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Semiconducting diamonds made in the USSR
Diamond and Related Materials, 1 (1992) 705 709 Elsevier Science Publishers RV.. Amsterdam
705
Semiconducting diamonds made in the USSR A. E. Alexenko and B. V. Spitsyn Institute q/Physical Chemistry ol Russian Academy of Sciences, Leninsk y Prospect 31, Moscow 117915 ~Russia )
Abstract p- and n-type diamond films (DF) were grown by vapour chemical transport method and fiom DC arc discharge plasma. Maximum contents of doping elements in epitaxial DF (EDF) were 2.5, 1 and 0.02 wt% for B-, P- and S-doped EDF, respectively. Specific resistances of the EDF were 10 3, l01 and 103 (2 cm, respectively. The doping changes the growth rate and chemical properties of EDF. Doped EDF were investigated by spark mass-spectrometry, X-ray spectrometry, Rutherford backscattering and Hall-effect measurements. Electroabsorption and cathodoluminescence spectra were studied. DF with a highly distorted structure and dislocation p-type conductivity were grown from glow discharge plasma. B- and As-doped I mm size crystals were obtained by an ultrahigh-pressure method. As produces multicharged donor centres with two energy levels. The crystals are prospective for sensors. Doping of diamond with B, C, P, Sb and Li was realised by ion implantation. The best p- and n-type layers doped with B and Li have carrier mobility of about 1000 cm2/V s. Acceptor dislocation centres can also be created by plastic deformation of diamond crystals. Specific resistance of such crystals can fall by up to 102 Q cm. Combining the above-mentioned methods seems to be thc most prospective way.
1. Introduction
Diamond is considered to be the most prospective material for fabrication of powerful compact electronic and optoelectronic devices which can operate under extreme conditions. There exist four methods for the preparation of semiconducting diamonds for this purpose: doping of diamond crystals during their growth under ultra-high pressure, doping of diamond thin films during their growth from vapour phase, doping of diamond by ion implantation and creation of electrically active defects in diamond by its plastic deformation. It seems the vapour phase growth method has some advantages over the ultra-high-pressure one, because it is carried out under more gentle and controllable conditions and enables the production of a material with assigned properties, configuration and surface morphology. The diamond films (DF) synthesised from vapour phase can also be a starting material with reproducible properties for doping by ion implantation. So, this paper is mainly devoted to the results in semiconducting DF, obtained in the USSR at the Institute of Physical Chemistry of USSR Academy of Sciences. Some results obtained by three other methods of preparing semiconducting diamonds are also mentioned.
and by crystallisation from DC-discharge plasma [2]. In the former method a closed sandwich system filled with pure hydrogen at a pressure of 12 torr (1596 Pa) was used (Fig. l(a)). The graphite plate at a temperature of 2000-2500 C was the source of carbon. In the latter method, an arc discharge between the refractory cathode and a substrate or substrate holder in gas mixture was used (Fig. l(b)). The current density was 2-4 A/cm z. The mixture of hydrogen (97%) and methane (3%) was passed through the reactor. Total gas flow was 2000 sccm/h and pressure in the reactor was 100 tort (13.30 kPa). Natural diamond and silicon wafers
(a)
(b)
g~aphi t e source of carbon (2000_2500Oc)
?
~jcavhode diamond
substrate
f./.,/
U / \
~
~ta+C~~
2. Experimental details optical pyrometry
DF were grown by the method of high-temperature, high-gradient chemical vapour transport of carbon [1]
0925 9635.9255.00
~' Elsevier Science Publishers B.V. All rights reserved
Fig. I. Schematic diagram of DF crystallisation (a) by the chemical transport method, and (b) from DC arc discharge plasma.
706
A, E. Alexenko, B. V, Spitsyn / Semiconducting diamonds made in the USSR
were the substrates in both methods. Substrate temperature was measured with an optical pyrometer through a hole in the substrate holder. In the case of diamond substrate, its face (opposite the growing one) was covered with an opaque carbon layer. The first successful attempts of doping DF with boron, phosphorus and sulphur were made in 1971-1972. To prepare p-type DF, boron carbide, B4C, and neocarborane, CzBloHa2, were used as doping agents. The former was placed at the surface of the source of carbon in the method of chemical transport (Fig. l(a)) and the latter was in a special crucible near the substrate. The crucible was equipped with a heater and a thermocouple. To prepare n-type DF, elementary phosphorus and sulphur placed in similar crucibles were used. The crucible temperature ranged between 50 and 300 °C.
1o 6
3
1°5
g
~
~o 3
"4
I oo
7;0
960 Io;o Ioo T
(oc)
Fig. 2. Resistance of epitaxial D F and boron contents in the DF as a function of crystallisation temperature.
