of tellurium trapping under high chlorineand tellurium pressures. Its appearance is as- sociated with the participation of tellurium atoms in the formation of nuclei, which make an additional contribution both to the growth velocity and to the impurity-trapping velocity. The maximum forms under definite pressures and with definite chemical forms of the compo- nents.

Near the (001) face an asymmetry was observed to appear in the anisotropy of impurity trapping as the chlorine and tellurium pressure were increased, which is a result of the difference in the distribution factor of the impurity on the (iii) and (ii0) faces.

Near the (Ii0) face the singular minimum in the impurity-trapping velocity is stable in the range of chlorine and telluriumpressures studied.

LITERATURE CITED

i. S.E. Toropov, L. P. Porokhvnichenko, and L. G. Lavrent'eva, Izv. Vyssh. Uchebn. Zaved., Fiz., No. 6, 46 (1983).

2. L.G. Lavrent'eva, S. E. Toropova, and L. P. Porokhovnichenko, Izv. Vyssh. Uchebn. Zaved., Fiz., No. Ii, 18 (1982).

3. H. Bakly, Growth of Crystals [Russian translation], Inostr. Lit., Moscow (1954), p. 250. 4. Modern Crystallography [in Russian], Vol. 3, Nauka Moscow (1980), p. 127.

COMPLEXING OF TELLURIUM-DOPED GALLIUM ARSENIDE IN

GAS-PHASE EPITAXY

I. A. Bobrovnikova, L. G. Lavrent'eva, and S. E. Toropov

UDC 621.315.592:548.552.22

The effect of the conditions of gas-phase epitaxy on complexing in tellurium-doped layers of gallium arsenide is studied. Measurements of the intensities of the cor- responding photoluminescence lines were used to estimate the concentration of op- tically-active complexes. A correlation was found between the processes involved in complexing, the growth velocity, and impurity trapping in the layers. The deter- mining role of the surface structure in the formation of complexes, including im- purity atoms, is demonstrated.

i. Introduction. Experimental Procedure

It is well known that the photoluminescence spectra of gallium arsenide have a number of charaGteristic bands, including bands which are interpreted as resulting from complexes of the type "impurity--impurity" and "impurity-vacancy" [1-5]. However, the mechanism re- sponsible for the formation of complexes of this type in gas-phase epitaxy has practically not been studied, and the stages responsible for complexing are unknown. In the analysis of the processes involved in complexing, aquasiequilibrium approach, whose applicability is nat, however, obvious, is often used.

It was previously established that the capture of donor and acceptor impurities in gas-phase epitaxy of gallium arsenide is limited by the rate of surface processes occurring in the absorption layer [6]. It is also shown that impurity trapping is accompanied by the formation of complex molecules, containing tellurium atoms and atoms of the main material, in the adsorption layer [7]. This provides a basis for the assumption that the formation of complexes, including the impurity, can occur at the crystallization stage. In this case, the concentration of complexes must dependon the growth mechanism and the surface structure. To check the possibilities of this approach, in this work we studied the dependence of the concentration of complexes in tellurium-doped layer s of galliu m arsenide on the impurity con- centration and the orientation of the substrate, and we compared the kinetics of trapping of tellurium in different forms with the growth kinetics of the layers.

V. D. Kuznetsov Siberian Physicotechnical Institute, Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 96-100, February, 1985. Orig- inal article submitted March 27, 1984.

172 0038-5697/85/2802-0172509.50 �9 1985 Plenum Publishing Corporation

<. IpL, relative units

0,5

~,e 1

b

f ,2 ~q hi,

Fig. 1

TABLE 1

Position of Emitting Reference PL band, defect eV

0,83 lCuGJ l l ] 0,93-0,96 [ VOa-- VAs ] [1, 2]

1,0--1,02 [Cu~a---VAs ] [1, 3] 1,2--1,22 [V(],,--TeAs ] [2, 4, 5]

1,28-1,31 [Cue3 a - ' r e s ] [1--3]

eV

_/0, relative units

�9 " A X

loL

I0 ~ a

L /~ ~ o- 0,1)4 , , - e ' ( l U ) 8 ,

10 I~ 2 " . q 6 8 10 ~ n, cr5 a

Fig. 2

In this work we measured the photoluminescence spectra (PL) of the gallium arsenide layers grown in the system GaAs--AsCI3--H2 , with a tellurium concentration in the source of NTe = 4"10 Is cm -3 and PAsCI3 pressures at the system inlet equal to 1.1-10 -4 and 8"10 -3 atmo

The temperature of the source and of the substrate was equal to 830 and 750~ GaAs sub- strates with the (III)A orientation and deviations from it by 2, 4, 6, and 8 ~ toward (001) and (ll0) were used. The electron density in the layers obtained under a low AsCI3 pressure was equal to (1-2.5)'1018 cm -3 and the electron density obtained under a high pressure was equal to (4-9.5)'1018 cm -3

