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The growth and photoluminescence of ZnSe on GaAs by VPE in the temperature range 300–500°C
Journal of Crystal Growth 66 (1984) 21—25 North-Holland, Amsterdam
21
THE GROWTH AND PHOTOLUMINESCENCE OF ZnSe ON GaAs BY VPE IN THE TEMPERATURE RANGE 300-500°C M. UMAR-SYED and P. LILLEY
*
Electrical Engineering Laboratories, The University, Manchester M13 9PL, UK
Received th October 1983
Blue band-edge photoluminescence at room temperature has been observed in ZnSe heteroepitaxial layers grown on GaAs at 300°C.The growth system included a large temperature gradient between the solid ZnSe source and the growth zone. A Li 2S source was used for doping the layers with suitable luminescent centres. Deep level emission, a characteristic of ZnSe, was particularly weak.
I. Introduction Despite the lack of significant progress towards high conductivity p-type ZnSe there is considerable interest in the use of this material for blue LEDs. VPE is a favoured growth technique, potentially providing for the control of conductivity and luminescence properties. A typical VPE systme [1] uses a solid ZnSe powder source and a GaAs or Ge substrate. Source temperatures above 950°C are required to give appreciable mass transport using H2 carrier gas flow. Deposition is normally restricted to the temperature range 550—800°C: the upper limit due to transport kinetics, and the lower limit due to a lack of flux (growth on the quartz reactor tube provides a large loss of flux in this temperature range). Although good quality layers can be grown, they are affected by the substrates. The elements of the substrate are incorporated as dopants in the ZnSe lattice, resulting in conductive n-type layers on GaAs and insulating layers on Ge [2]. In addition, the high substrate temperatures allow trace impurities to dominate the photoluminescence (PL) spectra. The 2 K PL spectra indicate shallow donors and shallow acceptors, whereas the room temperature spectra show a small band-edge blue emission dominated by a *
Currently seconded to the Open University, Milton Keynes, UK.
large broad-band deep-level orange/red emission [2]. Growth at lower tempertures may provide the solution to this problem. The OMCVD technique shows some progress but appears to be limited by the purity of the starting materials, as evident from band-edge PL spectra at 2 K. The purpose of this paper is to report a system designed for ZnSe growth at low temperatures by conventional VPE and the results of assessment of these layers by photoluminescence at room ternperature (RT) 77 and 2 K.
2. The growth system A schematic diagram of the experimental growth system is shown in fig. 1. The reactor and furnace tubes are fabricated from quartz. The system is designed to provide regions for three distinct functions: (a) Vaporisation of the ZnSe powder source. The temperature gradient in this region should be small to avoid nucleation on the reactor walls. (b) Growth on foreign substrates in a deposition zone with a small temperature gradient. (c) Mass transport between the above zones. A transition zone with large temperature gradient is required, linking the vaporisation and deposition zones, to minimise losses to the reactor walls (the
0022-0248/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
22
M. Umar - Syed, P. Lilley
V
c arr e r g a ~
/
Growth and photoluminescence of ZnSe on GaAs by VPE
deposition zone
~~
~
~ ~or
2
P
________
furnace
substrate
furnace heat shields
300 C
~
--
Fig. 1. A system for the low temperature growth of ZnSe by VPE.
onset of deposition in this system with suitable dilutant H flow is about 750°C). 2
The temperature profile of the system, as measured by a thermocouple in a thin quartz tube along the axis of the reactor tube, is shown in fig. 1. The profile was achieved by the use of two independent windings on separate quartz tubes. The high temperature furnace tube is shaped to provide efficient heating for the source material and radiation isolation from the lower temperature regions of the system. The problem of temperature isolation provided a further design restriction: the cross-sectional area of the reactor tube between the high temperature zone and the deposition zone (the transition region) had to be as small as possible to avoid the substrate being heated by radiation from the high temperature vaporisation chamber, being just large enough to allow a passage for the source material boat. Temperature measurements and control required thermocouples within the growth tube. Vertical quartz tubes forming parts of the growth reactor were used to house the temperature monitors. Slots were incorporated into the quartz furnace tubes to facilitate the positioning of the growth tube: the high temperature furnace contains a removable quartz plug with a heating element in series with the furnace winding, to bridge the slot. The furnace tubes are held firm by insulating supports and aluminium heat reflector plates packed at right angles to the tube axis, Ceramic wool is packed into any remaining spaces.