3. Results and discussion
The growth rate of non-doped DF in chemical transport method was about 0.3 gm/h. While doping with boron it sometimes increased up to 0.35-0.4 gin/h, perhaps because of forming carborans which could transport both boron and carbon. While doping with phosphorus and sulphur their partial pressures were comparable with hydrocarbons and the growth rate usually was falling by up to 0.2-0.1 gin/h, probably, due to dilution and/or deactivation of the growth agents. DF grown on diamond substrates were 1-2 gm thick. Their epitaxial nature was shown by reflective electron diffraction. Composition of the epitaxial DF (EDF) was studied by spark mass-spectrometry [3, 4]. Boron-doped EDF were also studied by X-ray spectrometry [4] and secondary ion mass-spectrometry [5]. It has been established that the maximum content of boron in EDF grown on (111) substrates is about 2.5 wt%, those of phosphorus and sulphur are 1 and 0.02 wt%, respectively. Doping of EDF on (110) and (100) substrates proved to be much less effective, so further study was carried out with the (111) substrates only. The boron contents in EDF and their resistance depended on the crystallisation temperature (Fig. 2). It seems that there forms a non-equilibrium solid solution of boron in diamond at temperatures below about 800 °C. At higher temperatures the forming process becomes more equilibrium and the boron contents fall [4]. The Rutherford backscattering study shows that when boron content does not exceed 0.1 wt%, boron atoms mainly occupy diamond lattice points in EDF. According to X-ray topography data, lattice parameters of such EDF are diminished by 0.0006 A in comparison with non-doped EDF. At higher contents, single boron atoms can enter tetrahedral interstices of the lattice. This causes
an expansion of the diamond lattice. At boron contents of I wt%, boron atoms mainly (80-90% of them) occupy the interstices and the lattice parameter approaches to that of non-doped diamond [4]. p-Type conductivity of boron-doped EDF and n-type conductivity of phosphorus- and sulphur-doped EDF were demonstrated by a sign of thermoelectromotive force and (for boron-doped EDF) by Hall-effect measurements. Specimens for Hall-effect measurements [5] were prepared using masking during the growth process so that EDF had the form of a stripe with 4:1 or 5:1 length:width ratio. Concentration of boron in these EDF was about 1019/cm3. Carrier concentration proved to be about 1016/cm3 at room temperature and its activation energy was 0.3 eV. Carrier mobility was 2 8 cmZ/V s at room temperature. The mobility rises with the measurement temperature up to 10-15 cmZ/V s and holds this value in the range of 500-700 K (Fig. 3). It is suggested that there exists hopping conductivity at low temperatures and the rise of mobility with temperature can be due to decreasing the contribution of the hopping conductivity. Low mobility values may be related to high contents of boron or structural defects (e.g. microtwins [-6]) in the EDF. Specific resistance of p-type EDF was measured using two gold electrodes, and that of n-type EDF was measured using two gallium ones which proved to have a nearly ohmic character. Minimum specific resistances of EDF a r e 10 - 3 , 101 and 103 ~ c m for boron-, phosphorus- and sulphur-doped EDF, respectively. That is, the resistance of phosphorus-doped EDF is higher by four orders than that of boron-doped ones at nearly the same maximum content of doping element. Hence, phosphorus atoms may enter electrically inactive positions
A. E. Alexenko, B. V. Spitsyn ,, Semiconducting diamonds made in the USSR
1.5
!
Ea=
10 4
/
'7 c',l
.el
0.10
707
eV
(a)
O
10 3
.el ,lo O
-,-t al
®
o
0 2
J
3o0
|
~u3o
!
i
500
60o
i
7oo
Fig. 3. Temperature dependence of carrier mobility in boron-doped epitaxial DF. {:,) Measurements made directly after the growth process, (@1 measurements made after annealing of the DF at 1400 C.
in the diamond lattice even to a higher degree than boron ones. Perhaps, phosphorus can form associations in diamond-like nitrogen. The activation energy of conductivity of doped EDF depends on the doping level (Figs 4 and 5). Maximum values measured in the constant mobility temperature range 500 700 K are 0.25, 0.10 and 0.16eV for boron-, phosphorus- and sulphur-doped EDF, respectively. Polycrystalline boron-doped D F grown by the same method on silicon substrate [7] have specific resistance l0 s 108Qcm and activation energy about 0.35eV. These and other data [8] suggest a complex relation between carrier concentration and ionisation energy of
(a)
Ea= 0.25 eg
10 2 O
(b)
101 0
Ea= 0.09 eg °
Ea= 0.05 eV
O .,4 -,..I O r~
~
10-1
(c)
,
!
i
•
1
2
3
o,
Iooo,,~ U<-1 ) Fig. 4. Temperature dependence of specific resistance of boron-doped epitaxial DF. The doping level rises from (a) to (c).
102
O -,.4 .,-I O
Ea= 0.03 eV (b) 101
!
I
!
i
I
2
3
4
Iooo~
(I< - I )
Fig. 5. Temperature dependence of specilic resistance of phosphorusdoped epitaxial DF. The doping level rises from {a) to (b).
boron atoms in diamond which is considered [8] to be 0.37 eV. To clear up the nature of the acceptor centre in the EDF, electroadsorption spectra were studied [5]. The spectrum of boron-doped EDF (Fig. 6(a)) is similar to that of natural |la type diamond doped with boron by ion implantation (Fig. 6(b)). But peaks in the former case are less intensive and much wider and there is little energy shift there. The differences may be related with higher boron contents, some structural defects in EDF and its non-uniformity. EDF with high boron contents were blue or darkblue coloured. Phosphorus- and sulphur-doped EDP were colourless. Cathodoluminescence spectra [10] of boron-doped EDF include one broad band whose appearance depends on the doping level (Fig. 7). There are not any narrow lines induced by point defects in the spectra. It seems boron atoms can fill vacancies in a diamond lattice and improve its perfection [11]. This effect may be related to some changes in chemical properties of diamond. The resistivity of boron-doped EDF to oxidation [4] with melted KNO3 at 700'=C proved to be five times as high as that of natural diamond substrate or non-doped EDF. So, it must be said that the described EDF are inferior to high-quality natural diamonds in some electrical parameters. Nevertheless, they may be applied in electronics and a few models of semiconducting devices (thermistor, p n diodes and metal dielectric-semiconductor structures) were made [12] on the base of these EDF.
