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Photostimulated epitaxy of II–VI and IV–VI layers
Journal of Crystal Growth 52 (1981) 141-145 © North-Holland Publishing Company
P H O T O S T I M U L A T E D E P I T A X Y OF H - V I A N D I V - V I L A Y E R S S.N. M A X I M O V S K Y ,
I.P. R E V O C A T O V A
and M . A . S E L E Z N E V A
Lebedev Physical Institute, Academy of Sciences of the USSR, Moscow, USSR
A method of preparation A2B6 and A4B6 layers with controlled properties is described. The method involves the exposure of the layer of light (0.3 < A < 1.20/~m) during the epitaxial process. The effect of the electromagnetic radiation on the electrical conductivity of the layers has been studied systematically. It has been found that this photostimulated epitaxy permits the growth of thin films with a wide variation of nonstoichiometry. The morphology of the grown films has been also studied as function of the wavelength of the electromagnetic radiation. It has been established that the depth of the Sn diffusion from a PbSnSe substrate into a PbSe film depends on the irradiation time. In this way control of the width of the transition layer is possible. closed value m a y be o p e n e d r e p e a t e d l y as required. T h e substrate, material source and c o m p o n e n t source are enclosed in this v o l u m e (fig. 1). T h e t e m p e r a t u r e s of material a n d c o m p o n e n t source are controlled independently. This permits the variation of the v a p o u r phase composition o v e r the growing surface, the substrate
1. Introduction It has b e e n shown in the past that elect r o m a g n e t i c radiation decreases the t e m p e r a t u r e of epitaxial deposition and increases the layer g r o w t h rate in V P E [1, 2]. H o w e v e r , the effect of e l e c t r o m a g n e t i c radiation on the epitaxial g r o w t h of A n B va a n d A I V B vI films has not b e e n studied in detail.
2. E x p e r i m e n t a l Thin films of C d T e , P b S e and P b T e w e r e g r o w n b o t h on c o n d u c t i n g and insulating substrates by a p h o t o s t i m u l a t e d epitaxy technique d e v e l o p e d in the L e b e d e v Physical Institute. T h e n e w technique has s o m e a d v a n t a g e s as c o m p a r e d with o t h e r m e t h o d s of sublimation in a closed v o l u m e b e c a u s e the substrate e x p o s e d to elect r o m a g n e t i c irradiation is cooled on the back side by a refrigerant [3]. A high pressure x e n o n lamp D K S R - 3 0 0 0 with the w a v e l e n g t h r a n g e 0.3-1.2/~m was used as the light source. A fiat cylindrical a m p o u l e with a g r o u n d joint a n d a special reservoir f o r variation of the v a p o r pressure of o n e of the c o m p o n e n t s can be tightly closed by pushing a piston in the g r o u n d joint. T h e piston is p u s h e d with approx. 1 a t m of h y d r o g e n over-pressure. It contains a tube for the cooling of the substrate by an air stream. T h e main a d v a n t a g e of this construction is that a
S OtL'IC,~
,~uSstz,
h~t'sa
Fig. 1. Schematic of apparatus for photostimulated epitaxy: (1) ampoule; (2) ground joint; (3) reservoir; (4) piston; (5) tube for cooling; (6) joint; (7) cooling camera; (8) metal holder; (9) source of refrigerant. 141
S.N. Maximovsky et al. / Photostimulated epitaxy of H - V I and I V - V I layers
142
W/C/'0¸ 5S 50 25 2d 45 /0 Y i
~
iJ
I
Ii
5
e
~
o
/
J
f
e 3 X, cm
Fig. 2. Curve of illumination profile: (a) in the focal plane and (b) at a distance of 6 c m from the focal plane. The position of substrate is also shown in the centre (-+0.7 cm).
temperature being kept constant. The substrate is cooled with the help of refrigerant passing through the inner channel of the shaft. T o control and focus the light beam a radiation source " Y P A H - I " was used [4]. The film growth was carried out in a light spot up to 3 cm in diameter with uniform illumination and dimensions exceeding those of the substrate (fig. 2). Optimum growth conditions were established experimentally through variations of the source temperature, the current through the lamp and the flow of the refrigerant.