Table 1 Source temperature
970 C
Substrate temperature Carrier H 2 flow rate . Dilutant H 2 flow rate
250—700°C 3/min 600 cm3
Substrate material Pre-growth etch
GaAs (100) 1 H2S04 :5 H2O2:1 1-120
Source material
(60 s at 50°C) Merck ZnSe powder or
600 cm /min
AWRE laser window ZnSe
The system design provides the facility for additional Se or Zn overpressure by placing pellets of the metal in the dilutant flow tube as shown in fig. 1. Table 1 lists the growth conditions: single crystal layers have been grown in this system at temperatures in the range 700—250°C;the effects of substrate temperature on the photoluminescence of the layers will be described in the next section. 3. The effects of substrate temperature on PL Inherent changes in growth rate gives rise to ambiguity in the effects of changes in substrate temperature. To obtain layers about 1 ~tm thick. growth times varied between 20 mm at 500°Cto 3 h at 240°C.
M. U,nar - Syed, P. Lilley / Growth andphotoluminescence of ZnSe on GaAs by VPE
With substrate temperatures above 500°C the room temperature (RT) PL was dominated by deep-level emission peaking in the yellow region of the visible spectrum. Lower substrate temperatures provided a remarkable change in the RT PL spectra: the luminescence was negligible throughout the visible spectrum (the exception being a weak deep-level emission from layers grown at 250°C. Emission at 77 K was generally weak, increasing with substrate temperatures below 300°C;in addition to the usual band-edge and deep-level emissions in the 530—650 nm range, a new emission band appeared between the wavelengths 470 and 500 nm: this band was dominant for layers grown at 450°C.The origin of this new emission band is unknown. The PL spectra at 77 K (a Hg lamp was used as excitation for PL measurements at 77 and 300 K)
23
and 2 K (40 mW argon-ion laser excitation) for layers grown at 480, 370 and 300°C are shown in figs. 2 and 3. The new emission band is seen to be due to several transitions as evident from the 2 K spectrum of a layer grown at 370°C. Similar spectra were obtained from layers grown using solid crystalline source material as those from the powder source. The band-edge spectra at 2 K becomes less complicated as the growth temperature is reduced. The high resolution band-edge spectrum for a
substrate temperature 480°C
substrate temperatur
j
~ I
___
0
H
________
I
H~]
~
450 550 650 nm Fig. 2. Photoluminescence spectra at 77 K for ZnSe layers grown on GaAs at 470, 370 and 300*C (the increasing intensity at wavelengths shorter than 430 nm is stray UV excitation radiation).
370C
____
300°C
,J 440
540
640nm
Fig. 3. Photoluminescence spectra at 2 K for the same layers as in fig. 2.
M. U.’nar-Syed, P. Lilley / Growth and photoluminescence of ZnSe on GaAs by VPE
24
ties from the Li2S powder on the PL characteristics were expected to be negligible if Li was to be incorporated in ppm quantities. Before use the Li2S was baked out in flowing H2 at 600°C to remove volatile impurities. Li was expected to
444.1 rn,
Q/P series
I
440
460
480
nm
Fig. 4. The band-edge 2 K spectrum of a layer grown at 300 C.
