Method of fabricating a P type nitride semiconductor layer doped with carbon

Abstract

A method of stably manufacturing a p type nitride semiconductor layer using a carbon dopant is provided. A crystal plane substrate is prepared having a main surface which has an offset angle in a range of +/0.1% with respect to a C-plane or a crystal plane equivalent to the C-plane; and during a time period in which a III-source gas and a V-source gas are supplied to grow a III-V group nitride semiconductor layer, carbon tetrabromide (CBr.sub.4), which is a carbon source gas, is supplied so as to introduce carbon into a V-group atom layer.

Claims

1. A method of growing, using metal organic vapor phase epitaxy (MOVPE), a uniform p type III-V group nitride semiconductor layer on a substrate directly or with a single or a plurality of intermediate layers being provided therebetween, the method comprising the steps of: (a) growing a layer consisting of 5 to 7 molecular layers of III-group atoms by supplying a III-source gas during a first time period; (b) growing a carbon-doped layer consisting of 5 to 7 molecular layers of V-group atoms by supplying a V-source gas during a second time period separate from the first time period and supplying a carbon source gas as a p type dopant during a time period that overlaps and is shorter than the second time period, alternately repeating the step of (a) and the step of (b), wherein: the substrate is either a sapphire substrate, a silicon substrate, a silicon carbide substrate, a gallium nitride substrate or an aluminum nitride substrate; and the substrate has a main surface having an offset angle in a range of +/0.1% with respect to a C-plane or a crystal plane equivalent to the C-plane.

2. The uniform III-V group nitride semiconductor layer growth method according to claim 1, wherein the uniform III-V group nitride semiconductor layer has a thickness of 0.1 m or more.

3. A method of growing, using metal organic vapor phase epitaxy (MOVPE), a uniform p type III-V group nitride semiconductor layer on a substrate directly or with a single or a plurality of intermediate layers being provided therebetween, the method comprising the steps of: (a) growing a layer consisting of 5 to 7 molecular layers of III-group atoms by supplying a III-source gas which is a source of Al.sub.xGa.sub.1-xN (0<x1) during a first time period, (b) growing a carbon-doped layer consisting of 5 to 7 molecular layers of V-group atoms by supplying a V-source gas which is a source of Al.sub.xGa.sub.1-xN (0<x1) during a second time period separate from the first time period, and alternately repeating the step of (a) and the step of (b), and further comprising supplying a carbon source gas as a p type dopant during a time period that overlaps with and is shorter than the second time period in which the V-source gas is supplied to introduce carbon into the layer of V-group atoms.

4. The uniform III-V group nitride semiconductor layer growth method according to claim 3, wherein an Mg source gas is supplied during the second time period in which the V-source gas is supplied.

5. The uniform III-V group nitride semiconductor layer growth method according to claim 3, wherein the Mg source gas is supplied during the first time period in which the III-source gas is supplied and during the second time period in which the V-source gas is supplied.

6. The uniform III-V group nitride semiconductor layer growth method according to claim 3, wherein an Mg source gas is supplied during the certain second time period in which the V-source gas is supplied.

7. The uniform III-V group nitride semiconductor layer growth method according to claim 3, wherein the substrate is either a sapphire substrate, a silicon substrate, a silicon carbide substrate, a gallium nitride substrate or an aluminum nitride substrate, and the carbon source gas is carbon tetrabromide (CBr.sub.4).

8. The uniform III-V group nitride semiconductor layer growth method according to claim 3, wherein the substrate has a main surface having an offset angle in a range of +/0.1% with respect to a C-plane or a crystal plane equivalent to the C-plane.

9. A method of growing, using metal organic vapor phase epitaxy (MOVPE), a uniform p type III-V group nitride semiconductor layer on a substrate directly or with a single or a plurality of intermediate layers being provided therebetween, the method comprising the steps of: supplying a III-source gas and a V-source gas, which are sources of Al.sub.xGa.sub.1-xN (0<x1) simultaneously, and supplying a carbon source gas as a p type dopant to the reactor together with the III-source gas and the V-source gas; wherein an amount ratio of V-source gas/III-source gas is 5 or higher and 600 or lower and the substrate has a main surface having an offset angle in a range of +/0.1% with respect to a C-plane or a crystal plane equivalent to the C-plane.

10. The uniform III-V group nitride semiconductor layer growth method according to claim 9, wherein: the substrate is either a sapphire substrate, a silicon substrate, a silicon carbide substrate, a gallium nitride substrate or an aluminum nitride substrate; and the carbon source gas is carbon tetrabromide (CBr.sub.4).

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the description serve to explain the principles of the invention.