3. Resuit~ and discussion
Thin films of I I - V I and I V - V I materials have been grown by photostimulated epitaxy in the whole range of homogeneity and following temperatures of substrate: 650°C (CdTe) and 720°C (PbSe, PhTe). Deviations from stoichiometry were achieved through changes of the vapour phase composition using an auxiliary vapour source of one of the components. The deviation from stoichiometry was estimated by measuring the cartier concentration which varied from p = 4 × 1 0 1 6 c m -3 to n = 2 × 1 0 1 7 c m -3 for CdTe films [5], from p = 1.3 × 10 TM c m -3 t o n = 1 × 1018cm -3 for PbSe [6] and from p = 6 × 10 TM to n = 7.2 x 1018cm -3 for PbTe [7]. X-ray investigations proved that all the films obtained were single crystalline and had the same orientation as the substrates.
The conditions of epitaxy have been also investigated. It has been established by experiments with ampoules having different distances between source and substrate that rates for all the three compounds depend on this distance and not on the orientation of the substrate. This indicates that the process is diffusion-limited [7]. No data was previously available about the preparation of CdTe, PbSe or PbTe films, with compositions close to the phase boundaries of these compounds, by sublimation in a closed system. According to theoretical [8] and experimental [9] work, this method was not successful because the film growth rate and film perfection decrease in the course of the growth process due to constitutional supercooling at the vapour-solid interface. It is well known that, due to decrease of the sticking coefficient of the substrate for one of the components, constitutional supercooling is established in diffusionlimited growth when the vapour phase composition deviates from the stoichiometry. No constitutional supercooling was observed in our experiments of photostimulated epitaxy. Therefore we can conclude that growth rate and film perfection do not depend on the vapour phase composition above the growing layer. This is probably due to changes in the sticking coefficient of the substrate for one of the components. In principle, there are two ways of changing this coefficient: (a) Additional dissociation of the vapour species, e.g. change of the ratio of the equilibrium partial pressure over the growing interface due to photodissociation. This leads also to a change of the vapour phase composition according to the expression. (b) Change of the adsorption properties of the surface. Within the wavelength range used, an intrinsic photoeffect is expected [10, 11] for all three compounds, which may change the equilibrium in the adsorption layer, as well as the adsorption kinetics for all vapour phase components. As a result, the sticking coefficient of the surface can increase and become closer to unity. PbTe films were grown on (100) KC1 500/zm thick. The source temperature was 750°C and
S.N. Maximovsky et al. / Photosfimulated epitaxy of H - V I and I V - V I layers
that of the substrate 720°C, the thickness of the layer being 60-70/zm. The composition of the PbTe film can change even if all other growth conditions are kept constant due to substrate and source temperature fluctuations. Therefore special attention was paid to temperature stabilization during the experiment. Temperature control was realized by a contactless m e t h o d [11] using P t / P t - R h thermocouples, the accuracy being 0.5°C. During irradiation the lamp power (0.6kW) was maintained constant for all the experiments. In the case of decrease of the light flux intensity at the substrate, the heat deficit was compensated with the aid of an extra resistance heater. According to the Wien law the maximum of furnace radiation corresponded to A -- 5 pm. Table 1 gives the carrier concentration of single crystal layers of PbTe as a function of the Pb vapour pressure and the irradiation conditions. Four types of irradiation experiment have been performed: (i) irradiation of both gas phase and substrate; (ii) irradiation of the gas phase only (light beam tangential to the substrate); (iii) no irradiation at all; (iv) radiation with A < 0.65/zm eliminated by a filter (UKC-17). One can see that different carrier concentrations and conductivity types can be obtained with the same Pb vapour pressure. If the substrate itself is not exposed to the radiation (table 1, case 1), increase of the Pb vapour pressure does not change significantly the