708
A. E. Alexenko, B. V. Spitsyn / Semiconducting diamonds made in the USSR
conductivity induced, probably, by the dislocations. Specific resistance at room temperature ranges from 0.1 to 5 ~ cm, activation energy of the conductivity is 2-10 meV.
(a) photon 0.25,
0.~
0
-1
4. High-pressure synthesis of diamond
-3 ,,,4 -5
(b)
o.I-
I
_ /~ 0.3~9 o.a5 ~ o . ~ \o.35 /o~4
o
t
1 -0.2-o.
O.o1-~ _ ~ o. 023 o. 31o o.36o
Fig. 6. Electroabsorption spectra of (a) natural Ila type diamond doped by ion implantation of boron, and (b) boron-doped epitaxial DF.
.r4 t/l
o.
4~
c
Synthesis of semiconducting diamond by this method is being investigated by a number of organisations in the USSR, in particular, at the Institute of Superhard Materials of the Ukrainian Academy of Sciences and the All-Union Institute for Synthesis of Minerals. Obtained there are p-conducting boron-doped crystals [14] of size up to 1 mm having a specific resistance of 10 4 10s f~ cm, activation energy of conductivity of 0.160.35 eV and a carrier mobility of about 300 cmZ/V s at room temperature [16]. Crystals with high boron contents [16] (1021/cm 3) have a metal type of conductivity and specific resistance of 10 - ~ ~ cm. Specific resistance of n-conducting As-doped crystals [14] is higher than 10 6 ~ cm. The n-conductivity has two values of activation energy in the region above room temperature: 0.25eV and 0.58 eV. There is a suggestion that an As atom fills a divacancy in the diamond lattice and gives a multicharged donor centre with the two above-mentioned energy levels. Carrier mobility in these crystals [17] is usually in the range of 90 230 cmZ/V s but sometimes it reaches 1100 cmZ/V s. These n-type crystals with high resistance and high temperature sensitivity are thought to be prospective for sensing devices [14]. Precise diamond thermistors are also described in ref. 18. By growing semiconducting diamond on opposite conductivity type seed crystal p n diodes were fabricated [19].
a) 5. Ion implantation in diamond !
i
4o0
50o
i
i
6oo 7o0 wave length (]am)
Fig. 7. Cathodoluminescencespectra of boron-doped epitaxial DF. The doping level rises from (a) to (c). Semiconducting carbon films can also be grown from glow discharge plasma. The material obtained at the Institute of Superhard Materials of the Ukrainian Academy of Sciences [13] has a highly distorted diamond structure and may be classified as an intermediate phase between diamond and well-known so-called diamondlike carbon. The films grown on quartz substrates at 800-1000 °C were 30-50 ~m thick. Its density is about 3.3 g/cm 3. Grain size in the films is 5 10 nm, density of dislocations reaches 10~3/cm 2. The films have p-type
Investigations in this field have been carried out at the Physical Institute of the USSR Academy of Sciences since 1970. p-Conducting layers were obtained there by implantation of boron, n-conducting ones were obtained by implantation of carbon, phosphorus, antimony and lithium [20]. Boron- and lithium-doped layers withstand annealing at 1400°C, have carrier concentrations 101"7-1019 and carrier mobility up to 1000 cmZ/V s. Activation energy of conductivity ranges from 0.17 to 0.45 eV for boron-doped layers and is about 0.1 eV for lithiumdoped ones. On the base of the ion-implanted layers a number of electronic devices was created (p-n diodes [21], lightsensitive [22] and light-emitting structures [23], and nuclear particle counters [24]). Structures like diodes and field-effect transistors were also made at Byelorus-
A. E. Alexenko, B. V. Spitsyn
Semieonduetin# diamonds made in the USSR
sian State University on the base of boron-implanted diamonds [125].
7
6. Plastic deformation of diamond 8
This method of creation of semiconducting regions in diamond was worked out at Donetsk State University. The semiconducting regions are created by the pressing of natural diamond crystals with small ones 1-26] or by a laser focused beam effect on the crystals [27]. This treatment produces some dislocations which act as acceptors in diamond. The resistance of the semiconducting regions was measured with two electrodes introduced into the craters formed by laser shots. The resistance ranged from l02 to 1014 ~'-'~ while the distance between the craters varied from 0.05 to 0.5 ram. Minimum specific resistance was 102~ cm. p-Type conductivity was demonstrated by a sign of asymmetry of current voltage characteristics. The activation energy of conductivity is 0.26 0.29 eV.
9 l0
II 12
13 14
15 16 17
7. Conclusions Semiconducting diamonds of both the p- and n-type can be prepared by several methods and each of them has its own advantages. It seems that a combination of these methods is the most prospective way. So, the investigations in such directions as growing of DF on semiconducting synthetic diamond crystals and on ion implantation-doped diamonds, ion implantation in DF are in progress in the USSR now.