(2) FIG. 1A shows one cycle of a gas supply method in a first example for epitaxially growing Al.sub.xGa.sub.1-xN, which uses an alternating supply method and FIG. 1B illustrates the alternate growth of 5 to 7 molecular layers of III-group atoms and 5 & 7 molecular layers of V-group atoms;

(3) FIG. 2 shows one cycle of a gas supply method in a second example for epitaxially growing Al.sub.xGa.sub.1-xN, which uses the alternating supply method;

(4) FIG. 3 shows one cycle of a gas supply method in a third example for epitaxially growing Al.sub.xGa.sub.1-xN, which uses the alternating supply method;

(5) FIG. 4 shows an ionized acceptor concentration of an Al.sub.xGa.sub.1-xN (x=0.55) semiconductor layer, which is grown by the gas supply method in the first example, in a film depth direction which is measured by a C-V measurement method (ionized dopant concentration measurement);

(6) FIG. 5 shows an ionized acceptor concentration of an Al.sub.xGa.sub.1-xN (x=0.55) semiconductor layer, which is grown by the gas supply method in the second example, in the film depth direction which is measured by the C-V measurement method (ionized dopant concentration measurement);

(7) FIG. 6 shows one cycle of a gas supply method in a fourth example for epitaxially growing Al.sub.xGa.sub.1-xN, which uses a simultaneous supply method;

(8) FIG. 7 shows the relationship between the flow rate of CBr.sub.4, which is carbon source gas, and an effective ionized acceptor concentration of carbon-doped Al.sub.xGa.sub.1-xN (x=0.1);

(9) FIG. 8 shows the relationship between the flow rate of CBr.sub.4, which is carbon source gas, and an effective ionized acceptor concentration of carbon-doped Al.sub.xGa.sub.1-xN (x=0.55);

(10) FIG. 9 shows the measurement results of the I-V characteristics of carbon-doped Al.sub.xGa.sub.1-xN (x=0.27) according to the present invention and Mg-doped Al.sub.xGa.sub.1-xN (x=0.27);

(11) FIG. 10 shows the measurement results of contact resistance, sheet resistance, resistivity, carrier mobility, sheet carrier density, and carrier density of carbon-doped Al.sub.xGa.sub.1-xN (x=0.27) according to the present invention, which was grown while changing the Al composition rate, the flow rate of the carbon source and the layer thickness;

(12) FIG. 11 is a conceptual view of an example of layer structure of a nitride semiconductor light emitting device using carbon-doped Al.sub.xGa.sub.1-xN according to the present invention; and

(13) FIG. 12 is a conceptual view of another example of layer structure of a nitride semiconductor light emitting device using carbon-doped Al.sub.xGa.sub.1-xN according to the present invention.

DESCRIPTION OF EMBODIMENTS

First Embodiment

(14) Hereinafter, a method of manufacturing a III-V group nitride semiconductor doped with carbon as a p type dopant according to the present invention will be explained.

(15) A III-V group nitride semiconductor layer doped with carbon as a p type dopant according to the present invention can be formed using a known deposition method which deposits a film on a substrate surface, such as metal organic chemical vapor deposition (MOCVD) or low pressure chemical vapor deposition (LPCVD).

(16) In an embodiment of the present invention, an Al.sub.xGa.sub.1-xN layer is manufactured as a III-V group nitride semiconductor layer by use of MOCVD (also referred to as MOVPE (metal organic vapor phase epitaxy)).

(17) First, a substrate on which Al.sub.xGa.sub.1-xN is to be epitaxially grown is prepared. Any crystal plane substrate having a main surface which has an offset angle in a range of +/0.1% with respect to a C-plane or a crystal plane equivalent to the C-plane is usable; therefore, the substrate is not limited to a sapphire substrate but may be any of various types of substrates including a silicon substrate, a silicon carbide substrate, a gallium nitride substrate and an aluminum nitride substrate. In the present specification, an intermediate layer refers to a layer grown on a substrate.

(18) When the substrate has a main surface having an offset angle in a range of +/0.1% with respect to a C-plane or a crystal plane equivalent to the C-plane, 5 to 7 molecular layers of III-group atoms and 5 to 7 molecular layers of V-group atoms can be deposited alternately on the substrate using a deposition method. More specifically, 5 to 7 molecular layers of III-group atoms, 5 to 7 molecular layers of V-group atoms, another 5 to 7 molecular layers of III-group atoms, another 5 to 7 molecular layers of V-group atoms, and the like are deposited.

(19) As sources of a III-group element for growing Al.sub.xGa.sub.1-xN, trimethylgallium (TMG) and triethylaluminum (TMA; (CH.sub.3).sub.3Al), for example, are used. As a source of Mg, cyclopentadienyl magnesium (Cp.sub.2Mg), for example, is used. As a source of a V-group element for growing Al.sub.xGa.sub.1-xN, ammonia (NH.sub.3), for example, is used. A carrier gas for transporting the sources may be, for example, hydrogen (H.sub.2). It should be noted that these materials are merely examples and the present invention is not limited to using these materials.

(20) Materials usable as a source of a carbon (C) dopant (hereinafter, referred to as a carbon source) include, for example, carbon tetrabromide (CBr.sub.4). In the present invention, the carbon source is not limited to carbon tetrabromide (CBr.sub.4). However, it is not preferred to use acetylene as a carbon source because it has high reactivity and is dangerous. It is not preferred, either, to use carbon tetrachloride as a carbon source because it has etching effects, and when the flow rate thereof is too high, the crystal growth rate decreases significantly and a semiconductor layer no longer grows. Carbon tetrabromide is a halide like carbon tetrachloride and also has etching effects but is preferably used as a carbon source because bromine has a greater atomic number than that of chlorine, and therefore a chemical reaction thereof occurs slightly more slowly than that of carbon tetrachloride.

