AU614090B2 - Semiconductor base material - Google Patents

Description

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J 11111- IIf iIi'i~ 11111 1.25 1.4 a' 11111- II--- AU-AI-19351/88' WELTORGANISATION, FOR GEISTIGES EIljENTUM IflLAMD, on~ Bur INTERNATIONALE t IUN4IyE F NA DEM VERTRAG OIBER DIE INTERNATIONALE Z E AU g E BIET DES PATENTWESENS (PCT) (51) Internationale Patentklassifikation 4 (11) Internationale Veriiffentlichungsnummer: WO 89/ 01237 HOlIL 21/16,52C 62 Al (43) Internationales H~l 2/36 .Veroffen'llichungsdatum: 9. Februar 1989 (09.02.89) (21) Internationales Aktenzeichen: PCT/EP88/00526 (81) Bestimmungsstaaten: AT (europ~isches Patent), AU, (22) Interuationales Annieldedatum: 14. Juni 1988 (14.06.88) BE (europliisches Patent), BR, CH (europaisches Patent), DE (europaisches Patent), FR (europ~isches Patent), GB (europ~Iisches Patent), IT (europ~isches Pa- (31) Prioritaitsaktenzelchen.: P 37 25 700.5 tent), JP, KR, LU (europaisches Patent), NL (euro- (32)Priritisdaurn:3. ugus 197 (0.0887) pdisches Patent), SE (europ~isches Patent), SU, US.

(33) Priorititslard: DE Veroffentlicht Mil internationalem Recherchenibericht.

(71) Anmelder (ftir alle Bestimmungsslaaten ausser US): SIE- MENS AKTIENGESELLSCHAFT [DE/DE]; Wit- ~Jp 2~AR18 telsbacherplatz 2, D-8000 Mflnchen 2 20AR18 (72) Erfinder;und

AUSTRALIAN

Erfinder/Anmelder (nur fir US) BIRKLE, Siegfried EDE/DE], Veit-StoB-Str. 46, D-8552 Hbchstadt 1MR18 KAMTAERMAIER, Johann [DE/DE]; Ziehr-r Str. 19,-1 A 99 j D-8025 Unterhaching SCHULTE, Riolf [DE/PAETOFC DE]; Georg-Kraug-Str. 2, D-8520 Erlangen (DE).PAETOFC WINNACKER, Albrecht [DE/DE]; Puch~astr. 6, D- 8520 Erlangen RITTMAYER, Gerhard [DE/ DE]; Eskilstunastr. 8, D-8520 Erlangen (DE).

(54) Title: SEMICONDUCTOR BASE MATERIAL (54) Bezeichnung: HALBLEITERGRUNDMATERIAL 101 (57) Abstract x GaAs (n) A new semiconductor base material is manufactured t by thin-film technology using strip processes and has a si (n charge carrier mobility of at least I cm 2 .V-l.s- 1 It consists of 0 thin films of amorphous carbon containing hydrogen T GaAs (p) having a specifir, electrical resistance between 101 and rsS (p) 108 D and a charge carrier concentration (n p) between 1010 and 1018 cm- 3 each at ambient temperature. Application in semiconductor components.

Zusammenfasasng schichttechnologie tinter Einsatz von Bandprozessen her-- 101 stellbar sein und cine Ladungstragerbewegliclikeit von we-/ nigstens I CM 2

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1 l.s-1 aufweisen. ErfindungsgemiAi besteht das Halbititeigrundmaterial aus Diinnschichten aus amerphem, wasserstoftbaltigem Kohleristoff mit eiriem speziischen elektrischen Widerstand zwischen 101 un-d 108 f2.cm und einer Ladungstragerkonzentration (n p) zwischen 1010 und 1018 cm- 3 jeweils bei Raumitemperatur.

Bauelemente.

S (0 log 9 Declaration was/were, the first application(s-made in a Convention country in respect of the invention (s-the subject of the application.

Declared at Sydney this 2nd day of February 19 SFP4 To: The Commissioner of Patents Sigr/cure of Declarant(s) John Gordon Hinde 1 8 1 Siemens Aktiengesellschaft A Novel Semiconductor Base Material The invention relates to a novel semiconductor base material with great capability of exhibiting semiconductor properties, consisting of thin layers of amorphous hydrogen-containing carbon and to method of producing such a semiconductor base material.

Particularly single-crystal silicon (Si) and gallium arsenide (GaAs) with very high n and p charge carrier mobilities 1 cm 2 .V .sec have been known as semiconductor base materials. The shortcoming of these materials is that they cannot be produced in the form of thin layers in a strip technique using flexible supports and that high-temperature processes are required for their preparation and working.

