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Efficient photoemission from GaAs epitaxial layers
Solid State Communications,
Vol. 7, pp.615—617, 1969.
Pergamon Press.
Printed in Great Britain
EFFICIENT PHOTOEMISSION FROM GaAs EPITAXIAL LAYERS S. Garbe and G. Frank Philips Zentrallaboratorium GmbH Laboratorium Aachen (Received 10 February 1969 by G.W. Rathenau)
A method is described for obtaining clean GaAs surfaces by low energy argon ion bombardment of epitaxially grown GaAs layers. After annealing and depositing a cesium oxide layer a maximum white light (2800°K)sensitivity of 423~A/lmwas obtained.
SINCE Scheer and van Laar’s’ report on Cs—GaAs photoemitters the white light sensitivities of GaAs crystals cleaved and cesiated in ultrahigh vacuum have been improved from 500~A/lm to 9O0~A/lm2and even to 1100~A/lm3 by using crystals with a long electron diffusion— recombination length in the bulk and an optimum cesium oxide coverage to provide the required low work function. Nevertheless, there has been limited success in obtaining similar results from air-cleaved crystals4 or from epitaxial layers of GaAs. Steinberg5 described an evaporation technique to grow epitaxially GaAs on doped substrates in ultrahigh vacuum. Photoemission sensitivities of 220~A/lmwere reported, but the samples had not been handled in air, In this work a modified sputter technique has been used for cleaning to obtain photoemission from epitaxial layers. These layers were grown in an open tube by Ga/AsC1 3/H2 vapour transport on (110)-oriented undoped or n-type GaAs substrates. The techniques of ‘growing zinc-doped layers were similar to those described in. 6 Electron diffraction analysis indicated the layers to be monocrystalline. * After deposition
of the epitaxial layers the samples were removed from the system, attached to the sample holder and degreased with boiling trichloroethane. No etching was applied to the surfaces. Layer thickness of the samples was between 1 and 10g. Low bake out temperatures (2hr at 250°C and l5hr at 200°C)were applied to avoid loss of Zn and excessive oxidation of the layers. After cooling down, the pressure in the system for photoemission measurement was in the 10_b Torr range. The argon used for sputtering was cleaned by means of titanium gettering in a separate u.h.v. system, connected with the main system by a valve. A simple ion gun with hot filament was used to produce argon ions of variable energy. Sputtering was first performed at 150 V energy with 105—i0’ amp/cm2 for 1 hr, removing between 100 and 1000 molecular layers of the sample. Then the energy of the bombarding ions was lowered to 50 V with about the same amount of ions bombarding the sample. During this stage the sample was already heated for 10 mm to about 250°C, sufficient to remove the damage of 7 the surface left by the ion bombardment. After the ion bombardment the sample was annealed for 10 mm at the same temperature. It seems that almost all the damage produced by the initial 150 eV ion bombardment can be removed
____________
*We are indebted to J.Ungelenk for making the electron diffraction analysis. 615
616
EFFICIENT PHOTOEMISSION FROM GaAs EPITAXIAL LAYERS
under these annealing conditions. While the depth of 150 eV ion bombardment damage can be estimated to be of the order of 10 atomic layers 8 the threshold energy for surface roughening may be of the order of 20—30eV. Similar values were found for tungsten’ bomt~ardedby Cs1 ions and for the first appearance of spots, loops, and short lines on gold crystals bombarded by argon ions. ‘° The lowering of the bombarding voltage after removing the contaminated layer serves to remove most of the damaged layer, while the remaining surface damage can be removed by heating. In some cases this annealing was done at 150°Cfor 1 hr in the presence of cesium vapour. (It turned out, however, that the presence of Cs vapour is not a necessary condition, provided the pressure is low enough.) An optimum Cs,O coverage was applied. The Cs source consisted of an Al 203-covered tungsten heater, impregnated by cesium carbonate. During d~ gassing the carbonate is decomposed and a Cs’ ion current of about 10”~amp can be drawn at 1O~”° Torr. The ion emission of this ion emitter is entirely due to Cs ~ ~ while also some atomic Cs is evaporated well. A coverage of 3 while x 10 ‘~ 2 gives a as work function of 1.4eV, ions/cm the formation of three or four successive Cs,O layers lowers the work function to 1.2 eV. The oxygen influx of each alternate coverage is 2.5 x 10° Torr sec.