(21) The conditions for growing Al.sub.xGa.sub.1-xN using MOVPE are as follows, for example:

(22) Growth temperature: 1180 C. or higher and 1370 C. or lower

(23) Substrate surface temperature: 1070 C. or higher and 1250 C. or lower

(24) Source gas pressure during growth: 4000 Pa or higher and 20000 Pa or lower

(25) Amount ratio of V-group element source/III-group element source: 5 or higher and 600 or lower

(26) Supply amount of CBr.sub.4: 710.sup.8 mol/min or more and 1.710.sup.5 mol/min or less

(27) Supply amount of Cp.sub.2Mg: 1.310.sup.7 mol/min to 1.610.sup.7 mol/min

(28) Supply amount of source gas of III-group element (TMG and TMAl): 510.sup.5 mol/min

(29) The above-described growth conditions are examples only, and any other conditions are usable as long as Al.sub.xGa.sub.1-xN is grown using MOVPE. It should be noted that the dissociation rate of nitrogen atoms from ammonia gas molecules depends highly on the temperature. In the case where ammonia gas is used as a component of a source gas of a V-group element, the dissociation rate of nitrogen atoms from ammonia gas molecules is closely related to the growth temperature of a p type AlGaN semiconductor layer. Therefore, it is preferred that a source gas of a III-group element (hereinafter, referred to as a III-source gas) and a source gas of a V-group element (hereinafter, referred to as a V-source gas) are supplied under the conditions that the growth temperature is in a range of 1180 C. or higher and 1370 C. or lower and the substrate temperature is in a range of 1070 C. or higher and 1250 C. or lower.

(30) When the growth temperature is lower than 1180 C., it is difficult for carbon to enter the sites of nitrogen atoms within the AlGaN crystal. Therefore, it is preferred that the growth temperature is 1180 C. or higher. By contrast, when the growth temperature is higher than 1370 C., gallium atoms vaporize. Therefore, it is preferred that the growth temperature is 1370 C. or lower.

(31) It is preferred that the optimal growth temperature of the AlGaN semiconductor layer to be formed is changed in accordance with the content (mol %) of aluminum in the AlGaN semiconductor layer. For example, in the case of AlGaN having an aluminum content of a few mol percent to 25 mol %, the optimal growth temperature is 1180 C. or higher and 1230 C. or lower. However, when the content (mol %) of aluminum in the AlGaN layer to be formed is higher, the growth temperature needs to be higher from the viewpoints of crystal quality and dopant characteristics. The optimal growth temperature in this case is 1180 C. or higher and 1370 C. or lower.

(32) The amount ratio of V-group element source/III-group element source is set to a range of 5 or higher and 600 or lower because with the ratio in this range, carbon enters the sites of nitrogen atoms within the AlGaN crystal easily. With the ratio in this range, the amount of carbon which can be doped into the sites of nitrogen atoms within the AlGaN crystal can be maximized. When the growth temperature is 1250 C. or higher, an amount ratio of V-group element source/III-group element source of 5 or higher allows a sufficient amount of carbon to enter the sites of nitrogen atoms within the AlGaN crystal. By contrast, when the growth temperature is lower than 1250 C., it is preferred that the amount ratio of V-group element source/III-group element source is 200 or higher and 600 or lower.

(33) Carbon tetrabromide, which is a carbon source, is preferably supplied to a reactor as follows. Carbon tetrabromide is dissolved in a solvent having a low vapor pressure and accommodated in a bubbler. A carrier gas is bubbled to supply the resultant carbon tetrabromide to the reactor. This method is preferable because it allows the carbon source to be supplied to the reactor stably and thus is suitable to a method of forming a p type semiconductor by putting carbon into the sites of nitrogen atoms within the AlGaN crystal.

(34) Hereinafter, specific methods of supplying a gas usable in the method of manufacturing a III-V group nitrogen semiconductor layer doped with carbon as a p type dopant according to the present invention will be explained with reference to FIG. 1A through FIG. 3 and FIG. 6.

(35) As methods of supplying gas for growing a III-V group nitrogen semiconductor layer using MOVPE, a simultaneous supply method of supplying a III-source gas and a V-source gas simultaneously, and an alternating supply method of supplying a III-source gas and a V-source gas alternately, are available. FIG. 1A shows one cycle of a gas supply method in a first example for epitaxially growing Al.sub.xGa.sub.1-xN. In the first example, the alternating supply method is used.

(36) As is shown in FIG. 1A, a reactor is charged with trimethylgallium (TMG) and triethylaluminum (TMA; (CH.sub.3).sub.3Al), which are respectively sources of Ga and Al as III-group elements, for a time period T.sub.1. Cyclopentadienyl magnesium (Cp.sub.2Mg), which is a source of Mg, is supplied to the reactor for a time period t.sub.1, which overlaps and is shorter than the time period T.sub.1.

