Short Notes K207

phys. stat. sol. (a) E, K207 (1986) Subject classification: 72.40; S7.12

Departamento Flsica de la Materia Condensada, Facultad de Ciencias, Universidad de Valladolidl) (a) and Centre d’ Electronique de Montpellier, UniversitC des Sciences et Techniques du Languedocz) (b)

Photoconductivity Propert ies of Vanadium Doped Bulk GaAs

BY J. JIMkNEZ (a), J. BoNNAFk (b), M.A. GONZALEZ (b), P. HERNANDEZ (a), and J.C. MERINO (a)

In the last yea r s a great deal of attention has been paid to the doping of GaAs

with vanadium in order to obtain semi-insulating material , which is crucial

for integrated circuit technology. The doping with vanadium should improve

the thermal stability of the high resistivity GaAs relative to the chromium

doped material . This is due to the lower diffusivity of the vanadium impurity. It is assumed that vanadium atoms enter substitutionally on gallium sites. Several studies dealing with thg location of vanadium related levels in the band

gap of GaAs have been carried out in different laboratories, however, the role

played by these levels in the semi-insulating performances of GaAs has not yet

been well established. Our aim in this short note is to contribute to the study of

the influence of vanadium doping in the semi-insulating properties of GaAs.

Studies on the characterization of vanadium related levels in GaAs and

other III-V compounds have been performed by several authors, experimental data on optical absorption /1, 2/, photoluminescence /3/, EPR /4/, DLTS /5/, etc. of vanadium doped GaAs can be found in the l i terature of GaAs. In

this note we report experimental data concerning the photoconductivity and TSC (thermally stimulated currents) of bulk GaAs:V.

3 Measurements were performed on samples (5x2x0.5 mm ) cut from two

ingots manufactured by Wacker Chemitronic and RTC (Radio Technique

Compelec), respectively. These ingots were grown in quartz crucibles by the LEC (liquid encapsulated Czochralski) and HB (horizontal Bridgman) methods,

respectively. The samples from Wacker Chemitronic ingot were semi-in-

sulating (> 10 Qcm a t room temperature) whereas the RTC specimens presented lower resist ivity (< 10 .Q cm at room temperature).

6 4

1) 47011 Valladolid, Spain. 2) 34060 Montpellier, France.

physica status solidi (a)%

Fig. 1 . Photoconductivity spectra of V-doped GaAs. a) Wacker Chemitronic sample. The spectrum is shown for two different temperatures, below and above 135 K, the thermal quenching is clearly observed; b) RTC sample. The more relevant features of the spectra a r e indicated by a r rows

The photoconductivity spectra of both samples a r e shown in Fig. la and b, respectively. These spectra were performed by means of the thermal cycling

technique in order to ignore the influence the optically induced metastability

has on the spectral distribution of the photoconductivity in GaAs. This method has been previously described in other papers /6, 7/, where i t w a s demon- strated to provide very reliable photoconductivity spectra in GaAs.

The photoconductivity spectrum of the Wacker Chemitronic samples exhibits

similar features to those observed for other semi-insulating LEC samples non- intentionally doped with vanadium /8/. Only the photocurrent threshold observed

a t 1.37 eV seems to be related to the vanadium impurity (V

level). The broad photoconductivity band located between 1 and 1 . 2 5 eV cannot be ascribed to the vanadium doping, in fact i t exhibits the thermal and spectral

features usually found in vanadium-less semi-insulating LEC and HB samples,

which were extensively described in previous papers /8, 9/. These character-

is t ics are: a) strong enhancement of the photoconductivity for persistent illu-

mination with photons of energy ranging from 1 to 1 . 2 5 eV, b) thermal quenching

of the excess photoconductivity between 120 and 135 K. This phenomenon has

2+ /V3+ acceptor

Short Notes K2 09

been proved to occur in many semi-insulating GaAs specimens and basically

consists of a strong enhancement of the photosensitivity, yielding high photo-

conductivity level, after long time exposure to light of 1 to 1.25 eV photons.