The PL spectra were measured at the temperature of liquid nitrogen. The photolumines-

cence was excited with an LG-75 laser with a wavelength of ~ = 0.638 ~m and the radiation

173

I~, reLunits 2

Vte, reLunits [ b v , . �9 tel. units

Vg' ~m'h [ s - - ~ ~ c Vtc.10- ,

~m/h JTe" 10-~ , z g - ~ < ~ JTe" 10"Z~, :

cm.Z. sec_ l /9/ ~ Cn].2. secI 1

rel. units j "

]

I

.... ;J! n, cm -s ~ ~ . e n. cm "3 tOi .~ �9 ~ . d ~ " '~ I

~o:,~ ~ ~.is . . . . . . . . ,~ ~. (mlA L/ g 8 o 8 ~ (/~/),~ ~ 8 S (n~)- -(oo~) (//o]- .. -(~!.1

Fig. 3. Orientational dependences of the relative inten- sity Io (a, f), the velocity of trapping of complexes Vtc (b, g), growth velocity Vg (c, h), impurity-trapping ve- locity JTe (d, i), and electron density (e, j). The val- ues of PAsCI3 in atm are: a-e) 1.1"10 -4 , t-j) 8"10 -3 .

was detected with a germanium photoresistor. The spectra were separated into individual components with the help of a computer using a program based on the method proposed in [8].

It was assumed that the ratio of the intensities of the separate PL bands characterizes the ratio of the concentrations of the corresponding optically active defects.

2. Experimental Results and Discussion

Figure 1 shows the typical PL spectra of epitaxial layer of GaAs grown at low (a: i.i" 10 -4 atm) and high (b: 8"10 -3 atm) AsCI3 pressures in the system GaAs--AsCI3--H2.

The spectra of all samples contain the edge-emission band and a much more intense wide emission band in the impurity region of the spectrum, consisting of six elementary bands, whose maxima are situated at 0.8, 0.93, 1.02, 1.12, 1.22, and 1.31 eV, while the relative in- tensities depend on the growing conditions of the layers. The bands in the PL spectra were identified by comparing with data available in the literature [1-5], which are summarized in Table i. According to these data, the tellurium complexes of interest to use are character- ized by emission at energies of 1.22 and 1.31 eV (complexes of the type [VGa--TeAs] and [CuGa-- TeAs], respectively).

An analysis of the ratios of the intensities of different bands showed that on the whole the regularities in the changes in Io = Ij/I i (where Ij, Ii are the intensities of the bands associated with tellurium and bands not associated with tellurium, respectively) accompany- ing a variation of the growing conditions are the same for different i and j, but are more distinct for the ratio Io = I~.22/Io.9. This value will be used in further discussions as the parameter characterizing the concentration of complexes with the participation of tellurium.

A comparison of the relative intensities of the impurity PL bands in the spectra of lay- ers grown under different conditions shows that the concentration of the complexes with the participation of tellurium increases as the tellurium and chlorine pressure increase in the gas phase (Fig. i).

The dependence of the relative intensity of the impurity bands on the electron density (Fig. 2, value of PAsC13in atm:a) l.l.10-4,b) 8,10 -3 ) exhibits a growth in the emission intensity accompanying an increase in the charge-carrier density up ton ~ 2.5-10 Is cm -3 (Fig. 2a)and then

a decrease (Fig. 2b). Therefore the concentration of complexes including the impurity in- creases as the impurity concentration increases in the range (I-2.5)'i018 cm -3

The decrease in the emission intensity in the region 1.2-1.3 eV with a further increase in the impurity concentration in the layers could be associated with the transformation of complexes of the type [VGa--TeAs] into more complicated associates, which are centers of non- radiative recombination. This phenomenon is well known for single crystals [10, ll].

174

For disoriented layers such a clear dependence of the intensity of the impurity bands (concentration of complexes with the participation of tellurium) on the impurity concentra- tion is not observed. Evidently, the processes involved in the formation of optically active complexes in layers are affected not only by the impurity concentration but also by the or- ientation of the growth surface.

Figure 3a, f shows the dependences of I0 on the angle of deviation relative to the (iii) A face for two pressures of AsCI3 (and therefore of tellurium also) in the gas phase of the gas-transport crystallization system~

It is evident that the (III)A point corresponds to a minimum, whose depth is greater,

the higher the tellurium pressure.

As is well known, dependences of this type show that the steps in the growth process, whose density increases with the deviation from the singular face, have a determining effect on the process under analysis (in this case complexing). From here it may be concluded that the formation of impurity-vacancy associates (complexes) evidently occurs at the growth steps In this case the process must be limited by the kinetics of mass-transfer at the faces of the

steps.