(37) After the time period T.sub.1 elapses and then an interval time period I.sub.1 elapses, ammonia (NH.sub.3), which is a source of N as a V-group element, is supplied to the reactor for a time period T.sub.2. Carbon tetrabromide (CBr.sub.4), which is a carbon source, is supplied to the reactor for a time period t.sub.2, which overlaps and is shorter than the time period T.sub.2.

(38) After the time period T.sub.2 elapses and then an interval time period I.sub.2 elapses, the III-group element source is supplied for the time period T.sub.1 and the Mg source is supplied for the time period t.sub.1. Then, after the interval time period I.sub.1 elapses, the V-group element source is supplied for the time period T.sub.2, and the carbon source is supplied for the time period t.sub.2. This cycle is repeated multiple times. In this way, Al.sub.xGa.sub.1-xN is grown to a desired film thickness.

(39) The interval time period I.sub.1 and the interval time period I.sub.2 are each 2 seconds at the maximum. The interval time period I.sub.1 and the interval time period I.sub.2 may be 0 seconds. However, with neither the interval time period I.sub.1 nor I.sub.2, there is a possibility that the III-source gas mixes with the V-source gas. Therefore, it is preferable to provide the interval time periods I.sub.1 and I.sub.2. By contrast, when the interval time periods I.sub.1 and I.sub.2 are set to be longer than 2 seconds, the crystal quality at the interface of the grown film may degrade significantly due to re-evaporation of the components in the grown film from the interface, or adsorption or incorporation of a residual gas. Therefore, it is not preferred to set each of the interval time periods I.sub.1 and I.sub.2 to be longer than 2 seconds.

(40) Under the growth conditions by MOVPE described above, the AlGaN semiconductor layer grown by the gas supply method in the first example stably has p type conductivity. A reason for this is that since a substrate having a main surface which has an offset angle in a range of +/0.1% with respect to a C-plane or a crystal plane equivalent to the C-plane is used, as shown in FIG. 1B, 5 to 7 molecular layers of III-group atoms are grown at 102 and 5 to 7 molecular layers of V-group atoms are grown at 104. The process is repeated a number of times so that they are alternately stacked on the substrate and thus carbon securely enters the V-group atom layers.

(41) Carbon, when entering a III-group atom layer, becomes an n type dopant, and when entering a V-group atom layer, becomes a p type dopant. According to the growth method of the present invention, as shown in FIG. 1B, about 5 to 7 molecular layers of III-group atoms are grown at 102 and about 5 to 7 molecular layers of V-group atoms are grown at 104, are alternately stacked on the substrate, and carbon enters the V-group atom layers. Therefore, according to the method of the present invention, a p type AlGaN semiconductor layer can be formed stably.

(42) FIG. 2 shows one cycle of a gas supply method in a second example for epitaxially growing Al.sub.xGa.sub.1-xN. In the second example, the alternating supply method is used.

(43) In the first example, cyclopentadienyl magnesium (Cp.sub.2Mg) is supplied for the time period t.sub.1, which overlaps and is shorter than the time period T.sub.1. By contrast, in the second example, cyclopentadienyl magnesium (Cp.sub.2Mg) is supplied for the time period t.sub.2, in which carbon tetrabromide (CBr.sub.4) as a carbon source is supplied. The other steps in the second example are substantially the same as those in the first example.

(44) FIG. 3 shows one cycle of a gas supply method in a third example for epitaxially growing Al.sub.xGa.sub.1-xN. In the third example, the alternating supply method is used.

(45) In the third example, unlike in the first and second examples, cyclopentadienyl magnesium (Cp.sub.2Mg) is supplied to the reactor for the time period t.sub.1 and the time period t.sub.2. The other steps in the third example are substantially the same as those in the first and second examples.

(46) An Mg source gas, even if being supplied to the reactor continuously, does not cause any particular problem in the process of growing the AlGaN semiconductor layer. Since the Mg source gas can be supplied continuously, no precise control on the timing to supply the Mg source gas is necessary. This can simplify the manufacturing process.

(47) FIG. 4 shows an ionized acceptor concentration of an Al.sub.xGa.sub.1-xN (x=0.55) semiconductor layer in a film depth direction which is measured by a C-V measurement method (ionized dopant concentration measurement). The Al.sub.xGa.sub.1-xN semiconductor layer is grown by the gas supply method in the first example under the following growth conditions.

(48) Growth temperature: 1180 C. or higher and 1230 C. or lower

(49) Substrate surface temperature: 1070 C. or higher and 1110 C. or lower

(50) Source gas pressure during growth: 4000 Pa or higher and 20000 Pa or lower

(51) Amount ratio of V-group element source/III-group element source: 5 or higher and 600 or lower

(52) Supply amount of CBr.sub.4: 710.sup.8 mol/min or more and 1.710.sup.5 mol/min or less.

(53) Supply amount of Cp.sub.2Mg: 1.310.sup.7 mol/min to 1.610.sup.7 mol/min.

(54) Supply amount of III-source gas (TMG and TMAl): 510.sup.5 mol/min.

(55) The term ionized dopant concentration means the concentration of dopants, among the introduced dopants, which generate carriers and are negatively or positively ionized. The ionized dopants are not free carriers but ions fixed to the crystal. Thus, the C-V measurement method can accurately analyze the doping process without being influenced by an internal electrical field of the crystal. The device used in this measurement is ECV-Pro manufactured by Nanometrics.

