METHOD OF MANUFACTURING P-TYPE
COMPOUND SEMICONDUCTOR
BACKGROUND OF THE INVENTION 5
1. Field of the Invention
The present invention relates to a method of manufacturing a II-VI Group compound semiconductor device and a III V Group compound semiconductor device used as a light-emitting device, for example, a UV- 10 emitting laser diode, blue light-emitting laser diode, UV-emitting diode, or blue light-emitting diode and more specifically, to a method of manufacturing a lowresistance p-type compound semiconductor from a III-V Group compound semiconductor and a II-VI 15 Group compound semiconductor from by doping ptype compounds thereinto as impurities.
2. Description of the Related Art
Studies on blue light-emitting elements have been generally conducted using ZnSe, which is a II-VI 20 Group compound, SiC, a IV-IV Group compound, or GaN, a III-V Group compound.
Of the types of compounds mentioned above, it was recently found that a gallium nitride series compound [GaxAli_xN (where 0Sx= 1) semiconductor exhibits 25 excellent semiconductor light emission at room temperature, and therefore much attention is now being paid to the GaN series semiconductor.
A blue light-emitting basically has a structure in which n type, and i-type or p-type GaN series semicon- 30 ductors each represented by general formula Ga^. Ali_xN (where OSxg 1) are stacked in turn on a sapphire substrate.
There are several well-known methods for growing a III-V Group compound, such as the metalorganic 35 chemical vapor deposition (MOCVD) method, the molecular beam epitaxy method, and the hydride vapor phase epitaxy method. As an example, the MOCVD method will be briefly described. In this method, a metalorganic compound gas serving as a reaction gas 40 (for example, trimethyl gallium (TMG), trimethyl aluminum (TMA), or ammonium) is introduced into a reaction container (vessel) in which a sapphire substrate is placed. Then, while mamtaining the epitaxial growth temperature as high as 900° C.-llOO0 C., an epitaxial 45 film of a III-V Group compound is grown on the substrate. By supplying suitable impurity gas during the growth of the film according to circumstances, a multilayer made of the n-type and p-type III-V Group compound semiconductors can be manufactured. In general, 50 Si is a well-known n-type impurity; however in the case of a GaN series compound semiconductor, there is a tendency for the semiconductor to exhibit the n-type characteristics even without doping an n-type impurity. Some of the well-known examples of p-type impurities 55 are Mg and Zn.
There can be proposed a method described below, as an improved version of the MOCVD method. When a III-V Group compound semiconductor is directly epitaxial-grown on a sapphire substrate at a high tempera- 60 ture, the surface condition of the crystals, and the crystallinity will be extremely degraded. In order to avoid this, before the compound is grown at the high temperature, an A1N buffer layer is formed on the substrate at a temperature as low as about 600" C, and then the com- 65 pound is grown on the buffer layer at a high temperature. The fact that the crystallinity of GaN can be remarkably improved by the above-mentioned technique
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is disclosed in Published Unexamined Japanese Patent Application No. 2-229476. Meanwhile, the authors of the present invention disclosed in Japanese Patent Application No. 3-89840, prior to the present application, that a gallium nitride compound semiconductor having a better crystallinity can be formed when a GaN buffer layer is used than when a conventional A1N buffer layer is used.
However, a blue light-emitting device employing a blue color-emitting element including a GaN series compound semiconductor has not yet been developed as a practical device. This is because p-type III-V Group compound semiconductor having a sufficientlylow-resistance cannot be produced by any of the conventional techniques, and therefore a light-emitting element having various types of structure such as p-type double hetero, single hetero, etc. cannot be manufactured. In the case where an epitaxial film is formed by the conventional chemical vapor deposition method, even if the film is grown while doping p-type impurities, it is impossible to make III-V Group compound semiconductor characteristic p-type. And also a semi-insulation material having a high resistivity of 108 fl-cm or higher, i.e., an i-type semiconductor may be obtained. Consequently, at present, the blue-light-emitting element having a structure of the p-n junction diode cannot be achieved, but a so-called MIS structure is the only one known structure for the blue-color-emitting element, in which structure, a buffer layer, an n-type film, and an i-type film are formed on a substrate in the mentioned order.