carrier concentration of the PbTe films; the conductivity is p-type up to a pressure of 7 × 10-2 Torr. Irradiation of both gas phase and substrate in the spectral range A < 0.65/zm (table 1, case 3) has only a minor effect on the carrier concentration. Radiation of the substrate and gas phase in the whole spectral range of the lamp leads at Pb pressure 7 x 10-2Torr to electron conduction and therefore to the greatest deviation of the layer composition from stoichiometry. Detailed results have been given in other publications [5-7]. According to ref. [11], the filtered radiation can activate surface reactions but not the direct dissocaition of molecules in the gas phase. We can see from table 1 that filtered radiation does not allow large deviations from stoichiometry. On the other hand, as we have mentioned above, small deviations from stoichiometry are the result of a low sticking coefficient. Thus one can assume that the major factor increasing the sticking coefficient of the substrate (in case 2, table 1) is the change in adsorption properties of the surface and not the dissociation of the gas phase components. A l o n g with the changes of carrier concentration, the irradiation by the xenon lamp changes the surface morphology and the degree of film perfection. Fig. 3a shows the surface morphology of PbTe films grown under irradiation of both gas phase and the substrate. The electron micrograph shows flat low-index faces with continuous growth steps.
Table 1 Carrier concentration in PbTe single crystalline layers (epitaxy in (100)KCI) obtained under different experimental conditions (P = partial vapor pressure of lead, temperature substrate 720°C, lamp power 0.6 kW, thickness 60-70/zm) Irradiation conditions (1) Gas phase (2) Substrate and gas phase (3) Substrate and gas phase irradiation through filter (4) No irradiation
143
Carrier concentration (cm-3) P = 3 x 10-5 Torr P = 10-2 Ton"
P = 7 × 10-2 Torr
p = 6.7x 1017 p = 1.3 × 10TM p = 6.2 × 1017
p=5×1017 p = 9 × 1016 p = 5 x 1017
p = 6 x 1016, droplets, whiskers n = 1 x 10TM p = 8 x 1016, droplets, whiskers
Polycrystal
Polycrystal
Polycrystal
144
S.N, Maximovsky et al. / Photostimulated epitaxy of H - V I and 1 V - V I layers
Fig. 3. Electron micrograph of PbTe layer. Vapour pressure 3 x 10-5 Torr: (a) irradiation substrate and gas phase (x 1600); (b) only gas phase (×640); (c) radiation with A <0.65/zm, substrate and gas phase (x640).
When the substrate itself is not irradiated, its morphology deteriorates although the films are still single crystalline (fig. 3b). When both the gas phase and substrate are exposed to radiation with A < 0.65/~m, further morphology deterioration is observed, the layer consisting of still smaller oriented crystallites (fig. 3c). With increase of Pb pressure to 7 x 10-2 T o r r in the system the conductivity type changes, and whiskers appear on the surface. In all the experiments source and substrate temperatures were maintained constant and therefore the diffusion rate in the gas phase was also constant. The study of surface morphology of the films obtained under different conditions proves that the structural perfection is influenced mainly by the short wavelength range of the spectrum of the xenon lamp; its absence or attenuation results in film formation through lateral growth of oriented crystallites instead of continuous layer growth mechanism realized under irradiation [7]. In the absence of light, polycrystalline films are formed which can be attributed to smaller rates of surface diffusion of adatoms. P - n junctions grown by the above technique contain impurities from the substrate. This results in a spread of the p - n junction [12]. Two possible explanations can be discussed: (a) Capture of etching products by the growing surface as the latter is first exposed to intense photostimulated vapour etching. (b) Nonequilibrium vacancies appear in the layer close to the surface during its exposure to light with quantum energy exceeding the band gap of the material. These vacancies promote the diffusion of substrate atoms into the film [10]. T o reveal the origin of the substrate atoms in the layer, epitaxial PbSe layers were grown on PbSnSe substrates 500/~m thick at constant source and substrate temperatures. The growth process was carried out in three regimes (fig. 4, curves 1, 2 and 3): (1) without preliminary photo-etching of the Substrate and irradiation of the film during its cooling; (2) with preliminary photo-etching of the substrate surface and with irradiation; (3) without preliminary photo-etching of the
S.N. Maximovsky et al. / Photostimulated epitaxy of H - V I and I V - V I layers
/.l 321 Fig. 4. Sn profiles in the transition layer of PbSe/PhSn~
measured by microprobe. For details see text. substrate and with irradiadiation of the film during its cooling. T h e Sn distribution in the h e t e r o j u n c t i o n region was d e t e r m i n e d f r o m the c h a n g e of intensity of X - r a y S n L a m e a s u r e d with a microanalyser type MS-46. T h e excitation region is ~< 1 / z m and thus the distribution slope of Is, d e p e n d s mainly on the Sn distribution in the region of h e t e r o j u n c t i o n . O n e can see (fig. 4) that the width of the transition region d e p e n d s on the irradiation time of the growing film and not on the p r e s e n c e of substrate c o m p o u n d s in the gas phase. T h e r e f o r e m e c h a n i s m Co) seems to be active.
4. Conclusions It has b e e n shown that I I - V I and I V - V I films with m a x i m u m deviation f r o m stoichiometry can be g r o w n for the first time by p h o t o s t i m u l a t e d epitaxy d u e to the c h a n g e of a d s o r b t i o n p r o p e r ties of the surface. This c a n g e can be explained by the increase of sticking coefficient of the surface at any compositions of the gas phase a n d by dissociation of the v a p o r species on the substrate surface u n d e r the influence of light.
145
T h e g r o w t h process is limited by the rate of substance transport to the surface. T h e perfection of the grown layers is influenced mainly by the short-wave range of the spectrum. T h e analysis of Sn distribution in the transitional region of P b S e / P B S n S e p - n junctions shows that the width of this region d e p e n d s on the duration of the e x p o s u r e to electromagnetic radiation.
References [1] [2] [3] [4]
R.G. Friesher, J. Electrochem. Soc. 115 (1968) 401. J.S. Grossman, Patent USA, N3.200.018. S.N. Maximovsky et al., Patent USA, N4.115.163. V.P. Sasorov, Eiektrovakuumnaya Tekhnika 0glectrovacuum Technology) (Energiya Publ. House, 1967) pp. 42, 87 (Russian). [5] S.N. Maximovsky, I.P. Revocatova, V.M. Salman and M.A. Selezneva, Rev. Physique Appl. 12 (1977) 161. [6] A.S. Averushkin, O.V. Alexandrova, K.V. Kiseleva, S.N. Maksimovsky, I.P. Revokstova and E.P. Shchebnev, Neorgan. Mater. 15 (1979) 380. [7] S.N. Maximovsky and I.P. Revokatova, Elektronnaya Tekhnika, Set. 6, Materialy 4 (1979) 48. [8] M.M. Factor and S. Garret, J. Crystal Growth 9 (1971) 3. [9] S.A. Medvedev and Yu.V. Klevkov, Izv. Akad. Nauk. SSSR, Set. Neorg. Mater. 7 (1971) 753. [10] V.T. Baru and F.R. Volkenshtein, Vliyaniye Oblucheniya na Poverhnostnye Svoistva Poluprovodnikov (Radiation and Surface Phenomena in Semiconductors) (Nauka, Moscow, 1978). [11] V.D. Chesteakov, Obzory po Elektronnoi Tekhnike, Ser. Poluprovodnikovye Pribory 5 (1975) 369. [12] Yu.I. Gorina, G.A. Kaluzhnaya, K.V. Kiseleva, V.N. Salman and N.I. Strogankova, Fiz. Tekh. Poluprovodnikov 13 (1979) 305.