100
(h10)~0~crys~~~ ‘~‘~
/
~.ayer
ii~
--~--~----~-~
—
..-
~------~
~
2.0
25 —
E (eV)
F’IG. 1. Quantum yield per incident quantum 77 for epitaxial GaAs layer (corresponding to 423~A/lm)and for a cleaved crystal (575~A/lm)
/ -
~ ~
=
These results show that the optimum possible conditions have by no means been reached, but there is at least an improvement of a factor of 2 as compared with previously reported photoemission from epitaxial layers of GaAs using other techniques.
i& ~ 0
~----
(11O)epitoxiot
~
The white light sensitivity for a Zn-doped layer (p 1.5 x 10’s cm3) was 423~A/lm.The spectral yield (Fig.1) was used to evaluate escape probability and escape lengths ~23(Fig.2). The absorption data of Kudman and SeideI13 were used in the region of the fundamental absorption edge. The escape depth for the r-electrons was 1.45 x 10~cm and and the probability of escape was 0.11, whereas the escape depth for the X-electrons was 1 x 10~ cm with a probability of 0.47. An escape probalility of 0.18 was obtained using the same techniques with an air-cleaved crystal of a doping concentration of 5.1 x 1019 cm’3.
Vol. 7, No. 8
/ /~
/
~—H.
1
-
/ __________________
-1.0 Lr
/~
/1 (
—
~o
to 1/~ (10 cm)
FIG. 2. Reciprocal value of quantum yield per
absorbed quantum 1/Y vs. reciprocal absorption coefficient 1/a for epitaxial GaAs layer, covered by Cs,O. The intercept of the abscissa gives the escape depth L while the intercept at the ordinate gives the reciprocal of the escape probability 1/P.
Vol. 7, No. 8
EFFICIENT PHOTOEMISSION FROM GaAs EPITAXIAL LAYERS
617
REFERENCES 1. 2.
SCHEER J.J., VAN LAAR J., Solid State Commun. 3, 189 (1965). UEBBING J.J., BELL R., Proc. I.E.E.E. 56, 1624 (1968).
3.
GARBE S., to be published.
4.
TURNBULL A.A., EVANS G.B., Br. J. app!. Phys. Ser. 2, 1, 155 (1968).
5. 6.
STEINBERG R.F., Appi. Phys. Lett. 12, 63 (1968). CONRAD R.W., HAISTY R.W., J. electrochem. Soc. 113, 199 (1966).
7.
ANDERSON G.S., J. app!. Phys. 37, 3455 (1966).
8.
MACDONALD R.J., HANEMAN D., J. app!. Phys. 37, 1609 (1966).
9.
STRAYER R.W., COOPER E.C., SWANSON L.W., Paper presented at the 14th Field Emission Symp., Washington 1967.
10.
OGILVIE G.J., SANDERS J.V., THOMSON A.A., Phys. Chem. 24, 247 (1963).
11.
KLOPFER A., private communication.
12.
SPICER W.E., Quarterly Report Nr. 8, Stanford University, Contract Nr. DA—44—009—AMC—1474, April 1968. KUDMAN I., SEIDEL T., J. app!. Phys. 33, 771 (1962).
13.
Es wird eine Methode beschrieben, urn saubere GaAs-Oberflächen durch Zerstäuben von epitaxial gewachsenen GaAs-Schichten mit niederenergetischen Argon-lonen zu erhalten. Nach dern Tempera und Aufbringen einer Cãsiurnoxyd-Schicht wurde eine maximale Empfindlichkeit von 423~A/lmfür weifies Licht (2800°K)erreicht.