layer grown at 300°C is shown in fig. 4: apparently a single bound excitonic transition at a wavelength associated with acceptor impurities, dominates the spectra. The line is considered to be too broad for a single exciton transition [3]: most likely, several transitions are involved and it is possible that they are due to donor impurities such as In, Ga, and Al, the emission lines (12) being shifted by strain in the epitaxial layer. It would appear that the layers are relatively free from shallow compensating impurities: the impurities are either donors or acceptors, not both types, representing a significant advance in material characteristics,
4. Doping experiments
diffuse readily into the quartz reactor tube: the loss of Li flux was minimised by placing the quartz boat loaded with Li2S within the main reactor chamber near to the substrate at the required dopant temperature. Flux from the dopant source was varied by its temperature (400—600°C) and by using different diameters of capillary tubing for the Li2S boat. This technique provided immediate success: the room temperature PL spectra indicated band-edge emission with comparatively negligible deep-level emission (fig. 5). As far as the authors are aware. this represents a significant advance in ZnSe for LED devices. However, the luminescence shown in fig. 5 is not strong enough for such applications (the sensitivity of the human eye is weak in the blue region of the visible spectrum). The 2 K PL spectra of such layers indicate particularly strong exciton emission; shallow donor—acceptor pair emission is comparatively weak, and deep level emission negligible. The strong exciton peak is at a wavelength similar to the single line for an undoped (unintentionally doped) layer as in fig. 4. A considerable number of the growth system variables were changed in an attempt to increase the blue emission, but with little success; dopant temperature.
The 2 K PL spectra of layers grown at 300°C suggest that this growth temperature may be suitable for controlled doping to produce well behaved semiconducting material; such material requires negligible concentration of compensating and deep levels; subsequent luminescence will then be at band-edge wavelengths. The dopant should be an element involved in the band-edge spectrum of fig. 3c. The transitions may be due to acceptors (Li, Na) or donors (In, Al Ga); Li2S and In were chosen as materials for these experiments.
uv
exciiation
/
4.1. Li doping
temperature.
zinc
and
room
temperature
band edge emission
A
~
450
The Li2S used was not as pure as electronic grade materials: the effects of sulphur and impuri-
substrate
selenium overpressure, and capillary size were all varied. There are several possible reasons for this: (1) the solubility for Li in ZnSe had been reached:
550
650
nm
Fig. 5. Room temperature photoluminescence from a ZnSe layer grown at 300°Cdoped from a Li,S source.
M. Umar - Syed, P. Lilley / Growth and photoluminescence of ZnSe on GaAs by VPE
(2)
a surface barrier forms on the Li2S source, preventing an increase in sublimation or reaction rate; (3) similar arguments to (1) and (2) would apply to a donor impurity from the Li 2~ (4) a donor impurity from sources other than the Li2S is present in the system at these low temperatures, and its incorporation mechanism is enhanced by the Li2S. 4.2. Indium doping Indium was incorporated into the gas stream at 550°Cat the opposite end of the furnace (fig. 1) using an elemental source. The RT PL spectrum included a blue peak of strength similar to the layers doped from a Li2S source, and in addition a broad deep-level band from yellow to red, the latter a characteristic of impure ZnSe (indium may be involved with emission in the region of 620 nm, whereas Cu is generally associated with the broad emissions at about 550 nm). The 2 K PL spectrum is similar to the Li2S doped layer spectra with strong exciton emission
25
5. Conclusions The design of a conventional VPE growth systern optimised for ZnSe growth at low temperatures has been successful: single crystal films have been obtained at temperatures as low as 250°C from a solid source at 970°C.As a consequence, growth conditions have been found suitable for introducing dopant elements into ZnSe, resulting in near band-edge blue photoluminescence at room temperature without deep-level emission. Photoluminescence at 2 K indicates the transitions are either shallow donors or shallow acceptors; the inherent stress in the heteroepitaxial layers causes shifting of the emission wavelengths, and ambiguity in interpretation.
Acknowledgements The authors are grateful to Dr. J.E. Nicholls, Department of Physics, Hull University, for the use of his 2 K photoluminescence equipment.
at the same wavelength. Several conclusions are
possible: (1) In is incorporated in the lattice; (2) group I trace impurities from the indium source
References
are incorporated, the In condensing out of the gas
[2] P. Lilley, MR. Czerniak, J.E. Nicholls and J.J. Davies, J. Crystal growth 59 (1982) 161. [3] P.J. Dean, RSRE, UK, private communication.
stream at high temperatures, perhaps in the form of InSe.