P H O T O S T I M U L A T E D E P I T A X Y OF H - V I A N D I V - V I L A Y E R S S.N. M A X I M O V S K Y ,
I.P. R E V O C A T O V A
and M . A . S E L E Z N E V A
Lebedev Physical Institute, Academy of Sciences of the USSR, Moscow, USSR
A method of preparation A2B6 and A4B6 layers with controlled properties is described. The method involves the exposure of the layer of light (0.3 < A < 1.20/~m) during the epitaxial process. The effect of the electromagnetic radiation on the electrical conductivity of the layers has been studied systematically. It has been found that this photostimulated epitaxy permits the growth of thin films with a wide variation of nonstoichiometry. The morphology of the grown films has been also studied as function of the wavelength of the electromagnetic radiation. It has been established that the depth of the Sn diffusion from a PbSnSe substrate into a PbSe film depends on the irradiation time. In this way control of the width of the transition layer is possible. closed value m a y be o p e n e d r e p e a t e d l y as required. T h e substrate, material source and c o m p o n e n t source are enclosed in this v o l u m e (fig. 1). T h e t e m p e r a t u r e s of material a n d c o m p o n e n t source are controlled independently. This permits the variation of the v a p o u r phase composition o v e r the growing surface, the substrate
1. Introduction It has b e e n shown in the past that elect r o m a g n e t i c radiation decreases the t e m p e r a t u r e of epitaxial deposition and increases the layer g r o w t h rate in V P E [1, 2]. H o w e v e r , the effect of e l e c t r o m a g n e t i c radiation on the epitaxial g r o w t h of A n B va a n d A I V B vI films has not b e e n studied in detail.
2. E x p e r i m e n t a l Thin films of C d T e , P b S e and P b T e w e r e g r o w n b o t h on c o n d u c t i n g and insulating substrates by a p h o t o s t i m u l a t e d epitaxy technique d e v e l o p e d in the L e b e d e v Physical Institute. T h e n e w technique has s o m e a d v a n t a g e s as c o m p a r e d with o t h e r m e t h o d s of sublimation in a closed v o l u m e b e c a u s e the substrate e x p o s e d to elect r o m a g n e t i c irradiation is cooled on the back side by a refrigerant [3]. A high pressure x e n o n lamp D K S R - 3 0 0 0 with the w a v e l e n g t h r a n g e 0.3-1.2/~m was used as the light source. A fiat cylindrical a m p o u l e with a g r o u n d joint a n d a special reservoir f o r variation of the v a p o r pressure of o n e of the c o m p o n e n t s can be tightly closed by pushing a piston in the g r o u n d joint. T h e piston is p u s h e d with approx. 1 a t m of h y d r o g e n over-pressure. It contains a tube for the cooling of the substrate by an air stream. T h e main a d v a n t a g e of this construction is that a
S OtL'IC,~
,~uSstz,
h~t'sa
Fig. 1. Schematic of apparatus for photostimulated epitaxy: (1) ampoule; (2) ground joint; (3) reservoir; (4) piston; (5) tube for cooling; (6) joint; (7) cooling camera; (8) metal holder; (9) source of refrigerant. 141
S.N. Maximovsky et al. / Photostimulated epitaxy of H - V I and I V - V I layers
142
W/C/'0¸ 5S 50 25 2d 45 /0 Y i
~
iJ
I
Ii
5
e
~
o
/
J
f
e 3 X, cm
Fig. 2. Curve of illumination profile: (a) in the focal plane and (b) at a distance of 6 c m from the focal plane. The position of substrate is also shown in the centre (-+0.7 cm).
temperature being kept constant. The substrate is cooled with the help of refrigerant passing through the inner channel of the shaft. T o control and focus the light beam a radiation source " Y P A H - I " was used [4]. The film growth was carried out in a light spot up to 3 cm in diameter with uniform illumination and dimensions exceeding those of the substrate (fig. 2). Optimum growth conditions were established experimentally through variations of the source temperature, the current through the lamp and the flow of the refrigerant.