Vol. 7, pp.615—617, 1969.
Pergamon Press.
Printed in Great Britain
EFFICIENT PHOTOEMISSION FROM GaAs EPITAXIAL LAYERS S. Garbe and G. Frank Philips Zentrallaboratorium GmbH Laboratorium Aachen (Received 10 February 1969 by G.W. Rathenau)
A method is described for obtaining clean GaAs surfaces by low energy argon ion bombardment of epitaxially grown GaAs layers. After annealing and depositing a cesium oxide layer a maximum white light (2800°K)sensitivity of 423~A/lmwas obtained.
SINCE Scheer and van Laar’s’ report on Cs—GaAs photoemitters the white light sensitivities of GaAs crystals cleaved and cesiated in ultrahigh vacuum have been improved from 500~A/lm to 9O0~A/lm2and even to 1100~A/lm3 by using crystals with a long electron diffusion— recombination length in the bulk and an optimum cesium oxide coverage to provide the required low work function. Nevertheless, there has been limited success in obtaining similar results from air-cleaved crystals4 or from epitaxial layers of GaAs. Steinberg5 described an evaporation technique to grow epitaxially GaAs on doped substrates in ultrahigh vacuum. Photoemission sensitivities of 220~A/lmwere reported, but the samples had not been handled in air, In this work a modified sputter technique has been used for cleaning to obtain photoemission from epitaxial layers. These layers were grown in an open tube by Ga/AsC1 3/H2 vapour transport on (110)-oriented undoped or n-type GaAs substrates. The techniques of ‘growing zinc-doped layers were similar to those described in. 6 Electron diffraction analysis indicated the layers to be monocrystalline. * After deposition
of the epitaxial layers the samples were removed from the system, attached to the sample holder and degreased with boiling trichloroethane. No etching was applied to the surfaces. Layer thickness of the samples was between 1 and 10g. Low bake out temperatures (2hr at 250°C and l5hr at 200°C)were applied to avoid loss of Zn and excessive oxidation of the layers. After cooling down, the pressure in the system for photoemission measurement was in the 10_b Torr range. The argon used for sputtering was cleaned by means of titanium gettering in a separate u.h.v. system, connected with the main system by a valve. A simple ion gun with hot filament was used to produce argon ions of variable energy. Sputtering was first performed at 150 V energy with 105—i0’ amp/cm2 for 1 hr, removing between 100 and 1000 molecular layers of the sample. Then the energy of the bombarding ions was lowered to 50 V with about the same amount of ions bombarding the sample. During this stage the sample was already heated for 10 mm to about 250°C, sufficient to remove the damage of 7 the surface left by the ion bombardment. After the ion bombardment the sample was annealed for 10 mm at the same temperature. It seems that almost all the damage produced by the initial 150 eV ion bombardment can be removed
____________
*We are indebted to J.Ungelenk for making the electron diffraction analysis. 615
616
EFFICIENT PHOTOEMISSION FROM GaAs EPITAXIAL LAYERS
under these annealing conditions. While the depth of 150 eV ion bombardment damage can be estimated to be of the order of 10 atomic layers 8 the threshold energy for surface roughening may be of the order of 20—30eV. Similar values were found for tungsten’ bomt~ardedby Cs1 ions and for the first appearance of spots, loops, and short lines on gold crystals bombarded by argon ions. ‘° The lowering of the bombarding voltage after removing the contaminated layer serves to remove most of the damaged layer, while the remaining surface damage can be removed by heating. In some cases this annealing was done at 150°Cfor 1 hr in the presence of cesium vapour. (It turned out, however, that the presence of Cs vapour is not a necessary condition, provided the pressure is low enough.) An optimum Cs,O coverage was applied. The Cs source consisted of an Al 203-covered tungsten heater, impregnated by cesium carbonate. During d~ gassing the carbonate is decomposed and a Cs’ ion current of about 10”~amp can be drawn at 1O~”° Torr. The ion emission of this ion emitter is entirely due to Cs ~ ~ while also some atomic Cs is evaporated well. A coverage of 3 while x 10 ‘~ 2 gives a as work function of 1.4eV, ions/cm the formation of three or four successive Cs,O layers lowers the work function to 1.2 eV. The oxygen influx of each alternate coverage is 2.5 x 10° Torr sec.