[11P. Lilley, J. Crystal Growth 44 (1978) 452.
21
THE GROWTH AND PHOTOLUMINESCENCE OF ZnSe ON GaAs BY VPE IN THE TEMPERATURE RANGE 300-500°C M. UMAR-SYED and P. LILLEY
*
Electrical Engineering Laboratories, The University, Manchester M13 9PL, UK
Received th October 1983
Blue band-edge photoluminescence at room temperature has been observed in ZnSe heteroepitaxial layers grown on GaAs at 300°C.The growth system included a large temperature gradient between the solid ZnSe source and the growth zone. A Li 2S source was used for doping the layers with suitable luminescent centres. Deep level emission, a characteristic of ZnSe, was particularly weak.
I. Introduction Despite the lack of significant progress towards high conductivity p-type ZnSe there is considerable interest in the use of this material for blue LEDs. VPE is a favoured growth technique, potentially providing for the control of conductivity and luminescence properties. A typical VPE systme [1] uses a solid ZnSe powder source and a GaAs or Ge substrate. Source temperatures above 950°C are required to give appreciable mass transport using H2 carrier gas flow. Deposition is normally restricted to the temperature range 550—800°C: the upper limit due to transport kinetics, and the lower limit due to a lack of flux (growth on the quartz reactor tube provides a large loss of flux in this temperature range). Although good quality layers can be grown, they are affected by the substrates. The elements of the substrate are incorporated as dopants in the ZnSe lattice, resulting in conductive n-type layers on GaAs and insulating layers on Ge [2]. In addition, the high substrate temperatures allow trace impurities to dominate the photoluminescence (PL) spectra. The 2 K PL spectra indicate shallow donors and shallow acceptors, whereas the room temperature spectra show a small band-edge blue emission dominated by a *
Currently seconded to the Open University, Milton Keynes, UK.
large broad-band deep-level orange/red emission [2]. Growth at lower tempertures may provide the solution to this problem. The OMCVD technique shows some progress but appears to be limited by the purity of the starting materials, as evident from band-edge PL spectra at 2 K. The purpose of this paper is to report a system designed for ZnSe growth at low temperatures by conventional VPE and the results of assessment of these layers by photoluminescence at room ternperature (RT) 77 and 2 K.
2. The growth system A schematic diagram of the experimental growth system is shown in fig. 1. The reactor and furnace tubes are fabricated from quartz. The system is designed to provide regions for three distinct functions: (a) Vaporisation of the ZnSe powder source. The temperature gradient in this region should be small to avoid nucleation on the reactor walls. (b) Growth on foreign substrates in a deposition zone with a small temperature gradient. (c) Mass transport between the above zones. A transition zone with large temperature gradient is required, linking the vaporisation and deposition zones, to minimise losses to the reactor walls (the
0022-0248/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
22
M. Umar - Syed, P. Lilley
V
c arr e r g a ~
/
Growth and photoluminescence of ZnSe on GaAs by VPE
deposition zone
~~
~
~ ~or
2
P
________
furnace
substrate
furnace heat shields
300 C
~
--
Fig. 1. A system for the low temperature growth of ZnSe by VPE.
onset of deposition in this system with suitable dilutant H flow is about 750°C). 2
The temperature profile of the system, as measured by a thermocouple in a thin quartz tube along the axis of the reactor tube, is shown in fig. 1. The profile was achieved by the use of two independent windings on separate quartz tubes. The high temperature furnace tube is shaped to provide efficient heating for the source material and radiation isolation from the lower temperature regions of the system. The problem of temperature isolation provided a further design restriction: the cross-sectional area of the reactor tube between the high temperature zone and the deposition zone (the transition region) had to be as small as possible to avoid the substrate being heated by radiation from the high temperature vaporisation chamber, being just large enough to allow a passage for the source material boat. Temperature measurements and control required thermocouples within the growth tube. Vertical quartz tubes forming parts of the growth reactor were used to house the temperature monitors. Slots were incorporated into the quartz furnace tubes to facilitate the positioning of the growth tube: the high temperature furnace contains a removable quartz plug with a heating element in series with the furnace winding, to bridge the slot. The furnace tubes are held firm by insulating supports and aluminium heat reflector plates packed at right angles to the tube axis, Ceramic wool is packed into any remaining spaces.