Furthermore, amorphous hydrogen-containing silicon (a-Si:H) has been known as a semiconductor base material. True, this material can be obtained on thin-film basis but its n and p charge-carrier mobility is usually significantly below 1 cm 2

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1 .sec-1 (see in this connection, J. Dresner in "Semiconductors and Semimetals," vol. 21, part C, pp. 193 (1984)).

Japanese Displayed Specification 59-26906 (of February 13, 1984) describes an amorphous carbon material which, in the form of thin layers, is produced with the plasma CVD process (CVD standing for chemical vapour deposition) from hydrocarbons subjected to an RF high-freouency field. This material has luminescence and semiconductor properties and high hardness. The luminescence peak is at 2.6 eV and the spacing of the optical bands is 3.0 eV (in a material prepared from propane under a pressure of 40 mtorr), with the 1 7 electrical resistivity being comparatively high and amounting to 1012 or 1013 2.cm.

1- 182z/JRB T~L_ h I nl i -2- The goal of the invention is to provide a semiconductor base material which can be produced in thin-layer form with the aid of band processes and without high-temperature processes and which, in the undoped state, has an n .and p charge-carrier mobility comparable with that of crystalline semiconductor materials such as Si and GaAs, wherein 2 -1 -1 the charge-carrier mobility is to re, h at least 1 cm .V .sec at room temperature.

In accordance with the present invention there is disclosed a process for producing a semiconductor base material on a substrate, said process comprising the step of using high-frequency plasma deposition of gaseous hydrocarbons to provide on said substrate amorphous, hydrogen-containing carbon in a thin layer, wherein: the average retention time of the hydrocarbons in the plasma is at least 15 msec, with the retention time being defined as the ratio of the product of plasma volume times gas pressure to mass throughput; the power density in the plasma is between 0.2 and _3 .cm 3; and the substrate is not heated.

goo In a preferred embodiment, this is achieved by a semiconductor base 20 material which consists of thin layers of amorphous, hydrogen-containing carbon with an electrical resitivity between 10 and 2.cm and an (n p) charge-carrier concentration ranging from 10 to 1018 cm 3 in each case at room temperature.

To date semiconductive thin layers with an n and p charge-carrier 2 -I -1 mobility of more than 1 cm .V .sec like in the material of the invention, have not been known in the case of undoped amorphous semiconductors.

In a semiconductor material, there exists a high mobility of the two types of charge carriers, which is important for many applications, if the ratio of the respective Hall constants to the electrical resistivity is as large as possible. This is the case in the semiconductor base material according to the invention, in the particular a-C:H. In this material, in which significantly less than 68% of the carbon atoms have diamond-like tetrahedral bonds (sp 3 hybridization) and significantly more than 30% have graphite-like 2 trigonal bonds (sp hybridization), and which has a hydrogen 1124o -i- 2A concentration of 10 to 30 atomic the requirement of a high charge-carrier mobility is satisfied in a planned fashion, and even to an optimal value, via a specific concentration of the n and p charge carriers and a certain electrical resistivity. When the numerical value of the product of charge- carrier concentration times the electrical resistivity decreases, the ratio of the Hall constant to the electrical resistivity increases and, hence, an increasing mobility of the charge carriers in the a-C:H layer develops.

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SO 0 0 0 S '11240 p~ L-s, i I- II i 3 -3graphite-like trigonal bonds (sp 2 hybridization), and which has a hydrogen concentration of 10 to 30 atomic the requirement of a high charge-carrier mobility is satisfied in a planned fashion, and even to an optimal value, via a specific concentration of the n and p charge carriers and a certain electrical resistivity. When the numerical value of the product of chargecarrier concentration times the electrical resistivity decreases, the ratio of the Hall constant to the electrical resistivity increases and, hence, an increasing mobility of the charge carriers in the a-C:H layer develops.

The semiconductor material according to the invention has advantageously an electrical resistivity ranging from about 102 to 10 7 a.cm.

Such a material has an n and p charge-carrier mobility in excess of 2 cm 2 .sec The electrical resistivity preferably amounts to values ranging from 5.10 3 to 5.106 Q.cm. In such a material, the n and p charge-carrier mobility exceeds values of 10 3 cm 2 .Vl .sec Besides this, the semiconductor material according to the invention has a spacing of 1.4 to 2.4 eV of the optical bands.