18
19
20
21
References 1 B. V. Spitsyn, L. L. Bouilov and B. V. Derjaguin, J. Cryst. Growth, 52 (1981) 219. 2 L. L. Bouflov, A. E. Alexenko, A. A. Botev and B. V. Spitsyn, Ookl. Akad. Nauk SSSR, 287 (1986) 888 (in Russian). 3 B. V. Spitsyn and A. E. Alexenko, Proe. 7th Int. Con[i on Crystal Growth, Stuttgart, September. 1983, pp. 1 24. 4 B. V. Spitsyn and A. E. Alexenko, Archiwum Nauki o Materialaeh, 7 {19861 201 (in Russian). 5 A. E. Alexenko, V. S. Vavilov, B. V. Derjaguin, M. A. Gukasyan, T. A. Karatygina, E. A. Konorova, V. F. Sergienko, B. V. Spitsyn and S. D. Tkachenko, Dokl. Akad. Nauk SSSR, 233 (1977) 334 (in Russian). 6 B. V. Derjaguin, B. V, Spitsyn, A. E. Alexenko, A. E. Gorodetsky,
22
23 24 25
26 27
709
A. P. Zakharov and R. I. Nazarova, Dokl. Akad, Nauk SSSR, 213 (I973) 1059 (in Russian). B. V. Spitsyn, A. E. Alexenko, G. A. Sokolina and V. A. Laptev, Proc. 2nd Int. Co~[i on New Diamond Science and Teehnoloj~y, Washington, September, 1990, Hyatt Regency Crystal City, Arlington. 1990, p. 39. N. Fujimori, H. Nakahata and T. lmai..lap..I, o! Appl. Phys.. 29 (I 990) 824. A. T. Collins and E. C. Lightowlers, In J. Field led.) Properties ~! Diamond, Academic Press, New York. 1979, p. 79. V. S. Vavilov, A. A. Gippius, A. M. Zaytsev, B. V. Derjaguin, B. V. Spitsyn and A. E. Alexenko. Fiz. Tekn. Paluprot'odn., 14 (1980) 1811 (in Russian). S. Matsumoto, Archiwum Nauki o Materialach. 7(1986) 179. A. E. Alexenko and B. V. Spitsyn, Proe. NATO-Advaneed Study Institute on Diamond and Diamond-like Films and Coatin~,,s, Casteh:eeeio Paseoli, July-August, 1990. p. 93. V. D. Andreyev, V. A. Semenovich, Yu. I. Sozin, T. A. Nachalnaya and V. 1. Torishny, Sverhtt:erdye Materialy, 6 (1987) 19 (in Russian1. V. A. Laptev, Proe. Ist Int. Con.['. on the Applieations o[ Diamond Fihns and Related Materials, Auburn. 4ugust, 1991. Amsterdam, 199I, p. 9. P. I. Baransky, V, G. Malogolovets, V. 1. Torishny and G. V. Chipenko, Fiz. fekn. Poluprovodn.. 21 (1987) 75 (in Russian). A. G. Gontar, A. S. Vishnevsky, A. A. Shulzhenko and V. 1. Torishny, Fiz. Tekn. Poluprot'odn., 15 (198II I145 {in Russian). V. A. Kriachkov, Yu. A. Detchuyev, S. A. Nosuhin, S. E. Hriapenkov, N. G. Sanzharlinsky and M. I. Samoylovich, Proc. 1st CopT!i on Applyin~'~ gf Diamonds in Electronics, Moscow, March, 1991. Energoatomizdat, Moscow, 1991, p. 80 (in Russian). N. V. Novikov and A. G. Gontar, In V. B. Kvaskov (ed.) 41maz z, Elektronnoy Tehnike, Energoatomizdat. Moscow, 1990, p. 57 (in Russian). Yu. M. Rotner, S. M. Rotner, V. A. Presnov, V. A. Laptev and M V. Samoylovich, In N. V. Novikov (ed.) Sterhtt:erdye Materialy." Sintez, Cvoystva, Primenenye. Naukova dumka, Kiev, 1983, p. I06 (in Russian). E. A. Konorova, Proc. 2nd Col~/~ on Wide-Gap Semiconductor.s, Leningrad, January, 1979, Fiziko-Tehnichesky lnstitut, 1979, p. 35 (in Russian). V. S. Vavilov, M. A. Gukasjan, E. A. Konorova and Yu. V. Milutin, Fiz. Tekn. Poluprovodn., 6 (1972) 2384 (in Russian). V. S. Vavilov and E. B. Stepanova, Proc. 1st Co~!!i on Applying gf Diamonds in Electronics, Moscow, March, 199l, Encrgoatomizdat, Moscow, 199I, p. 52 (in Russian). M. I. Guseva, E. A. Konorova, Yu. A. Kuznetsov and V. F. Sergienko, Fiz. Tekn. Poluprovodn., 12 (1978) 505 (in Russian). E. A. Konorova and S. F. Kozlov, l=i= Tekn. Poluproi'odn., 4 {1970) 1865 (in Russian). A. A. Melnikov, A. M. Zaytsev, V. I. Kurgansky, A, Ja. Shilov, V. S. Varichenko and V. F. Stelmah, in V. B. Kvaskov (ed.) 41maz I' Elektronnoy Tehnike. Energoatomizdat, Moscow, 1990, p. 228 (in Russian). N. D. Samsonenko, B. I. Timchenko, Yu. A. Litvin and G. B. Bokiy, Dokl. Akad. Nauk SSSR, 242 (1978)826 (in Russian). N. D. Samsonenko. B. I. Timchenko, V. A. Erects and G. B. Bokiy, Kristallo,~rc~llva, 25 (1980) 1300 (in Russian).