Table 1 Source temperature
970 C
Substrate temperature Carrier H 2 flow rate . Dilutant H 2 flow rate
250—700°C 3/min 600 cm3
Substrate material Pre-growth etch
GaAs (100) 1 H2S04 :5 H2O2:1 1-120
Source material
(60 s at 50°C) Merck ZnSe powder or
600 cm /min
AWRE laser window ZnSe
The system design provides the facility for additional Se or Zn overpressure by placing pellets of the metal in the dilutant flow tube as shown in fig. 1. Table 1 lists the growth conditions: single crystal layers have been grown in this system at temperatures in the range 700—250°C;the effects of substrate temperature on the photoluminescence of the layers will be described in the next section. 3. The effects of substrate temperature on PL Inherent changes in growth rate gives rise to ambiguity in the effects of changes in substrate temperature. To obtain layers about 1 ~tm thick. growth times varied between 20 mm at 500°Cto 3 h at 240°C.
M. U,nar - Syed, P. Lilley / Growth andphotoluminescence of ZnSe on GaAs by VPE
With substrate temperatures above 500°C the room temperature (RT) PL was dominated by deep-level emission peaking in the yellow region of the visible spectrum. Lower substrate temperatures provided a remarkable change in the RT PL spectra: the luminescence was negligible throughout the visible spectrum (the exception being a weak deep-level emission from layers grown at 250°C. Emission at 77 K was generally weak, increasing with substrate temperatures below 300°C;in addition to the usual band-edge and deep-level emissions in the 530—650 nm range, a new emission band appeared between the wavelengths 470 and 500 nm: this band was dominant for layers grown at 450°C.The origin of this new emission band is unknown. The PL spectra at 77 K (a Hg lamp was used as excitation for PL measurements at 77 and 300 K)
23
and 2 K (40 mW argon-ion laser excitation) for layers grown at 480, 370 and 300°C are shown in figs. 2 and 3. The new emission band is seen to be due to several transitions as evident from the 2 K spectrum of a layer grown at 370°C. Similar spectra were obtained from layers grown using solid crystalline source material as those from the powder source. The band-edge spectra at 2 K becomes less complicated as the growth temperature is reduced. The high resolution band-edge spectrum for a
substrate temperature 480°C
substrate temperatur
j
~ I
___
0
H
________
I
H~]
~
450 550 650 nm Fig. 2. Photoluminescence spectra at 77 K for ZnSe layers grown on GaAs at 470, 370 and 300*C (the increasing intensity at wavelengths shorter than 430 nm is stray UV excitation radiation).
370C
____
300°C
,J 440
540
640nm
Fig. 3. Photoluminescence spectra at 2 K for the same layers as in fig. 2.
M. U.’nar-Syed, P. Lilley / Growth and photoluminescence of ZnSe on GaAs by VPE
24
ties from the Li2S powder on the PL characteristics were expected to be negligible if Li was to be incorporated in ppm quantities. Before use the Li2S was baked out in flowing H2 at 600°C to remove volatile impurities. Li was expected to
444.1 rn,
Q/P series
I
440
460
480
nm
Fig. 4. The band-edge 2 K spectrum of a layer grown at 300 C.