3. Resuit~ and discussion
Thin films of I I - V I and I V - V I materials have been grown by photostimulated epitaxy in the whole range of homogeneity and following temperatures of substrate: 650°C (CdTe) and 720°C (PbSe, PhTe). Deviations from stoichiometry were achieved through changes of the vapour phase composition using an auxiliary vapour source of one of the components. The deviation from stoichiometry was estimated by measuring the cartier concentration which varied from p = 4 × 1 0 1 6 c m -3 to n = 2 × 1 0 1 7 c m -3 for CdTe films [5], from p = 1.3 × 10 TM c m -3 t o n = 1 × 1018cm -3 for PbSe [6] and from p = 6 × 10 TM to n = 7.2 x 1018cm -3 for PbTe [7]. X-ray investigations proved that all the films obtained were single crystalline and had the same orientation as the substrates.
The conditions of epitaxy have been also investigated. It has been established by experiments with ampoules having different distances between source and substrate that rates for all the three compounds depend on this distance and not on the orientation of the substrate. This indicates that the process is diffusion-limited [7]. No data was previously available about the preparation of CdTe, PbSe or PbTe films, with compositions close to the phase boundaries of these compounds, by sublimation in a closed system. According to theoretical [8] and experimental [9] work, this method was not successful because the film growth rate and film perfection decrease in the course of the growth process due to constitutional supercooling at the vapour-solid interface. It is well known that, due to decrease of the sticking coefficient of the substrate for one of the components, constitutional supercooling is established in diffusionlimited growth when the vapour phase composition deviates from the stoichiometry. No constitutional supercooling was observed in our experiments of photostimulated epitaxy. Therefore we can conclude that growth rate and film perfection do not depend on the vapour phase composition above the growing layer. This is probably due to changes in the sticking coefficient of the substrate for one of the components. In principle, there are two ways of changing this coefficient: (a) Additional dissociation of the vapour species, e.g. change of the ratio of the equilibrium partial pressure over the growing interface due to photodissociation. This leads also to a change of the vapour phase composition according to the expression. (b) Change of the adsorption properties of the surface. Within the wavelength range used, an intrinsic photoeffect is expected [10, 11] for all three compounds, which may change the equilibrium in the adsorption layer, as well as the adsorption kinetics for all vapour phase components. As a result, the sticking coefficient of the surface can increase and become closer to unity. PbTe films were grown on (100) KC1 500/zm thick. The source temperature was 750°C and
S.N. Maximovsky et al. / Photosfimulated epitaxy of H - V I and I V - V I layers
that of the substrate 720°C, the thickness of the layer being 60-70/zm. The composition of the PbTe film can change even if all other growth conditions are kept constant due to substrate and source temperature fluctuations. Therefore special attention was paid to temperature stabilization during the experiment. Temperature control was realized by a contactless m e t h o d [11] using P t / P t - R h thermocouples, the accuracy being 0.5°C. During irradiation the lamp power (0.6kW) was maintained constant for all the experiments. In the case of decrease of the light flux intensity at the substrate, the heat deficit was compensated with the aid of an extra resistance heater. According to the Wien law the maximum of furnace radiation corresponded to A -- 5 pm. Table 1 gives the carrier concentration of single crystal layers of PbTe as a function of the Pb vapour pressure and the irradiation conditions. Four types of irradiation experiment have been performed: (i) irradiation of both gas phase and substrate; (ii) irradiation of the gas phase only (light beam tangential to the substrate); (iii) no irradiation at all; (iv) radiation with A < 0.65/zm eliminated by a filter (UKC-17). One can see that different carrier concentrations and conductivity types can be obtained with the same Pb vapour pressure. If the substrate itself is not exposed to the radiation (table 1, case 1), increase of the Pb vapour pressure does not change significantly the