The new photosensitivity state, which is optically generated, presents meta-

stable features. The recovery of the normal state is achieved by heating above

140 K, a strong thermal quenching of the photoconductivity between 120 and

135 K is observed simultaneously to the recovery of the normal photosensitivity

state. The physico-chemical origin of this band has been largely discussed pr ior

to this in samples not vanadium doped 1 8 , 9/. It has been argued that this band

is related to the EL2 midgap family of levels, which is the main deep t rap in

undoped semi-insulating GaAs. The RTC specimen presents thoroughly different photoconductivity proper-

ties. F i r s t a t all, no photomemory effects a r e observed, in other words, the

steady state photoconductivity level is quickly reached in the full scanned

spectral range (0.7 to 1.51 ev). The existence of metastable s ta tes can be

ignored, so that i t can be argued that no high concentration of EL2 levels should

exist. In these samples the photoconductivity threshold a t 1.38 eV is also ob- served. This threshold has been also found in optical absorption experiments

/1, 2/ and is ascribed to the V2'/V3' acceptor level which captures electrons

optically pumped from the valence band. A weak band centered a t 1 . 2 eV is also observed, which could be associated

to the transition from the E c - 0.23 eV vanadium acceptor level to the conduction

band this level has been detected in DLTS experiments, and has been proved to

undergo a strong lattice relaxation with a photoionization threshold of 1.15 eV,

which is in agreement with our photoconductivity peak. On the other hand, the density of this center has been found to be small a s compared to that of the E - 0.14 eV trap responsible from the 1.38 eV photoconductivity threshold, this could explain the weakness of the associated photoconductivity band. This

level is usually found in low resistivity vanadium doped material.

C

The most important features of these photoconductivity spectra a r e in good agreement with the absorption spectra reported elsewhere for vanadium doped

GaAs/2, 3/.

On the other hand, the temperature evolution of the photoconductivity

spectrum of this sample does not show recovery effects similar to t&se of the

K210 physica s ta tus solidi (a) 96

100 150 200

Fig. 2. TSC (thermally stimulated currents) spectra of V-doped GaAs. a) Wacker Chemitronic sample. The spectrum is shown for fundamental (1 .51 ev) (curve 1) and fundamental +YAG (1 .17 eV) (curve 2) excitation a t 4 K. The dashed line is the da rk current (I ); b) RTC sample. TSC spectrum for fundamental excitation a t 77 K (full line? and dark current, I. (dashed line)

Wacker Chemitronic specimen. This can be considered as a proof of the non- relation between the 1 to 1 , 2 5 eV photoconductivity band of the Wacker Chemi-

tronic sample and the 1.2 eV band of the RTC sample.

The experimental resul ts obtained by TSC a r e reported in Fig. 2a and b.

In Fig. 2a are shown the TSC spectra obtained for the Wacker Chemitronic specimen by exciting with either fundamental light (curve 1) o r fundamental light + high intensity 1 . 1 7 eV (YAG laserbeam) (curve 2) a t liquid helium tem-

perature, A high intensity TSC peak, whose maximum is located a t 85 K, is

clearly observed. The activation energy of this peak can be estimated as 0.14 eV / lo / , furthermore the temperature range in which this peak is ob-

served is the same as that reported in DLTS experiments. I t is noteworthy

to r emark the high intensity of this peak, which is in agreement with DLTS measurements which reveal a high concentration of the V2+/V3+ acceptor

level. The temperature of the maximum is shifted in curve 2, this is due to the effect induced by 1. 17 eV light from the YAG laser inthetrapdistribution

as it has been stated elsewhere / l l / . The RTC specimen presents a poorly resolved TSC spectrum, due to i t s

low resistivity, the TSC is shadowed by the high equilibrium dark current.