Published Unexamined Japanese Patent Application No. 2-257679 discloses a method for reducing the resistance of a high-resistance i-type semiconductor as little as possible to convert into a type close to a p-type one. In this method, a high-resistance i-type GaN compound semiconductor layer into which Mg was doped as a p-type impurity is formed on the top of the multilayer of the GaN compound semiconductor. Then, while maintaining the temperature of the compound not higher than 600° C, electron beams having an acceleration voltage of 5 kV-15 kV are irradiated on the surface so as to reduce the resistance of the layers located in the surface portion within a depth of about 0.5 fi,m. However, with this method, reduction of the resistance can be achieved only up to the point where electron beams can reach i.e. a very thin surface portion. Further, in the method, the electron beams cannot be irradiated on the entire wafer while scanning the beams, and consequently the resistance cannot be uniformly reduced in the desired surface. Further, this method entails the problem of a very low reproducibility, i.e., the resistance value changes every time electron beam is irradiated to the same sample. With this method, it is impossible to constantly produce blue-light-emitting elements having a high efficiency.
Study is being directed not only to III-V Group compounds, but also to II-VI Group compounds in order that they can be put into practical use. As in the case of the GaN compound production method, the chemical vapor deposition method such as the MOCVD can be used to form a II-VI Group compound semiconductor.
Growth of ZnSe by the MOCVD method will be briefly described. In this method, an metalorganic compound gas (diethylzinc (DEZ), hydrogen selenide (H2Se), etc.) is introduced as reaction gas into a reaction vessel in which a GaAs substrate is placed. Then, while
3 4
maintaining the epitaxial growth temperature at about With the method recited in the invention, III-V 350' C, ZnSe is grown on the substrate. During the Group compound semiconductors and II-VI compound growth, an appropriate impurity gas is supplied to the semiconductors, which conventionally cannot be convessel to form an n-type or p-type ZnSe semiconductor. verted into low-resistance p-type semiconductors even Examples of the type of substrate are GaAs and ZnSe. 5 though p-type impurities are doped thereinto, can be Further, CI is a well known n-type impurity, and N is converted into low-resistance p-type semiconductors also the well-known p-type impurity. with high yield. Accordingly, elements having a vanHowever, as in the case of the before-mentioned 0us types of structure can be produced at a high yield, p-type GaN compound, a sufficiently low-resistance Further, with the conventional electron-beam irradip-type ZnSe compound cannot be produced by this 10 ation method, reduction of the resistance can be conventional technique, and therefore a light-emitting achieved only in the surface portion of the uppermost element having various types of structure such as dou- iayer. rn the invention, the entire portion of the p-typeble hetero, single hetero, etc. cannot be manufactured. impurity-doped III-V Group compound semiconductor In the case where eptaxial-growing is performed by the or the II VI Group compound semiconductor can be conventional chemical vapor deposition method while 15 converted into p-type, uniformly within the surface area doping p-type impurities, the obtained ZnSe compound ^ weu as in the thickness direction. In addition, it is semiconductor will be a compound having a high resis- possible to form thick layers of these semiconductors by tivity of 108 ft-cm or higher. this method, and therefore blue-light or green-light SUMMARY OF THE INVENTION 20 emittin8 elements with a high level of brightness can be
manufactured.
The purpose of the invention is to provide an im- Additional objects and advantages of the invention
provement of a method of manufacturing a semiconduc- wn] ^ xt forth m the description which follows, and in
tor element from a II-VI group compound, or III-V part wi]1 ^ obvious from the description, or may be
group compound, which can be obtained by doping a 2J jearned by practice of the invention. The objects and
p-type impurity thereto, more specifically to a method advantages of the invention may be realized and ob
of manufacturing a low-resistivity p-type GaN com- tained by means of the instrumentalities and combina
pound semiconductor element having an uniform resis- tions particular,y ... out b the appended claims, tance value over its entirety regardless of film thickness,
and having a structure usable as a light-emitting element 3Q BRIEF DESCRIPTION OF THE DRAWINGS
with a double or single hetero constitution. ^ acc0mpanying drawingSj which are incorpo
According to the first aspect of the invention, there is rated in ^ constitute a part 0f the specification, illus
prov!de a method of manufacturing a p-type III-V trate presently preferred embodiments of the invention
Group compound semiconductor element by a vapor and( together whh the general description given above
phase epitaxy method, comprising the steps of: 35 ^ tne ... descnption of the preferred embodi
eptaxial-growing a III-V Group compound by intro- ments iven Moy/ ^ tQ lain the rinci les of the
ducing a reaction gas containing a p-type impunty on a invention
substrate; and FIG. 1 is a graph showing the relationship between
annealing the compound at a temperature of 400 C. ^ temperature for sealing and the resistivity of a
compound annealed at the temperature.,
or higher. 40
According to the second aspect of the invention, - . , , T . .. ,. . .