To describe the kinetics of complexing it is useful to introduce the velocity of trap- ping of complexes Vtc, defining it in terms of the number of complexes appearing per 1 cm 2 of the surface in 1 sec (in analogy to the velocity of imPurity trapping [9]). If N c (~Io) is the concentration of complexes in the bulk of the film and Vg is the velocity of growth of the film, then Vtc = Nc'Vg. Since the PL method was used to estimate only the relative concentration of complexes, Vtc is also determined in relative units.

Comparison of the trapping velocity for complexes of the type [VGa--TeAs] with the trap- ping velocity of electrically-active impurity TeAs(iT e = n'Vp) and the growth velocity of layers showed that in the case of low impurity pressure all velocities change synchronously (Fig. 3, b-d): They increase as the density of steps increases, though not to the same de- gree. Therefore, the kinetics of complexing is similar to the kinetics of growth and im-

purity trapping in the layers.

With regard to the relative quantities -- the electron density n = JTe/Vg and Nc = Vtc/Vg (~Io) (Figs. 3e and a) -- they vary differently. The first one has a maximum at the point (lll)A and the second, as already mentioned, has a minimum. Therefore the process of forma- tion of the complexes of interest to us in the disoriented layers is determined not only by the impurity concentration, but also by the orientation of the growth surface, i.e., by its

structure.

Figure 3f-g shows the orientational dependences of Io, Vtc, Vp, n, JTe for layers gro~ under a high tellurium pressure in the gas phase. In contrast to the preceding dependence (Figs. 3a-d) here a maximum velocity of growth and impurity trapping on the (III)A face and a minimum concentration and velocity of trapping of tellurium-containing complexes are ob- served~ The concentration of electrically active tellurium in these layers is close to the limiting solubility, and the anistropy of the doping level is hardly manifested at all

(Fig. 3j ).

As shown previously [8], the appearance of a maximum in the velocity of growth and im- purity trapping on the (III)A face is associated with the mechanism of trapping of tellurium in the form of complicated complexes, formed in the adsorption layer accompanying the inter- action of Te and GaCI and participating in the nucleation process. The fact that the con- centration of complexes of the type [VGa--TeAs] in layers of this series has as before a dis- tinct minimum at the point (III)A proves that the formation of optically active complexes occurs on growth steps introduced by disorientation and is not associated with the formation

of nuclei on the terraces.

3. Conclusions

An analysis of the experimental results obtained leads to the following conclusions.

The photoluminescence spectra of epitaxial layers of tellurium-doped gallium arsenide contain bands associated with complexes forming with the interaction of the impurity (Te)

with intrinsic point defects.

175

The concentration of these complexes and its dependence on the impurity concentration in the layers are determined by the conditions Of epitaxy.

A correlation was found between the velocity of growth and of impurity trapping and the concentration of impurity-containing complexes in the layers, proving that the complexes are formed at the crystallization stage.

It was shown that the concentration of optically active tellurium-containing complexes depends sharply on the orientation of the growth surface, which indicates that the surface structure, in particular the growth step, affects the formation of these complexes.

LITERATURE CITED

i. H.J. Guislain, L. De Wolf, and P. J. Claus, J. Electron. Mater., ~, i, 90 (1978). 2. K.D. Glinchuk, K. Lukat, and A. V. Prokhorovich, Optoelektron. Poluprovodn. Tekhn., No.

i, 39 (1982). 3. K.D. Glinchuk, A. V. Prokhorovich, and N. S. Zayats, Phys. Status Solidi A, 72, 2, 715

(1981). 4. Mutsuyuki Otsub, Hideyjiomiku, andShiger Mitsui, Jpn. J. Appl. Phys., 16, ii, 1957 (1977). 5. C.J. Hwang, J. Appl. Phys., 40, ii, 4584 (1969). 6. L.G. Lavrent'eva, Izv. Vyssh. Uchebn. Zaved., Fiz., No. i0, 31 (1983). 7. S.E. Toropov, L. P. Prokhorovnichenko, and L. G. Lavrent'eva, Izv. Vyssh. Uchebn.

Zaved., Fiz., No. 6, 46 (1983). 8. M.V. Fok, Trudy Fiz. Inst. Akad. Nauk SSSR, 59, 3-24 (1972). 9. L.G. Lavrent'eva, S. E. Toropov, and L. P. Prokhorovnichenko, Izv. Vyssh. Uchebn.

Zaved., Fiz,, No. 11, 18 (1982). i0. M. G. Mil'vidskii, O. V. Pelevin, and B. A. Sakharov, Physicochemical Foundations of

the Production of Dissociating Semiconducting Compounds [in Russian], Metallurgiya, Moscow (1974).

ii. V. T. Bublik, M. G. Mil'vidskii, and V. B. Osvenskii, Izv. Vyssh. Uchebn. Zaved., Fiz. No. i, 7 (1980).

176