layer grown at 300°C is shown in fig. 4: apparently a single bound excitonic transition at a wavelength associated with acceptor impurities, dominates the spectra. The line is considered to be too broad for a single exciton transition [3]: most likely, several transitions are involved and it is possible that they are due to donor impurities such as In, Ga, and Al, the emission lines (12) being shifted by strain in the epitaxial layer. It would appear that the layers are relatively free from shallow compensating impurities: the impurities are either donors or acceptors, not both types, representing a significant advance in material characteristics,
4. Doping experiments
diffuse readily into the quartz reactor tube: the loss of Li flux was minimised by placing the quartz boat loaded with Li2S within the main reactor chamber near to the substrate at the required dopant temperature. Flux from the dopant source was varied by its temperature (400—600°C) and by using different diameters of capillary tubing for the Li2S boat. This technique provided immediate success: the room temperature PL spectra indicated band-edge emission with comparatively negligible deep-level emission (fig. 5). As far as the authors are aware. this represents a significant advance in ZnSe for LED devices. However, the luminescence shown in fig. 5 is not strong enough for such applications (the sensitivity of the human eye is weak in the blue region of the visible spectrum). The 2 K PL spectra of such layers indicate particularly strong exciton emission; shallow donor—acceptor pair emission is comparatively weak, and deep level emission negligible. The strong exciton peak is at a wavelength similar to the single line for an undoped (unintentionally doped) layer as in fig. 4. A considerable number of the growth system variables were changed in an attempt to increase the blue emission, but with little success; dopant temperature.
The 2 K PL spectra of layers grown at 300°C suggest that this growth temperature may be suitable for controlled doping to produce well behaved semiconducting material; such material requires negligible concentration of compensating and deep levels; subsequent luminescence will then be at band-edge wavelengths. The dopant should be an element involved in the band-edge spectrum of fig. 3c. The transitions may be due to acceptors (Li, Na) or donors (In, Al Ga); Li2S and In were chosen as materials for these experiments.
uv
exciiation
/
4.1. Li doping
temperature.
zinc
and
room
temperature
band edge emission
A
~
450
The Li2S used was not as pure as electronic grade materials: the effects of sulphur and impuri-
substrate
selenium overpressure, and capillary size were all varied. There are several possible reasons for this: (1) the solubility for Li in ZnSe had been reached:
550
650
nm
Fig. 5. Room temperature photoluminescence from a ZnSe layer grown at 300°Cdoped from a Li,S source.
M. Umar - Syed, P. Lilley / Growth and photoluminescence of ZnSe on GaAs by VPE
(2)
a surface barrier forms on the Li2S source, preventing an increase in sublimation or reaction rate; (3) similar arguments to (1) and (2) would apply to a donor impurity from the Li 2~ (4) a donor impurity from sources other than the Li2S is present in the system at these low temperatures, and its incorporation mechanism is enhanced by the Li2S. 4.2. Indium doping Indium was incorporated into the gas stream at 550°Cat the opposite end of the furnace (fig. 1) using an elemental source. The RT PL spectrum included a blue peak of strength similar to the layers doped from a Li2S source, and in addition a broad deep-level band from yellow to red, the latter a characteristic of impure ZnSe (indium may be involved with emission in the region of 620 nm, whereas Cu is generally associated with the broad emissions at about 550 nm). The 2 K PL spectrum is similar to the Li2S doped layer spectra with strong exciton emission
25
5. Conclusions The design of a conventional VPE growth systern optimised for ZnSe growth at low temperatures has been successful: single crystal films have been obtained at temperatures as low as 250°C from a solid source at 970°C.As a consequence, growth conditions have been found suitable for introducing dopant elements into ZnSe, resulting in near band-edge blue photoluminescence at room temperature without deep-level emission. Photoluminescence at 2 K indicates the transitions are either shallow donors or shallow acceptors; the inherent stress in the heteroepitaxial layers causes shifting of the emission wavelengths, and ambiguity in interpretation.
Acknowledgements The authors are grateful to Dr. J.E. Nicholls, Department of Physics, Hull University, for the use of his 2 K photoluminescence equipment.
at the same wavelength. Several conclusions are
possible: (1) In is incorporated in the lattice; (2) group I trace impurities from the indium source
References
are incorporated, the In condensing out of the gas
[2] P. Lilley, MR. Czerniak, J.E. Nicholls and J.J. Davies, J. Crystal growth 59 (1982) 161. [3] P.J. Dean, RSRE, UK, private communication.
stream at high temperatures, perhaps in the form of InSe.
[11P. Lilley, J. Crystal Growth 44 (1978) 452.