A. • _, _, .jr r.- FIG. 2 is a graph showing the relationship between
there is provided a method of manufactunng a p-type . .f r, , ~. . . .. . f. . ,
m „ „r , , , i fu the wavelength and the relative intensity of the photo
III-V Group compound semiconductor element by a .. ° ... , • j .
, .. , <■ luminescence of the compound semiconductor
vapor phase epitaxy method, comprising the steps of: „ . ... , , • , • ,
Tti \r r* — J u • * J ■ FIG. 3 is a graph showing the relationship between
growing a III-V Group compound by introducing a 45 , , ... , • ?, ,„„.;„ „„„ „ . • • „ „ „ t„„; •,.' . _ the wavelength and the relative intensity of the photoreaction gas containing a p-type impunty on a substrate; , . e , / . v. and luminescence of a compound semiconductor having a
irradiating electron beams on the compound while C&Z}^el' ■ .... ....
maintaining a surface temperature of the compound at FIG' 4 1S a *TMPh show"1« the relationship between 600° C or higher 50 tne su"ace temperature of a III-V Group compound According to the third aspect of the invention, there semiconductor layer during electron beam irradiation is provided a method of manufacturing a p-type II-VI 811(1 the resistivity thereof annealed at the temperature; Group compound semiconductor element by a chemi- FIG- 5 ls a ^Ph showing the relationship between cal vapor deposition method, comprising the steps of: the surface temperature of a p-type III-V Group corngrowing a II-VI Group compound by introducing a 55 pound semiconductor during electron-beam irradiation reaction gas containing a p-type impurity on a substrate; 81113 the relative intensity of the photoluminescence of and the compound semiconductor irradiated at the temperaannealing the compound at a temperature of 300° C. ture! or higher. FIG. 6 is a graph showing the relationship between According to the fourth aspect of the invention, there 60 the surface temperature of a p-type III-V Group comis provided a method of manufacturing a p-type III-V pound semiconductor having a cap layer during elecGroup compound semiconductor element by a vapor tron-beam irradiation and the relative intensity of the phase epitaxy method, comprising the steps of: photoluminescence of the compound semiconductor
growing a II-VI compound by introducing a reaction irradiated at the temperature; and gas containing a p-type impurity on a substrate; and 65 FIG. 7 is a graph showing the relationship between
irradiating electron beams on the compound while the annealing temperature of a p-type impurity doped
maintaining a surface temperature of the compound at ZnSe compound semiconductor layer and the resistiv
300° C. or higher. ity thereof annealed at the temperature.
DETAILED DESCRIPTION OF THE
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density of 8Xl010/cm3, whereas after annealing, the layer had a resistivity of 2 ft-cm and a hole carrier denPREFERRED EMBODIMENTS sity of 2x 10i7/cm3. FiG. 1 shows a case of only the
The present invention is provided to an improved GaN layer, but it was confirmed that a p-type impuritymethod of manufacturing a p-type compound semicon- 5 doped Ga^Ali-xN (0=x^l) also exhibits a similar ductor. result.
According to the present invention a III-V Group Next, the 4 jim-thick GaN layer annealed at 700° C. compound semiconductor or a II-VI group compound was etched to reduce the thickness thereof to 2 jim, and semiconductor can be manufactured as a p-type com- the Hall measurement was performed for the GaN pound semiconductor. 10 layer. The result indicated that the GaN layer had a
Further, according to the invention, each layer of the resistivity of 3 Cl-cm and a hole carrier density of compounds is formed by the vapor phase epitaxy 2xl017/cm3, which were very close to those of the method, and then the formed layer is annealed at a same GaN layer before etching. From the results, it can predetermined temperature, while each layer is irradi- be concluded that a p-type impurity doped GaN layer ated by electron beam on the layer, with being kept the 15 having an uniform low resistivity in the entire area and surface temperature of the layer at a predetermined in the thickness direction, is obtained by annealing, temperature. Annealing of the p-type-impurity-doped GaN series
The first aspect of the invention is provided to a compound semiconductor layer may be conducted in method of manufacturing a p-type impurity doped the reaction vessel after forming the layer, or in an III-V Group compound semiconductor by a vapor 20 annealing equipment after transferring the substrate phase epitaxy method wherein after forming p-type having the compound semiconductor layer from the impurities doped compound layers, the formed layers reaction vessel thereinto
are annealed at a predetermined temperature. The annealing may be carried out in a vacuum, or in
In this method, the annealing step is carried out at a an N2 atmosphere, or in an inert gas atmosphere of He, temperature of 400° C. or higher. The annealing tern- 25 Ne, Ar or the like, or an atmosphere of a mixture gas perature is preferably 600° C.-1200" C. The annealing thereof. Most preferably, the annealing should be percan be performed at over 1200° C, but this may cause formed in a nitrogen atmosphere which is pressurized to high cost. In the annealing step, the temperature within a level or higher of the decomposition pressure for the the above-mentioned range is fixed constant, and the GaN compound semiconductor at the annealing temtime is not less than 1 minute, preferably 10 minutes or 30 perature With the nitrogen atmosphere pressurizing the more. GaN compound semiconductor, decomposition and the
Even if the annealing temperature is 1000° C. or compound and split-off of N therefrom during annealhigher, decomposition of the compound can be pre- ing can be prevented.