In a-C:H layers with an optical band spacing of less than 1.4 eV and an electrical resistivity of less than 10.cm, the mobility of the charge carriers is significantly lower by virtue of a reduced Hall constant so that such materials within the scope of the-present invention cannot be considered as semiconductor base materials. On the other hand, in a-C:H layers with an electrical resistivity in excess of 108 2.cm, the resistance is detrimental to an increase in the Hall mobility of the charge carriers. As far as the electrical resistivity is concerned, such a-C:H layers are comparable with a-C:H layers in which the charge-carrier 2 -1 -1 mobility is below 1 cm .sec.

The invention also relates to amorphous, hydrogen-containing carbon, to a-C:H, which has a Hall mobility 1 cm2.V -l.sec of the n and p charge carriers. Such a semiconductor base material can be obtained by 182z/JRB

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-4high-frequency plasma deposition of gaseous hydrocarbons with an average retention time of at least '5 msec of the hydrocarbons in the plasma; the retention time is defined as the ratio of the product of plasma volume times gas pressure to mass throughput.

The semiconductor base material according to the invention, the amorphous, hydrogen-containing carbon, is obtained by high-frequency plasma deposition of gaseous hydrocarbons. It is an essential feature of the invention that the average retention time of the hydrocarbons in the plasma amounts to at least 15 msec. The retention time t is defined as above, i.e., t p.V.m, where p denotes the gas pressure (in Pa); V, the plasma volume (in cm3); and m v the mass throughput (in Pa.cm .sec As stated above, semiconductive a-C:H layers with a charge carrier mobility 1 cm2.V -l.sec I (when undoped) have not been known. Accordingly, to date no process of producing such layers has been known and, hence, indications of relevant process parameters have not been available. In particular, the state of the art does not suggest that, in order to achieve a certain Hall mobility of the charge carriers in a-C:H layers, the reaction gas used in the plasma deposition, the hydrocarbons, must have a well-defined retention time in the plasma, a retention time of at least 15 msec. With 2 -1 -1 a retention time 215 msec, charge-carrier mobilities 21 cm .V .sec are attained. The retention time of the-hydrocarbon molecules in the plasma zone preferably amounts to 50 500 msec.

With a given cross section of the reaction vessel used for plasma deposition, a retention time of the above-cited order of magnitude can be obtained only with a certain relation between the gas pressure, the partial pressure of the reaction gas, and the mass throughput. In the process according to the invention, the gas pressure therefore advantageously amounts to 400 Pa, preferably to 20 200 Pa, and the mass throughput preferably amounts to 0.05 2.10 Pa.cm .sec Furthermore, it is essential in the process according to the invention that the substrate onto which the.a-C:H is deposited is not heated.

182z/JRB -JI I

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A plasma deposition with hf excitation (hf means high frequency) serves to produce the semiconductive a-C:H layers according to the invention, with their great capability of exhibiting semiconductor properties by virtue of the high charge-carrier mobilities. The plasma deposition is preferably effected at radio frequencies in the range from 0.1 to 100 MHz, at 14.36 MHz. Plasma deposition can also be effected with microwaves i.e., in the range from 0.1 to 1000 GHz, at 2.45 GHz.

In the process according to the invention, the gaseous hydrocarbons are preferably alkanes, saturated aliphatic hydrocarbons such as methane, ethane, and propane. Apart from these there can be employed alkenes, i.e., unsaturated aliphatic hydrocarbons such as ethene and propene, as well as acteylene, cycloalkanes, saturated cyclic hydrocarbons such as cyclohexane, and, in the vapour state, aromatic hydrocarbons in the form of benzene and derivatives of benzene. The above-cited hydrocarbons can be used alone or in mixtures. Hydrogen and/or inert gases such as helium and argon may be admixed to the hydrocarbons.

When the internal electrodes of the discharge unit are of different size (surface area ratio 0.5, preferably between 0.25 and 0.05), a dc voltage component (bias voltage or "self bias vo-tage") of up to about 1 kV, pulsating with the high frequency, develops as a consequence of space charge in hf discharges, particularly upon excitation with rf. This dc voltage component is superimposed on the hf ac voltage and makes the smaller electrode the cathode. In this way the charged C Hy particles, which are generated by ionization and fragmentation of the reaction gas, are accelerated toward the cathode and deposited, with high kinetic energy and under formation of a-C:H, on the substrate placed before the cathode. A "self bias" effect of this kind is effective also in mw-induced deposition plasmas, though to a much smaller extent because of the lack of internal electrodes, as a potential difference exists in each case between the plasma and the substrate surface.