(56) FIG. 4 shows that the area of the layer from the surface down to a level having a depth of 0.1 m exhibits p type conductivity. However, some areas deeper than this level have n type ions and p type ions inverted and thus are unstable.

(57) FIG. 5 shows an ionized acceptor concentration of an Al.sub.xGa.sub.1-xN (x=0.55) semiconductor layer in the film depth direction which is measured by the C-V measurement method (ionized dopant concentration measurement). The Al.sub.xGa.sub.1-xN semiconductor layer is grown by the gas supply method in the second example under the same growth conditions as above. FIG. 5 shows that the layer stably exhibits p type conductivity down to a level having a depth of 1.3 m. This indicates that when an Mg source gas and a carbon source gas are supplied simultaneously along with a V-source gas, carbon enters the V-group atom layer more actively.

(58) In the second example, the Mg source gas is supplied together with the carbon source gas. As is understood from the measurement results in FIG. 4 and FIG. 5, when a test is performed of growing an AlGaN semiconductor film by the gas supply method in the first example and an AlGaN semiconductor film by the gas supply method in the second example while the other conditions including the growth temperature and the like are the same, the AlGaN semiconductor film grown by the gas supply method in the second example exhibits a high ionized acceptor concentration more stably than the AlGaN semiconductor film grown by the gas supply method in the first example.

(59) A reason why carbon enters the V-group atom layer more easily in the second example than in the first example is as follows. According to the second example, cyclopentadienyl magnesium (Cp.sub.2Mg) is supplied together with the V-source gas, and thus Mg is doped into the V-group atom layer. Mg is considered to have an effect of introducing defects to the AlGaN crystal. As a result, when Mg is doped into the V-group atom layer at a much lower concentration than the carbon concentration, Mg can increase the number of defects within the AlGaN crystal to such a level that carbon can enter the V-group atom layer, without Mg damaging the hetero interface of the AlGaN crystal.

(60) Like the gas supply method in the second example, the gas supply method in the third example allows an Mg source gas to be supplied together with a carbon source gas and thus provides substantially the same effect as that of the gas supply method in the second example.

(61) FIG. 6 shows one cycle of a gas supply method in a fourth example for epitaxially growing Al.sub.xGa.sub.1-xN. In the fourth example, the simultaneous supply method is used.

(62) In the fourth example, trimethylgallium (TMG) and triethylaluminum (TMA; (CH.sub.3).sub.3Al), which are respectively sources of Ga and Al as III-group elements, are supplied to a reactor for a time period T.sub.1. Ammonia (NH.sub.3), which is a source of N as a V-group element, is supplied to the reactor simultaneously in the same time period (time period T.sub.2). Cyclopentadienyl magnesium (Cp.sub.2Mg), which is a source of Mg, and carbon tetrabromide (CBr.sub.4), which is a carbon source, are supplied to the reactor for a time period t.sub.2, which overlaps and is shorter than the time periods T.sub.1 and T.sub.2. In this way, Al.sub.xGa.sub.1-xN is grown to a desired film thickness.

(63) In the fourth example, unlike in the case of the alternating supply method, it is preferred that the amount ratio of V-group element source/III-group element source is as low as possible within a range of 5 or higher and 600 or lower. Carbon enters the V-group atom layer more easily when the V-group atoms are supplied while being depleted as much as possible.

(64) The gas supply methods in the plurality of examples have been explained above. In either the alternating supply method or the simultaneous supply method, when a substrate having a main surface which has an offset angle in a range of +/0.1% with respect to a C-plane or a crystal plane equivalent to the C-plane is used, about 5 molecular layers of III-group atoms and about 5 molecular layers of V-group atoms are alternately stacked on the substrate and carbon enters the V-group atom layers. Therefore, as long as a substrate having a main surface which has an offset angle in the above-described range is used, a p type AlGaN semiconductor layer can be stably grown regardless of whether the alternating supply method is used or the simultaneous supply method is used.

(65) However, in the case of the simultaneous supply method, V-group atoms need to be supplied while being depleted as much as possible as described above. Therefore, the simultaneous supply method requires strict control on the amount ratio of V-group element source/III-group element source. By contrast, in the case of the alternating supply method, the control on the amount ratio of V-group element source/III-group element source can be significantly relaxed, and carbon enters the V-group atom layers much more actively than in the simultaneous supply method.

(66) FIG. 7 shows the relationship between the flow rate of CBr.sub.4, which is a carbon source gas, and an effective ionized acceptor concentration of carbon-doped Al.sub.xGa.sub.1-xN (x=0.1). It can be understood from FIG. 7 that when the flow rate of CBr.sub.4 exceeds at least 12 mol/min, the effective ionized acceptor concentration reaches 10.sup.16 cm.sup.3; and after this, the effective ionized acceptor concentration increases in proportion to the flow rate of CBr.sub.4.