vented by pressurizing the compound with nitrogen. For example, in the case of GaN, the decomposition Thus, p-type III-V Group compound semiconductors 35 pressure is about 0.01 atom at a temperature of 800° C, each having an excellent crystallinity can be stably about 1 atom at 1000° C, and about 10 atoms at 1100° C. obtained. Consequently, the GaN series compound semiconduc
FIG. 1 shows a property of a p-type-impurity-doped tor is annealed at 400° C, some decomposition may GaN series compound semiconductor as a III-V Group occur during annealing at 400° C. If decomposition compound semiconductor, and is a graph showing the 40 occurs, the crystallinity of the GaN compound semirelationship between an annealing temperature and the conductor tends to be degraded. Therefore, as stated resistance value of the GaN series compound semicon- before, the decomposition can be prevented by mainductor annealed at the temperature. As can be seen in taming a pressure of the nitrogen atmosphere no lower FIG. 1, the high-resistivity GaN series compound semi- than the decomposition pressure at the annealing temconductor can be converted into a low-resistivity p- 45 perature.
type impurity compound semiconductor by annealing. FIG. 2 is a graph showing the difference in crystallinIn the graph, the resistivity obtained by the Hall mea- ity between GaN series compound semiconductors as surement on the annealed GaN series compound semi- III-V Group compound semiconductors one of which conductor is plotted as a function of the annealing tern- annealed under a pressurized condition and the other perature. The used GaN series compound semiconduc- 50 under an atmospheric pressure condition. Each GaN tor was formed by growing a GaN buffer layer on a series compound semiconductor is prepared by forming sapphire substrate by the MOCVD method, followed a GaN buffer layer and a Mg-doped 4 /xm-thick GaN by formation of a 4 fim thick GaN layer on the buffer layer on sapphire substrate, and annealing at 1000° C. in layer while doping Mg thereinto as a p-type impurity. a nitrogen atmosphere for 20 minutes under 20 atm of a The data plotted on FIG. 1 were obtained as results of 55 pressurized condition, or in an atmospheric pressure annealing the substrate having these layers in a nitrogen condition. The p-type GaN layers were irradiated with atmosphere for 10 minutes at- various temperatures He-Cd laser beams from He-Cd laser beam source as an using an annealing equipment. excitation light beam source, so as to measure the inten
As is clear from this graph, the resistivity of the Mg- sity of photoluminescence as an evaluation of the crysdoped GaN layer sharply dropped around the point 60 tallinity. The evaluation is based on the fact, i.e., the where the temperature exceeded 400° C. When the higher the blue-light-emitting intensity of the photolutemperature was increased to higher than 700° C, the minescence at 450 nm, the higher the crystallinity. In GaN layer exhibited a substantially constant low-resis- FIG. 2, a curve 201 indicates the property of the p-type tivity p-type property, indicating the effect of anneal- GaN layer annealed under a pressure of 20 atoms, and a ing. For comparison, the Hall measurement was carried 65 curve 202 indicates the case of annealing at atmospheric out for a GaN layer before annealing and after anneal- pressure.
ing at 700° C. or higher. Before annealing, the GaN As is clear from FIG. 2, in the case where annealing layer had a resistivity of 2X 105 ft-cm and a hole carrier is carried out at a temperature of 1000° C. or higher, a
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