak is related to the Cr2+ (3d configuration) charge state of the chromium impurity which is the dominant one in n-type GaAs:Cr samples /4/. The peak

5 2+ corresponds toanelectronic transition from the T2 level of C r duction band. The sharpness of this peak indicates that the stable charge state of chromium in this sample is Cr2+. In other experiments carried out in GaAs:Cr samples, highly oxygen doped, this peak is either absent o r very weak. For excitation energies higher than 0.88 eV the photoconductivity de-

creases rapidly; due to the fact that, in our measurement procedure, the initial charge state is restored by heating before photoconductivity recording, the decrease can only be explained by a strong diminution of the optical cross section in the T2 level.

Some controversy exists at the present time about the nature of the EL2 t r ap and how it relates to the oxygen impurity in GaAs. In the spectra obtained by us, one can observe a low energy threshold at about 0.75 eV, which is related to an electronic transition from a doner state, presumably the EL2

trap, to the

with previously reported results and it suggests that this level could be the

major source of steady-state electrons. This optical transition is thermally stimulated with activation energy of 0.024 eV, in the low temperature range (80 to 120 K). This energy is not related to a shallow t rap in this temperature range, but rather to a shift in the threshold energy of the optical cross section. Effectively the low temperatures slope of the thermally stimulated current

4

to the con-

5

minimum of the conduction band / 5 / . This observation agrees

Kl92

(TSC) spectra is la rger than the value of 0.024 eV obtained in photoconductivity

for temperatures higher than 80 K (Fig. 2).

physica status solidi (a) 73

Some other electronic transitions can b e observed, ranging from the EL2 t rap to the other sub-band minima of the conduction band. Photoconductivity maxima a r e obtained for 1 .03 and 1.18 eV, corresponding, respectively, to the

electronic transition from the EL2 t r a p to the L and X minima of the conduction band /6/. Photoconductivity associated t o the EL2 +X band transition is thermal-

l y activated with an energy of 0.024 eV, the same that was obtained for the EL2-Wband transition. F o r energies above 1.18 eV the thermal activation

energy was also 0.024 eV. This is in agreement with the fact that in this energy

range the photoconductivity must be related to the electronic transition from the

EL2 t r a p level to high energy s ta tes of the X conduction band. This is not so for the 1.03 eV maximum due to the mixed nature of the photoconductivity in this ex-

citation energy interval. Hole transition at this energy is responsible of the photoconductivity maximum at around 1 eV in p-type samples /7/.

In conclusion, the results obtained in this note show that reliable photocon- ductivity spectra can be obtained when the photomemory is neglected; this can

be achieved by thermally erasing the t race of previous excitations and then measuring the photoconductivity under steady-state conditions.

References

/1/ A.L. LIN and R.H. BUBE, J. appl. Phys. -7 47 1859 (1976). /2/ J .C. MANlFACIER, J. BONNAFE, J. JIMENEZ, M. DE MURCIA, and

J. P. FILLARD, Second Lund Conference, Ste. Maxime 1979. /3/ T. OKUMURA, Y. ITOH, and T. IKOMA, J. electron. Mater. g, 865

(1 9 79). /4/ D.C. LOOK, Solid State Commun. - 24, 825 (1977). /5/ E.H. TYLER, M. JAROS, and C.M. PENCHINA, Appl. Phys. Let ters - 31,

208 (1977). /6/ A. CHANTRE, G. VINCENT, and D. BOIS, Phys. Rev. B - 23, 5335 (1981). /7/ W. PLEISIEWICZ, J. Phys. Chem. Solids -, 38 1079 (1977).

(Received June 28, 1982)