En spite & this a weak structure is observed a t 136 K, which could be ascribed

Short Notes K211

to the E - 0.23 eV acceptor level. I t is,important to r e m a r k that a n equilibrium

electron concentration analysis reveals that in this sample the F e r m i level is

pinned by the V2+/V3+ acceptor level, E - 0.14 eV, which controls the re-

sistivity properties of the sample. The same analysis for the Wacker sample reveals a n activation energy higher than 0.6 eV, which is typically found i n other semi-insulating GaAs samples, where the compensation is ensured by midgap levels of the EL2 family.

c

C

F r o m our analysis we can confirm that the only levels introduced by va- nadium impurity in GaAs are located a t Ec- 0.14 eV and E - 0,23 eV, respective-

ly. No evidence of deeper levels associated to the vanadium impurity which could

be responsible of the high resistivity of this material is found.

C

From our TSC and photoconductivity data i t is deduced that the high re-

sistivity of Wacker samples is mostly due to the usual compensation mechanism of undoped semi-insulating GaAs samples, in other words, this compensation

is related to the existence of midgap levels belonging to the EL2 family. On the

other hand, the absence of any of these levels leaves the compensation of back- ground shallow levels as mainly due to the vanadium, which does not introduce

enough deep levels in the band gap to ensure high resistivity material , this is

observed in RTC sample for which the Fe rmi level is pinned by the V

acceptor level, In conclusion, vanadium doping of GaAs does not produce by it-

self semi-insulating material . Another aspect to consider is related to whether vanadium doping confers more stability or not to the semi-insulating GaAs rel-

ative to both Cr doped and undoped material , or in other words does vanadium make more stable the EL2 t rap /12/? This item would be treated in a forth-

coming paper.

2+ 3+ /V

References /1/ w. ULRICI, K. FRIEDLAND, L. EAVES, and D. P. HALLIDAY, phys. stat.

sol, (b) 131, 719 (1985). - / 2 /B . CLERJAUD, C , NAUD, B. DEVEAUD, B. LAMBERT, B. PLOT,

G, BREMOND, C . BENJEDDOU, G. GUILLOT, andA. NOUAILHAT,

J. appl. Phys. - 58, 4207 (1985). /3/ G. ARMELLES, J. BARRAU, and D THEBAULTa J. Phw: C 17, 6883 -

(1 984) . /4/ U. KAUFMANN, E. ENNEN, J. SCHNEW ER, R . WORNER, J . WEBER,

K212 physica status sol idi (a) 96

and F. KOHL; Phys, Rev, B - 25, 5598 1982).

/5/ C.D. BRANDT, A.M. HENNEL, L.M. PAWLOWICZ, F.P. DABROWSKI,

J. LAGOWSKI, and H.C. GATOS, Appl Phys . Letters - 47, 607 (1985).

/6/ J. J IMkNEZ, M. A . GONZALEZ, L. F. SANZ, and J. BONNAFk, phys.

/7/ J. JIMkNEZ, M. A, GONZALEZ, J. A. D E SAJA, and J. BONNAFlf,

/8/ 3 . JIMkNEZ, M.A. GONZALEZ, P. HERNANDEZ, J. A. D E SAJA, and

/9/ J. J IMkNEZ, M.A. GONZALEZ, and L. F. SANZ, Solid S ta te Commun.

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

J. Mater . Sci. 19, 1207 (1984).

J. BONNAFk, J, appl. Phys . 57, 1152 (1985).

-

49, - 917 (1984). /lo/ J. BONNAFd, M. CASTAGNE, J. ROMESTAN, and J. P. FILLARD,

/11/ J.P. FILLARD, J. BONNAFd, and M. CASTAGNE, Appl. Phys . A - 35,

/12/ W. KUTT, D, BIMBERG, M. MAIER, H. KRAUTLE, F. KOHL, and

J. Phys. C 14, 2465 (1981). -

149 (1984).

E. BAUSE, Appl. Phys. L e t t e r s 44, 1078 (1984).

(Received J u n e 13, 1986)