705
Semiconducting diamonds made in the USSR A. E. Alexenko and B. V. Spitsyn Institute q/Physical Chemistry ol Russian Academy of Sciences, Leninsk y Prospect 31, Moscow 117915 ~Russia )
Abstract p- and n-type diamond films (DF) were grown by vapour chemical transport method and fiom DC arc discharge plasma. Maximum contents of doping elements in epitaxial DF (EDF) were 2.5, 1 and 0.02 wt% for B-, P- and S-doped EDF, respectively. Specific resistances of the EDF were 10 3, l01 and 103 (2 cm, respectively. The doping changes the growth rate and chemical properties of EDF. Doped EDF were investigated by spark mass-spectrometry, X-ray spectrometry, Rutherford backscattering and Hall-effect measurements. Electroabsorption and cathodoluminescence spectra were studied. DF with a highly distorted structure and dislocation p-type conductivity were grown from glow discharge plasma. B- and As-doped I mm size crystals were obtained by an ultrahigh-pressure method. As produces multicharged donor centres with two energy levels. The crystals are prospective for sensors. Doping of diamond with B, C, P, Sb and Li was realised by ion implantation. The best p- and n-type layers doped with B and Li have carrier mobility of about 1000 cm2/V s. Acceptor dislocation centres can also be created by plastic deformation of diamond crystals. Specific resistance of such crystals can fall by up to 102 Q cm. Combining the above-mentioned methods seems to be thc most prospective way.
1. Introduction
Diamond is considered to be the most prospective material for fabrication of powerful compact electronic and optoelectronic devices which can operate under extreme conditions. There exist four methods for the preparation of semiconducting diamonds for this purpose: doping of diamond crystals during their growth under ultra-high pressure, doping of diamond thin films during their growth from vapour phase, doping of diamond by ion implantation and creation of electrically active defects in diamond by its plastic deformation. It seems the vapour phase growth method has some advantages over the ultra-high-pressure one, because it is carried out under more gentle and controllable conditions and enables the production of a material with assigned properties, configuration and surface morphology. The diamond films (DF) synthesised from vapour phase can also be a starting material with reproducible properties for doping by ion implantation. So, this paper is mainly devoted to the results in semiconducting DF, obtained in the USSR at the Institute of Physical Chemistry of USSR Academy of Sciences. Some results obtained by three other methods of preparing semiconducting diamonds are also mentioned.
and by crystallisation from DC-discharge plasma [2]. In the former method a closed sandwich system filled with pure hydrogen at a pressure of 12 torr (1596 Pa) was used (Fig. l(a)). The graphite plate at a temperature of 2000-2500 C was the source of carbon. In the latter method, an arc discharge between the refractory cathode and a substrate or substrate holder in gas mixture was used (Fig. l(b)). The current density was 2-4 A/cm z. The mixture of hydrogen (97%) and methane (3%) was passed through the reactor. Total gas flow was 2000 sccm/h and pressure in the reactor was 100 tort (13.30 kPa). Natural diamond and silicon wafers
(a)
(b)
g~aphi t e source of carbon (2000_2500Oc)
?
~jcavhode diamond
substrate
f./.,/
U / \
~
~ta+C~~
2. Experimental details optical pyrometry
DF were grown by the method of high-temperature, high-gradient chemical vapour transport of carbon [1]
0925 9635.9255.00
~' Elsevier Science Publishers B.V. All rights reserved
Fig. I. Schematic diagram of DF crystallisation (a) by the chemical transport method, and (b) from DC arc discharge plasma.
706
A, E. Alexenko, B. V, Spitsyn / Semiconducting diamonds made in the USSR
were the substrates in both methods. Substrate temperature was measured with an optical pyrometer through a hole in the substrate holder. In the case of diamond substrate, its face (opposite the growing one) was covered with an opaque carbon layer. The first successful attempts of doping DF with boron, phosphorus and sulphur were made in 1971-1972. To prepare p-type DF, boron carbide, B4C, and neocarborane, CzBloHa2, were used as doping agents. The former was placed at the surface of the source of carbon in the method of chemical transport (Fig. l(a)) and the latter was in a special crucible near the substrate. The crucible was equipped with a heater and a thermocouple. To prepare n-type DF, elementary phosphorus and sulphur placed in similar crucibles were used. The crucible temperature ranged between 50 and 300 °C.
1o 6
3
1°5
g
~
~o 3
"4
I oo
7;0
960 Io;o Ioo T
(oc)
Fig. 2. Resistance of epitaxial D F and boron contents in the DF as a function of crystallisation temperature.