carrier concentration of the PbTe films; the conductivity is p-type up to a pressure of 7 × 10-2 Torr. Irradiation of both gas phase and substrate in the spectral range A < 0.65/zm (table 1, case 3) has only a minor effect on the carrier concentration. Radiation of the substrate and gas phase in the whole spectral range of the lamp leads at Pb pressure 7 x 10-2Torr to electron conduction and therefore to the greatest deviation of the layer composition from stoichiometry. Detailed results have been given in other publications [5-7]. According to ref. [11], the filtered radiation can activate surface reactions but not the direct dissocaition of molecules in the gas phase. We can see from table 1 that filtered radiation does not allow large deviations from stoichiometry. On the other hand, as we have mentioned above, small deviations from stoichiometry are the result of a low sticking coefficient. Thus one can assume that the major factor increasing the sticking coefficient of the substrate (in case 2, table 1) is the change in adsorption properties of the surface and not the dissociation of the gas phase components. A l o n g with the changes of carrier concentration, the irradiation by the xenon lamp changes the surface morphology and the degree of film perfection. Fig. 3a shows the surface morphology of PbTe films grown under irradiation of both gas phase and the substrate. The electron micrograph shows flat low-index faces with continuous growth steps.
Table 1 Carrier concentration in PbTe single crystalline layers (epitaxy in (100)KCI) obtained under different experimental conditions (P = partial vapor pressure of lead, temperature substrate 720°C, lamp power 0.6 kW, thickness 60-70/zm) Irradiation conditions (1) Gas phase (2) Substrate and gas phase (3) Substrate and gas phase irradiation through filter (4) No irradiation
143
Carrier concentration (cm-3) P = 3 x 10-5 Torr P = 10-2 Ton"
P = 7 × 10-2 Torr
p = 6.7x 1017 p = 1.3 × 10TM p = 6.2 × 1017
p=5×1017 p = 9 × 1016 p = 5 x 1017
p = 6 x 1016, droplets, whiskers n = 1 x 10TM p = 8 x 1016, droplets, whiskers
Polycrystal
Polycrystal
Polycrystal
144
S.N, Maximovsky et al. / Photostimulated epitaxy of H - V I and 1 V - V I layers
Fig. 3. Electron micrograph of PbTe layer. Vapour pressure 3 x 10-5 Torr: (a) irradiation substrate and gas phase (x 1600); (b) only gas phase (×640); (c) radiation with A <0.65/zm, substrate and gas phase (x640).
When the substrate itself is not irradiated, its morphology deteriorates although the films are still single crystalline (fig. 3b). When both the gas phase and substrate are exposed to radiation with A < 0.65/~m, further morphology deterioration is observed, the layer consisting of still smaller oriented crystallites (fig. 3c). With increase of Pb pressure to 7 x 10-2 T o r r in the system the conductivity type changes, and whiskers appear on the surface. In all the experiments source and substrate temperatures were maintained constant and therefore the diffusion rate in the gas phase was also constant. The study of surface morphology of the films obtained under different conditions proves that the structural perfection is influenced mainly by the short wavelength range of the spectrum of the xenon lamp; its absence or attenuation results in film formation through lateral growth of oriented crystallites instead of continuous layer growth mechanism realized under irradiation [7]. In the absence of light, polycrystalline films are formed which can be attributed to smaller rates of surface diffusion of adatoms. P - n junctions grown by the above technique contain impurities from the substrate. This results in a spread of the p - n junction [12]. Two possible explanations can be discussed: (a) Capture of etching products by the growing surface as the latter is first exposed to intense photostimulated vapour etching. (b) Nonequilibrium vacancies appear in the layer close to the surface during its exposure to light with quantum energy exceeding the band gap of the material. These vacancies promote the diffusion of substrate atoms into the film [10]. T o reveal the origin of the substrate atoms in the layer, epitaxial PbSe layers were grown on PbSnSe substrates 500/~m thick at constant source and substrate temperatures. The growth process was carried out in three regimes (fig. 4, curves 1, 2 and 3): (1) without preliminary photo-etching of the Substrate and irradiation of the film during its cooling; (2) with preliminary photo-etching of the substrate surface and with irradiation; (3) without preliminary photo-etching of the
S.N. Maximovsky et al. / Photostimulated epitaxy of H - V I and I V - V I layers
/.l 321 Fig. 4. Sn profiles in the transition layer of PbSe/PhSn~
measured by microprobe. For details see text. substrate and with irradiadiation of the film during its cooling. T h e Sn distribution in the h e t e r o j u n c t i o n region was d e t e r m i n e d f r o m the c h a n g e of intensity of X - r a y S n L a m e a s u r e d with a microanalyser type MS-46. T h e excitation region is ~< 1 / z m and thus the distribution slope of Is, d e p e n d s mainly on the Sn distribution in the region of h e t e r o j u n c t i o n . O n e can see (fig. 4) that the width of the transition region d e p e n d s on the irradiation time of the growing film and not on the p r e s e n c e of substrate c o m p o u n d s in the gas phase. T h e r e f o r e m e c h a n i s m Co) seems to be active.
4. Conclusions It has b e e n shown that I I - V I and I V - V I films with m a x i m u m deviation f r o m stoichiometry can be g r o w n for the first time by p h o t o s t i m u l a t e d epitaxy d u e to the c h a n g e of a d s o r b t i o n p r o p e r ties of the surface. This c a n g e can be explained by the increase of sticking coefficient of the surface at any compositions of the gas phase a n d by dissociation of the v a p o r species on the substrate surface u n d e r the influence of light.
145
T h e g r o w t h process is limited by the rate of substance transport to the surface. T h e perfection of the grown layers is influenced mainly by the short-wave range of the spectrum. T h e analysis of Sn distribution in the transitional region of P b S e / P B S n S e p - n junctions shows that the width of this region d e p e n d s on the duration of the e x p o s u r e to electromagnetic radiation.
References [1] [2] [3] [4]
R.G. Friesher, J. Electrochem. Soc. 115 (1968) 401. J.S. Grossman, Patent USA, N3.200.018. S.N. Maximovsky et al., Patent USA, N4.115.163. V.P. Sasorov, Eiektrovakuumnaya Tekhnika 0glectrovacuum Technology) (Energiya Publ. House, 1967) pp. 42, 87 (Russian). [5] S.N. Maximovsky, I.P. Revocatova, V.M. Salman and M.A. Selezneva, Rev. Physique Appl. 12 (1977) 161. [6] A.S. Averushkin, O.V. Alexandrova, K.V. Kiseleva, S.N. Maksimovsky, I.P. Revokstova and E.P. Shchebnev, Neorgan. Mater. 15 (1979) 380. [7] S.N. Maximovsky and I.P. Revokatova, Elektronnaya Tekhnika, Set. 6, Materialy 4 (1979) 48. [8] M.M. Factor and S. Garret, J. Crystal Growth 9 (1971) 3. [9] S.A. Medvedev and Yu.V. Klevkov, Izv. Akad. Nauk. SSSR, Set. Neorg. Mater. 7 (1971) 753. [10] V.T. Baru and F.R. Volkenshtein, Vliyaniye Oblucheniya na Poverhnostnye Svoistva Poluprovodnikov (Radiation and Surface Phenomena in Semiconductors) (Nauka, Moscow, 1978). [11] V.D. Chesteakov, Obzory po Elektronnoi Tekhnike, Ser. Poluprovodnikovye Pribory 5 (1975) 369. [12] Yu.I. Gorina, G.A. Kaluzhnaya, K.V. Kiseleva, V.N. Salman and N.I. Strogankova, Fiz. Tekh. Poluprovodnikov 13 (1979) 305.