1 82z/JRB 6 -6- In plasmas for a-C:H deposition, the bias voltage to a great extent determines the passive physicochemical properties of the layers produced, hardness, resistance to scratching, and refractive index, but to a lesser extent the mobility of the n and p charge carriers. A charge-carrier Hall mobility in excess of 1 cm .V .sec 1 [sic], desirable in undoped a-C:H for many applications, is obtained if, according to the invention, the ratio of the Hall constant to the electrical resistivity is large.

The above-cited condition can be satisfied in deliberate fashion through a certain chemical structure of the a-C:H layers, particularly through a specific ratio of sp 2 to sp 3 bonding fractions of the C atoms while free C' valencies ("dangling bonds") are saturated by H atoms. The chemical structure of the a-C:H layers, in turn, depends upon the relative concentrations of the species H, C 2 and CH in the plasma, which can be influenced and controlled by the plasma conditions, speficifically the electrical power applied, the frequency of the hf excitation, shape and size of the electrodes, the partial pressure of the reaction gas, mass throughput, and addition of inert gases.

In order to obtain an adequate concentration of the species H, C 2 and CH in the plasma, in the process according to the invention electrical power preferably ranging from 0.2 to 10 N.cm 3 is applied to the plasma.

The invention will be explained by-way of examples.

Methane (CH 4 as the reaction gas is introduced into an apparatus for plasma deposition by high-frequency discharge, into a cylindrical evacuated glass chamber. The reaction gas passes through a narrow annular gap into the plasma volume of about 45 cm 3 which develops between two plane electrodes of different size (surface ratio of the electrodes: The two electrodes, the grounded electrode (anode) and the hf electrode (cathode) are connected to an rf generator (13.56 MHz). At an hf power density of about 8 W.cm 3 in the plasma space, a bias voltage of about 1 kV develops between the two electrodes. The smaller of the two electrodes becomes the cathode on which the a-C:H deposition takes place.

182z/JRB "Z 11,e l-u -l-rr- I-i I -I I-I 7 Under the above-cited conditions (partial pressure of the reaction gas 100 Pa, mass throughput 0.88.105 Pa.cm3.sec-1, retention time 51 msec) and with an unheated substrate one obtains semiconductive a-C:H layers with an 2 -1 -1 n charge-carrier Hall mobility in excess of 10 cm .V .sec Methane as the reaction gas is discharged via a central inlet into the cylindrical evacuated glass chamber of an apparatus for plasma deposition. The cathode is flat, whereas the anode is pot-shaped; the surface ratio of the electrodes is 1:6. The a-C:H deposition is effected with the aid of radio frequency (13.56 MHz) under the following conditions: partial 3 pressure ofthe reaction gas: 20 Pa; plasma volume: 400 cm mass flow: 0.32.105 Pa.cm3.sec 1 With an unheated substrate, semiconductive a-C:H layers with an n charge-carrier Hall mobility of about 103 cm 2

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1 .sec 1 are obtained at a retention time of 250 msec and a power density of 0.8 N.cm in the plasma.

Figure 1 depicts the Hall mobility LH of the n and p charge carriers of the a-C:H layers according to the invention as a function of the electrical resistivity p. In addition, the values for crystalline Si and GaAS and a-Si:H are shown.

11 claims 1 iigure 182z/JRB

Claims (8)

1. A process for producing a semiconductor base material on a substrate, said process comprising the step of using high-frequency plasma deposition of gaseous hydrocarbons to provide on said substrate amorphous, hydrogen-containing carbon in a thin layer, wherein: the average retention time of the hydrocarbons in the plasma is at least 15 msec, with the retention time being defined as the ratio of the product of plasma volume times gas pressure to mass throughput; the power density in the plasma is between 0.2 and W.cm 3 and the substrate is not heated.

2. The process according to claim 1, wherein the average .retention time is between 50 and 500 msec.

3. The process according to claim 1 or 2, wherein the gas pressure is between 5 and 400 Pa.

4. The process according to claim 3, wherein the gas pressure is between 20 and 200 Pa.

The process according to any one of the preceding claims, wherein the mass throughput is between 0.05.10 and 2.10 20 Pa.cm 3 sec 1 S"

6. The process according to any one of the preceding claims wherein the plasma deposition is effected with the aid of radio frequency 0 energy.

7. The process according to any one of the preceding claims wherein alkanes are used as the hydrocarbons.

8. A process for producing a semiconductor base material on a substrate substantially as described herein with reference to the drawing. 1 .s DATED this ELEVENTH day of JUNE 1991 Siemens Aktiengesellschaft Patent Attorneys for the Applicant SPRUSON FERGUSON ,V 1 11240 j