100
(h10)~0~crys~~~ ‘~‘~
/
~.ayer
ii~
--~--~----~-~
—
..-
~------~
~
2.0
25 —
E (eV)
F’IG. 1. Quantum yield per incident quantum 77 for epitaxial GaAs layer (corresponding to 423~A/lm)and for a cleaved crystal (575~A/lm)
/ -
~ ~
=
These results show that the optimum possible conditions have by no means been reached, but there is at least an improvement of a factor of 2 as compared with previously reported photoemission from epitaxial layers of GaAs using other techniques.
i& ~ 0
~----
(11O)epitoxiot
~
The white light sensitivity for a Zn-doped layer (p 1.5 x 10’s cm3) was 423~A/lm.The spectral yield (Fig.1) was used to evaluate escape probability and escape lengths ~23(Fig.2). The absorption data of Kudman and SeideI13 were used in the region of the fundamental absorption edge. The escape depth for the r-electrons was 1.45 x 10~cm and and the probability of escape was 0.11, whereas the escape depth for the X-electrons was 1 x 10~ cm with a probability of 0.47. An escape probalility of 0.18 was obtained using the same techniques with an air-cleaved crystal of a doping concentration of 5.1 x 1019 cm’3.
Vol. 7, No. 8
/ /~
/
~—H.
1
-
/ __________________
-1.0 Lr
/~
/1 (
—
~o
to 1/~ (10 cm)
FIG. 2. Reciprocal value of quantum yield per
absorbed quantum 1/Y vs. reciprocal absorption coefficient 1/a for epitaxial GaAs layer, covered by Cs,O. The intercept of the abscissa gives the escape depth L while the intercept at the ordinate gives the reciprocal of the escape probability 1/P.
Vol. 7, No. 8
EFFICIENT PHOTOEMISSION FROM GaAs EPITAXIAL LAYERS
617
REFERENCES 1. 2.
SCHEER J.J., VAN LAAR J., Solid State Commun. 3, 189 (1965). UEBBING J.J., BELL R., Proc. I.E.E.E. 56, 1624 (1968).
3.
GARBE S., to be published.
4.
TURNBULL A.A., EVANS G.B., Br. J. app!. Phys. Ser. 2, 1, 155 (1968).
5. 6.
STEINBERG R.F., Appi. Phys. Lett. 12, 63 (1968). CONRAD R.W., HAISTY R.W., J. electrochem. Soc. 113, 199 (1966).
7.
ANDERSON G.S., J. app!. Phys. 37, 3455 (1966).
8.
MACDONALD R.J., HANEMAN D., J. app!. Phys. 37, 1609 (1966).
9.
STRAYER R.W., COOPER E.C., SWANSON L.W., Paper presented at the 14th Field Emission Symp., Washington 1967.
10.
OGILVIE G.J., SANDERS J.V., THOMSON A.A., Phys. Chem. 24, 247 (1963).
11.
KLOPFER A., private communication.
12.
SPICER W.E., Quarterly Report Nr. 8, Stanford University, Contract Nr. DA—44—009—AMC—1474, April 1968. KUDMAN I., SEIDEL T., J. app!. Phys. 33, 771 (1962).
13.
Es wird eine Methode beschrieben, urn saubere GaAs-Oberflächen durch Zerstäuben von epitaxial gewachsenen GaAs-Schichten mit niederenergetischen Argon-lonen zu erhalten. Nach dern Tempera und Aufbringen einer Cãsiurnoxyd-Schicht wurde eine maximale Empfindlichkeit von 423~A/lmfür weifies Licht (2800°K)erreicht.