3. Results and discussion
The growth rate of non-doped DF in chemical transport method was about 0.3 gm/h. While doping with boron it sometimes increased up to 0.35-0.4 gin/h, perhaps because of forming carborans which could transport both boron and carbon. While doping with phosphorus and sulphur their partial pressures were comparable with hydrocarbons and the growth rate usually was falling by up to 0.2-0.1 gin/h, probably, due to dilution and/or deactivation of the growth agents. DF grown on diamond substrates were 1-2 gm thick. Their epitaxial nature was shown by reflective electron diffraction. Composition of the epitaxial DF (EDF) was studied by spark mass-spectrometry [3, 4]. Boron-doped EDF were also studied by X-ray spectrometry [4] and secondary ion mass-spectrometry [5]. It has been established that the maximum content of boron in EDF grown on (111) substrates is about 2.5 wt%, those of phosphorus and sulphur are 1 and 0.02 wt%, respectively. Doping of EDF on (110) and (100) substrates proved to be much less effective, so further study was carried out with the (111) substrates only. The boron contents in EDF and their resistance depended on the crystallisation temperature (Fig. 2). It seems that there forms a non-equilibrium solid solution of boron in diamond at temperatures below about 800 °C. At higher temperatures the forming process becomes more equilibrium and the boron contents fall [4]. The Rutherford backscattering study shows that when boron content does not exceed 0.1 wt%, boron atoms mainly occupy diamond lattice points in EDF. According to X-ray topography data, lattice parameters of such EDF are diminished by 0.0006 A in comparison with non-doped EDF. At higher contents, single boron atoms can enter tetrahedral interstices of the lattice. This causes
an expansion of the diamond lattice. At boron contents of I wt%, boron atoms mainly (80-90% of them) occupy the interstices and the lattice parameter approaches to that of non-doped diamond [4]. p-Type conductivity of boron-doped EDF and n-type conductivity of phosphorus- and sulphur-doped EDF were demonstrated by a sign of thermoelectromotive force and (for boron-doped EDF) by Hall-effect measurements. Specimens for Hall-effect measurements [5] were prepared using masking during the growth process so that EDF had the form of a stripe with 4:1 or 5:1 length:width ratio. Concentration of boron in these EDF was about 1019/cm3. Carrier concentration proved to be about 1016/cm3 at room temperature and its activation energy was 0.3 eV. Carrier mobility was 2 8 cmZ/V s at room temperature. The mobility rises with the measurement temperature up to 10-15 cmZ/V s and holds this value in the range of 500-700 K (Fig. 3). It is suggested that there exists hopping conductivity at low temperatures and the rise of mobility with temperature can be due to decreasing the contribution of the hopping conductivity. Low mobility values may be related to high contents of boron or structural defects (e.g. microtwins [-6]) in the EDF. Specific resistance of p-type EDF was measured using two gold electrodes, and that of n-type EDF was measured using two gallium ones which proved to have a nearly ohmic character. Minimum specific resistances of EDF a r e 10 - 3 , 101 and 103 ~ c m for boron-, phosphorus- and sulphur-doped EDF, respectively. That is, the resistance of phosphorus-doped EDF is higher by four orders than that of boron-doped ones at nearly the same maximum content of doping element. Hence, phosphorus atoms may enter electrically inactive positions
A. E. Alexenko, B. V. Spitsyn ,, Semiconducting diamonds made in the USSR
1.5
!
Ea=
10 4
/
'7 c',l
.el
0.10
707
eV
(a)
O
10 3
.el ,lo O
-,-t al
®
o
0 2
J
3o0
|
~u3o
!
i
500
60o
i
7oo
Fig. 3. Temperature dependence of carrier mobility in boron-doped epitaxial DF. {:,) Measurements made directly after the growth process, (@1 measurements made after annealing of the DF at 1400 C.
in the diamond lattice even to a higher degree than boron ones. Perhaps, phosphorus can form associations in diamond-like nitrogen. The activation energy of conductivity of doped EDF depends on the doping level (Figs 4 and 5). Maximum values measured in the constant mobility temperature range 500 700 K are 0.25, 0.10 and 0.16eV for boron-, phosphorus- and sulphur-doped EDF, respectively. Polycrystalline boron-doped D F grown by the same method on silicon substrate [7] have specific resistance l0 s 108Qcm and activation energy about 0.35eV. These and other data [8] suggest a complex relation between carrier concentration and ionisation energy of
(a)
Ea= 0.25 eg
10 2 O
(b)
101 0
Ea= 0.09 eg °
Ea= 0.05 eV
O .,4 -,..I O r~
~
10-1
(c)
,
!
i
•
1
2
3
o,
Iooo,,~ U<-1 ) Fig. 4. Temperature dependence of specific resistance of boron-doped epitaxial DF. The doping level rises from (a) to (c).
102
O -,.4 .,-I O
Ea= 0.03 eV (b) 101
!
I
!
i
I
2
3
4
Iooo~
(I< - I )
Fig. 5. Temperature dependence of specilic resistance of phosphorusdoped epitaxial DF. The doping level rises from {a) to (b).
boron atoms in diamond which is considered [8] to be 0.37 eV. To clear up the nature of the acceptor centre in the EDF, electroadsorption spectra were studied [5]. The spectrum of boron-doped EDF (Fig. 6(a)) is similar to that of natural |la type diamond doped with boron by ion implantation (Fig. 6(b)). But peaks in the former case are less intensive and much wider and there is little energy shift there. The differences may be related with higher boron contents, some structural defects in EDF and its non-uniformity. EDF with high boron contents were blue or darkblue coloured. Phosphorus- and sulphur-doped EDP were colourless. Cathodoluminescence spectra [10] of boron-doped EDF include one broad band whose appearance depends on the doping level (Fig. 7). There are not any narrow lines induced by point defects in the spectra. It seems boron atoms can fill vacancies in a diamond lattice and improve its perfection [11]. This effect may be related to some changes in chemical properties of diamond. The resistivity of boron-doped EDF to oxidation [4] with melted KNO3 at 700'=C proved to be five times as high as that of natural diamond substrate or non-doped EDF. So, it must be said that the described EDF are inferior to high-quality natural diamonds in some electrical parameters. Nevertheless, they may be applied in electronics and a few models of semiconducting devices (thermistor, p n diodes and metal dielectric-semiconductor structures) were made [12] on the base of these EDF.