(67) FIG. 8 shows the relationship between the flow rate of CBr.sub.4, which is a carbon source gas, and an effective ionized acceptor concentration of carbon-doped Al.sub.xGa.sub.1-xN (x=0.55). FIG. 8 shows the results of a test performed as follows. Five substrates were prepared, and a plurality of layers were grown on each substrate while changing the flow rate of CBr.sub.4 for each layer. The effective ionized acceptor concentration was measured for each layer of each substrate. The marks , .circle-solid., .box-tangle-solidup., and specify the substrates, and the identical marks indicate the layers grown on the same substrate with different flow rates of CBr.sub.4. A plot of the identical marks indicates a change of the effective ionized acceptor concentration of the layers grown under the same growth conditions (offset angle of the main surface of the substrate, substrate surface temperature, source gas pressure during growth, etc.) apart from the flow rate of CBr.sub.4.

(68) As shown in FIG. 8, the effective ionized acceptor concentration increases generally in proportion to the flow rate of CBr.sub.4 in any of the growth conditions. It is understood that the effective ionized acceptor concentration can be controlled by controlling the flow rate of CBr.sub.4. In FIG. 8, like in the case of Al.sub.xGa.sub.1-xN (x=0.1), when the flow rate of CBr.sub.4 exceeds at least 11 mol/min, the effective ionized acceptor concentration reaches 10.sup.16 cm.sup.3; and after this, the effective ionized acceptor concentration increases in proportion to the flow rate of CBr.sub.4.

(69) It can be understood from the test described above that even when the Al composition rate in Al.sub.xGa.sub.1-xN is high, the effective ionized acceptor concentration can be adjusted by adjusting the flow rate of CBr.sub.4.

(70) FIG. 9 shows the measurement results of the I-V characteristics of carbon-doped Al.sub.xGa.sub.1-xN (x=0.27) according to the present invention and Mg-doped Al.sub.xGa.sub.1-xN (x=0.27).

(71) It can be understood from FIG. 9 that when a bias voltage of about 9V is applied to Mg-doped Al.sub.xGa.sub.1-xN (x=0.27), an injection current of only about 1 mA flows; whereas when a bias voltage of about 9V is applied to carbon-doped Al.sub.xGa.sub.1-xN (x=0.27) according to the present invention, an injection current of 20 mA flows.

(72) These results demonstrate that carbon-doped Al.sub.xGa.sub.1-xN (x=0.27) according to the present invention has significantly lower resistivity as compared to Mg-doped Al.sub.xGa.sub.1-xN (x=0.27). Therefore, in the case where carbon-doped Al.sub.xGa.sub.1-xN (x=0.27) according to the present invention is used for a p type cladding layer, a p type electrode can be stacked directly thereon without a contact layer being provided therebetween.

(73) FIG. 10 shows the measurement results of contact resistance, sheet resistance, resistivity, carrier mobility, sheet carrier density, and carrier density of carbon-doped Al.sub.xGa.sub.1-xN (x=0.27) according to the present invention, which was grown while changing the Al composition rate, the flow rate of the carbon source, and the and the layer thickness.

(74) According to the manufacturing method of the present invention, the aluminum composition rate in carbon-doped Al.sub.xGa.sub.1-xN was increased to as high as around 70% while the carrier density was raised to as high as (6.0 to 9.3)E+18/cm.sup.3.

Second Embodiment

(75) FIG. 11 is a conceptual view of a layer structure of a nitride semiconductor light emitting device using carbon-doped Al.sub.xGa.sub.1-xN according to the present invention. In addition to the layers shown in FIG. 11, a cap layer, for example, may be formed, when necessary, between a light emitting layer (active layer) 4 and a p type nitride semiconductor layer 5, so that a dopant doped into the p type nitride semiconductor layer 5 does not diffuse into the light emitting layer 4.

(76) Reference sign 1 refers to a substrate. The substrate 1 is a crystal plane substrate having a main surface which has an offset angle in a range of +/0.1% with respect to a C-plane or a crystal plane equivalent to the C-plane. The substrate 1 may be any of various types of substrates including a sapphire substrate, a silicon substrate, a silicon carbide substrate, a gallium nitride substrate and an aluminum nitride substrate as long as being a crystal plane substrate having a main surface which has an offset angle in the above-described range.

(77) Reference sign 2 refers to a buffer layer. The buffer layer 2 is provided for preventing defects from entering the crystal due to the difference between a lattice constant of the substrate 1 and a lattice constant of an n type nitride semiconductor layer 3 stacked on the substrate 1. The buffer layer 2 is formed of AlN or AlGaN, which has an intermediate lattice constant between those of the substrate 1 and the n type nitride semiconductor layer 3, so that the number of defects in the n type nitride semiconductor layer 3 is reduced. The buffer layer 2 may have a superlattice structure of AlN and AlGaN.

(78) Reference sign 3 refers to the n type nitride semiconductor layer. The n type nitride semiconductor layer 3 may be formed of AlGaN, GaN, GaInN or the like. Although not shown in FIG. 11, the n type nitride semiconductor layer 3 may have a stack of an n type contact layer on which an n type electrode 7 is to be stacked and an n type cladding layer provided on the side of the light emitting layer 4. The n type contact layer may also act as the n type cladding layer. As an n type dopant, Si, Ge or the like is preferably usable.

(79) In this example, the n type nitride semiconductor layer 3 includes the stack of the n type contact layer on which the n type electrode 7 is to be stacked and the n type cladding layer provided on the light emitting layer 4 side. The present invention is not limited to having such a structure. For example, the n type cladding layer provided on the light emitting layer 4 side, which is formed of an n type nitride semiconductor having a larger bandgap than that of the light emitting layer 4, may be replaced with an n type nitride semiconductor layer having the same bandgap as that of the light emitting layer 4.

(80) Reference sign 4 refers to the light emitting layer. The light emitting layer 4 is formed of GaN, InGaN, AlGaN, AlGaInN or the like, and may have either a single quantum well (SQW) structure or a multiple quantum well (MQW) structure, in which well layers and barrier layers are repeatedly stacked. The wavelength of the light to be emitted is adjusted so as to be shifted to the short wavelength side when the Al composition rate in the well layer(s) is increased and shifted to the long wavelength side when the In composition rate in the well layer(s) is increased. The composition of the light emitting layer 4 is appropriately selected in accordance with the wavelength of the light to be emitted by the nitride semiconductor light emitting device.

(81) Reference sign 5 refers to a p type nitride semiconductor layer. The p type nitride semiconductor layer 5 is formed of carbon-doped Al.sub.xGa.sub.1-xN according to the present invention. According to the manufacturing method of the present invention, the composition rate of aluminum in Al.sub.xGa.sub.1-xN can be increased up to 77%. When the aluminum composition rate is 77%, a p type cladding layer having a wider bandgap as compared to that of the light emitting layer 4 can be formed easily. It is ideal that the p type nitride semiconductor layer 5 has a thickness which is equal to, or larger than, the emission wavelength in order to provide a p type electrode 6 (described later) at a position sufficiently far from the active layer 4 so that there is no optical absorption loss. Specifically, the thickness of the p type nitride semiconductor layer 5 is 0.1 m or more in consideration of the emission wavelength. In consideration of the bulk resistivity of the p type nitride semiconductor layer 5, the thickness thereof is preferably 0.1 m or more and 4 m or less.

(82) The p type nitride semiconductor layer 5, which is formed of carbon-doped Al.sub.xGa.sub.1-xN according to the present invention, has more excellent I-V characteristics as compared to those of a p type nitride semiconductor layer formed of Mg-doped p type GaN or AlGaN. Therefore, in the case where carbon-doped Al.sub.xGa.sub.1-xN according to the present invention is used for the p type nitride semiconductor layer 5, the p type electrode 6 can be provided directly on the p type nitride semiconductor layer 5 without providing a current diffusion layer or a contact layer therebetween, although it is also possible to appropriately provide a current diffusion layer or a contact layer therebetween. The p type electrode 6 can be provided directly because the p type nitride semiconductor layer 5, which is formed of carbon-doped Al.sub.xGa.sub.1-xN according to the present invention, causes a Schottky contact easily with the p type electrode 6 when the effective ionized acceptor concentration of the p type nitride semiconductor layer 5 is appropriately adjusted.

(83) One of advantages of forming the p type electrode 6 directly on the p type nitride semiconductor layer 5 without forming a current diffusion layer or a contact layer is that ultraviolet light emitted from the light emitting layer 4 is output without being absorbed by the current diffusion layer or the contact layer. The current diffusion layer or the contact layer is usually provided for alleviating the contact resistance between the p type cladding layer and the p type electrode 6, and thus is formed of p type GaN doped with Mg at a high concentration (e.g., (13)E+18/cm.sup.3). A p type GaN layer doped with Mg at such a high concentration has an advantage of alleviating the contact resistance but also has a property of absorbing ultraviolet light. For this reason, the semiconductor device including no current diffusion layer or contact layer has a more desirable structure as a light emitting device.

(84) For realizing the Schottky contact between the p type nitride semiconductor layer 5 and the p type electrode 6, it is desirable to control the amount of carbon in the p type nitride semiconductor layer 5 so that the p type nitride semiconductor layer 5 has an effective ionized acceptor concentration of, for example, as high as (8.0 to 9.3)E+18/cm.sup.3. A reason for this is that when the effective ionized acceptor concentration is as high as (8.0 to 9.3)E+18/cm.sup.3, a depletion layer formed between the p type nitride semiconductor layer 5 and the p type electrode 6 provided thereon is narrowed, and as a result, a more ideal Schottky contact is realized.

(85) The p type nitride semiconductor layer 5 may have a superlattice structure of carbon-doped Al.sub.xGa.sub.1-xN according to the present invention. In this case, the resistivity of the p type nitride semiconductor layer 5 is decreased, for the following reason. When having a bulk structure, the p type nitride semiconductor layer 5 may possibly have high resistivity due to spontaneous polarization or an inner electric field such as a piezoelectric field or the like; whereas when having a superlattice structure, the p type nitride semiconductor layer 5 has the spontaneous polarization or the piezoelectric field weakened. Therefore, when the p type nitride semiconductor layer 5 has a superlattice structure of carbon-doped Al.sub.xGa.sub.1-xN, the Schottky barrier between the p type nitride semiconductor layer 5 and the p type electrode 6 can be lowered.