708
A. E. Alexenko, B. V. Spitsyn / Semiconducting diamonds made in the USSR
conductivity induced, probably, by the dislocations. Specific resistance at room temperature ranges from 0.1 to 5 ~ cm, activation energy of the conductivity is 2-10 meV.
(a) photon 0.25,
0.~
0
-1
4. High-pressure synthesis of diamond
-3 ,,,4 -5
(b)
o.I-
I
_ /~ 0.3~9 o.a5 ~ o . ~ \o.35 /o~4
o
t
1 -0.2-o.
O.o1-~ _ ~ o. 023 o. 31o o.36o
Fig. 6. Electroabsorption spectra of (a) natural Ila type diamond doped by ion implantation of boron, and (b) boron-doped epitaxial DF.
.r4 t/l
o.
4~
c
Synthesis of semiconducting diamond by this method is being investigated by a number of organisations in the USSR, in particular, at the Institute of Superhard Materials of the Ukrainian Academy of Sciences and the All-Union Institute for Synthesis of Minerals. Obtained there are p-conducting boron-doped crystals [14] of size up to 1 mm having a specific resistance of 10 4 10s f~ cm, activation energy of conductivity of 0.160.35 eV and a carrier mobility of about 300 cmZ/V s at room temperature [16]. Crystals with high boron contents [16] (1021/cm 3) have a metal type of conductivity and specific resistance of 10 - ~ ~ cm. Specific resistance of n-conducting As-doped crystals [14] is higher than 10 6 ~ cm. The n-conductivity has two values of activation energy in the region above room temperature: 0.25eV and 0.58 eV. There is a suggestion that an As atom fills a divacancy in the diamond lattice and gives a multicharged donor centre with the two above-mentioned energy levels. Carrier mobility in these crystals [17] is usually in the range of 90 230 cmZ/V s but sometimes it reaches 1100 cmZ/V s. These n-type crystals with high resistance and high temperature sensitivity are thought to be prospective for sensing devices [14]. Precise diamond thermistors are also described in ref. 18. By growing semiconducting diamond on opposite conductivity type seed crystal p n diodes were fabricated [19].
a) 5. Ion implantation in diamond !
i
4o0
50o
i
i
6oo 7o0 wave length (]am)
Fig. 7. Cathodoluminescencespectra of boron-doped epitaxial DF. The doping level rises from (a) to (c). Semiconducting carbon films can also be grown from glow discharge plasma. The material obtained at the Institute of Superhard Materials of the Ukrainian Academy of Sciences [13] has a highly distorted diamond structure and may be classified as an intermediate phase between diamond and well-known so-called diamondlike carbon. The films grown on quartz substrates at 800-1000 °C were 30-50 ~m thick. Its density is about 3.3 g/cm 3. Grain size in the films is 5 10 nm, density of dislocations reaches 10~3/cm 2. The films have p-type
Investigations in this field have been carried out at the Physical Institute of the USSR Academy of Sciences since 1970. p-Conducting layers were obtained there by implantation of boron, n-conducting ones were obtained by implantation of carbon, phosphorus, antimony and lithium [20]. Boron- and lithium-doped layers withstand annealing at 1400°C, have carrier concentrations 101"7-1019 and carrier mobility up to 1000 cmZ/V s. Activation energy of conductivity ranges from 0.17 to 0.45 eV for boron-doped layers and is about 0.1 eV for lithiumdoped ones. On the base of the ion-implanted layers a number of electronic devices was created (p-n diodes [21], lightsensitive [22] and light-emitting structures [23], and nuclear particle counters [24]). Structures like diodes and field-effect transistors were also made at Byelorus-
A. E. Alexenko, B. V. Spitsyn
Semieonduetin# diamonds made in the USSR
sian State University on the base of boron-implanted diamonds [125].
7
6. Plastic deformation of diamond 8
This method of creation of semiconducting regions in diamond was worked out at Donetsk State University. The semiconducting regions are created by the pressing of natural diamond crystals with small ones 1-26] or by a laser focused beam effect on the crystals [27]. This treatment produces some dislocations which act as acceptors in diamond. The resistance of the semiconducting regions was measured with two electrodes introduced into the craters formed by laser shots. The resistance ranged from l02 to 1014 ~'-'~ while the distance between the craters varied from 0.05 to 0.5 ram. Minimum specific resistance was 102~ cm. p-Type conductivity was demonstrated by a sign of asymmetry of current voltage characteristics. The activation energy of conductivity is 0.26 0.29 eV.
9 l0
II 12
13 14
15 16 17
7. Conclusions Semiconducting diamonds of both the p- and n-type can be prepared by several methods and each of them has its own advantages. It seems that a combination of these methods is the most prospective way. So, the investigations in such directions as growing of DF on semiconducting synthetic diamond crystals and on ion implantation-doped diamonds, ion implantation in DF are in progress in the USSR now.
18
19
20
21
References 1 B. V. Spitsyn, L. L. Bouilov and B. V. Derjaguin, J. Cryst. Growth, 52 (1981) 219. 2 L. L. Bouflov, A. E. Alexenko, A. A. Botev and B. V. Spitsyn, Ookl. Akad. Nauk SSSR, 287 (1986) 888 (in Russian). 3 B. V. Spitsyn and A. E. Alexenko, Proe. 7th Int. Con[i on Crystal Growth, Stuttgart, September. 1983, pp. 1 24. 4 B. V. Spitsyn and A. E. Alexenko, Archiwum Nauki o Materialaeh, 7 {19861 201 (in Russian). 5 A. E. Alexenko, V. S. Vavilov, B. V. Derjaguin, M. A. Gukasyan, T. A. Karatygina, E. A. Konorova, V. F. Sergienko, B. V. Spitsyn and S. D. Tkachenko, Dokl. Akad. Nauk SSSR, 233 (1977) 334 (in Russian). 6 B. V. Derjaguin, B. V, Spitsyn, A. E. Alexenko, A. E. Gorodetsky,
22
23 24 25
26 27
709
A. P. Zakharov and R. I. Nazarova, Dokl. Akad, Nauk SSSR, 213 (I973) 1059 (in Russian). B. V. Spitsyn, A. E. Alexenko, G. A. Sokolina and V. A. Laptev, Proc. 2nd Int. Co~[i on New Diamond Science and Teehnoloj~y, Washington, September, 1990, Hyatt Regency Crystal City, Arlington. 1990, p. 39. N. Fujimori, H. Nakahata and T. lmai..lap..I, o! Appl. Phys.. 29 (I 990) 824. A. T. Collins and E. C. Lightowlers, In J. Field led.) Properties ~! Diamond, Academic Press, New York. 1979, p. 79. V. S. Vavilov, A. A. Gippius, A. M. Zaytsev, B. V. Derjaguin, B. V. Spitsyn and A. E. Alexenko. Fiz. Tekn. Paluprot'odn., 14 (1980) 1811 (in Russian). S. Matsumoto, Archiwum Nauki o Materialach. 7(1986) 179. A. E. Alexenko and B. V. Spitsyn, Proe. NATO-Advaneed Study Institute on Diamond and Diamond-like Films and Coatin~,,s, Casteh:eeeio Paseoli, July-August, 1990. p. 93. V. D. Andreyev, V. A. Semenovich, Yu. I. Sozin, T. A. Nachalnaya and V. 1. Torishny, Sverhtt:erdye Materialy, 6 (1987) 19 (in Russian1. V. A. Laptev, Proe. Ist Int. Con.['. on the Applieations o[ Diamond Fihns and Related Materials, Auburn. 4ugust, 1991. Amsterdam, 199I, p. 9. P. I. Baransky, V, G. Malogolovets, V. 1. Torishny and G. V. Chipenko, Fiz. fekn. Poluprovodn.. 21 (1987) 75 (in Russian). A. G. Gontar, A. S. Vishnevsky, A. A. Shulzhenko and V. 1. Torishny, Fiz. Tekn. Poluprot'odn., 15 (198II I145 {in Russian). V. A. Kriachkov, Yu. A. Detchuyev, S. A. Nosuhin, S. E. Hriapenkov, N. G. Sanzharlinsky and M. I. Samoylovich, Proc. 1st CopT!i on Applyin~'~ gf Diamonds in Electronics, Moscow, March, 1991. Energoatomizdat, Moscow, 1991, p. 80 (in Russian). N. V. Novikov and A. G. Gontar, In V. B. Kvaskov (ed.) 41maz z, Elektronnoy Tehnike, Energoatomizdat. Moscow, 1990, p. 57 (in Russian). Yu. M. Rotner, S. M. Rotner, V. A. Presnov, V. A. Laptev and M V. Samoylovich, In N. V. Novikov (ed.) Sterhtt:erdye Materialy." Sintez, Cvoystva, Primenenye. Naukova dumka, Kiev, 1983, p. I06 (in Russian). E. A. Konorova, Proc. 2nd Col~/~ on Wide-Gap Semiconductor.s, Leningrad, January, 1979, Fiziko-Tehnichesky lnstitut, 1979, p. 35 (in Russian). V. S. Vavilov, M. A. Gukasjan, E. A. Konorova and Yu. V. Milutin, Fiz. Tekn. Poluprovodn., 6 (1972) 2384 (in Russian). V. S. Vavilov and E. B. Stepanova, Proc. 1st Co~!!i on Applying gf Diamonds in Electronics, Moscow, March, 199l, Encrgoatomizdat, Moscow, 199I, p. 52 (in Russian). M. I. Guseva, E. A. Konorova, Yu. A. Kuznetsov and V. F. Sergienko, Fiz. Tekn. Poluprovodn., 12 (1978) 505 (in Russian). E. A. Konorova and S. F. Kozlov, l=i= Tekn. Poluproi'odn., 4 {1970) 1865 (in Russian). A. A. Melnikov, A. M. Zaytsev, V. I. Kurgansky, A, Ja. Shilov, V. S. Varichenko and V. F. Stelmah, in V. B. Kvaskov (ed.) 41maz I' Elektronnoy Tehnike. Energoatomizdat, Moscow, 1990, p. 228 (in Russian). N. D. Samsonenko, B. I. Timchenko, Yu. A. Litvin and G. B. Bokiy, Dokl. Akad. Nauk SSSR, 242 (1978)826 (in Russian). N. D. Samsonenko. B. I. Timchenko, V. A. Erects and G. B. Bokiy, Kristallo,~rc~llva, 25 (1980) 1300 (in Russian).