(86) It is preferred that the superlattice structure of carbon-doped Al.sub.xGa.sub.1-xN includes about 5 or more and about 10 or less pairs of a III-group atom layer and a V-group atom layer.

(87) As described above, controlling the amount of carbon in the p type nitride semiconductor layer 5 to control the effective ionized acceptor concentration thereof, and forming the p type nitride semiconductor layer 5 of a superlattice structure, independently have an effect of decreasing the contact resistance between the p type nitride semiconductor layer 5 and the p type electrode 6. Moreover, when the p type nitride semiconductor layer 5 has a superlattice structure of Al.sub.xGa.sub.1-xN having an improved effective ionized acceptor concentration as a result of control on the amount of carbon, the contact resistance between the p type nitride semiconductor layer 5 and the p type electrode 6 is further decreased by a combined effect. The nitride semiconductor light emitting device having such a structure has a higher light emitting efficiency.

(88) Reference sign 6 refers to the p type electrode. The p type electrode 6 may be formed of a single film layer or a multiple film layer including 2 or more films which contains either Al, Pt, Ru, Ag, Ti, Au or Ni. The p type electrode 6 may be formed of an alloy containing at least two of these materials.

(89) Reference sign 7 refers to the n type electrode. The n type electrode 7 is formed on an exposed surface of the n type nitride semiconductor layer 3 that is obtained as a result of etching the p type nitride semiconductor layer 5, the light emitting layer 4 and a part of the n type nitride semiconductor layer 3. The n type electrode 7 may be formed of a multiple film layer including 2 or more films of either Cr, Ti, Au, Al or Ni.

Third Embodiment

(90) The layer structure of the nitride semiconductor light emitting device shown in FIG. 11 is merely an example. FIG. 12 shows an example of layer structure of a nitride semiconductor light emitting device which is different from that shown in FIG. 11. The same reference signs are used for parts substantially the same as, or equivalent to, the parts shown in FIG. 11.

(91) The nitride semiconductor light emitting device shown in FIG. 12 is manufactured by the following method. A buffer layer 2 is stacked on a substrate 1, and an n type nitride semiconductor layer 3, a light emitting layer 4 and a p type nitride semiconductor layer 5 are sequentially stacked in this order thereupon. On the p type nitride semiconductor layer 5, a reflecting electrode 8 formed of a metal such as Ag or the like is formed. The reflecting electrode 8 is provided for reflecting light, among light emitted from the active layer 4, which is propagating in a direction opposite to a direction toward the light extracting surface of the active layer 4 in order to improve the light extraction efficiency. The reflecting electrode 8 functions also as a p type electrode. A layer formed of a composition which demonstrates a function of preventing the diffusion of the component of the reflecting electrode 8 may be provided between the reflecting electrode 8 and the p type nitride semiconductor layer 5.

(92) The reflecting electrode 8 may be formed of Al because Al reflects light and thus improves the light extraction efficiency of the nitride semiconductor light emitting device.

(93) A conductive substrate 10 formed of silicon or the like that is separately prepared is bonded onto the reflecting electrode 8 via an adhesive layer 9 formed of Au or the like. Following this, the substrate 1 is removed by polishing or etching. At this time, the entirety or a part of the buffer layer 2 may be removed together with the substrate 1 also by polishing or etching.

(94) On the substrate 1 or a surface exposed as a result of the removal of the substrate 1 and the entirety or a part of the buffer layer 2, n type electrodes 7 are formed. The n type electrodes 7 may be transparent electrodes formed of ITO or the like.

(95) So far, the nitride semiconductor light emitting device having the structure shown in FIG. 12 has been explained. In this example also, when carbon-doped Al.sub.xGa.sub.1-xN according to the present invention is used for the p type nitride semiconductor layer 5, a p type cladding layer having a wider bandgap than that of the light emitting layer 4 is realized. Also in this example, a contact layer or the like, for causing an ohmic contact between the p type nitride semiconductor layer 5 and the reflecting electrode 8 provided thereon, does not need to be provided because the p type nitride semiconductor layer 5 has a low sheet resistance.

(96) In the nitride semiconductor light emitting device having the structure shown in FIG. 12 also, controlling the amount of carbon doped into Al.sub.xGa.sub.1-xN (p type nitride semiconductor layer 5) to control the effective ionized acceptor concentration thereof, and/or forming the p type nitride semiconductor layer 5 of a superlattice structure, independently have an effect of decreasing the contact resistance between the p type nitride semiconductor layer 5 and the p type electrode 8.

(97) As explained above, according the present invention, stable manufacture of a carbon-doped p type nitride semiconductor layer, which has been difficult so far, is made possible. This will be described more specifically. Since a carbon-doped gallium nitride semiconductor layer may become of a p type or an n type because of the property of carbon, it has been difficult to stably manufacture a p type gallium nitride semiconductor layer using carbon doping. However, according to the present invention, the carbon-doped p type nitride semiconductor layer is manufactured stably.

(98) In addition, since carbon is used as a dopant, the nitride semiconductor layer has low resistivity, unlike when Mg is used as a dopant. In the case where carbon-doped AlGaN is used for the gallium nitride semiconductor layer, a p type cladding layer with a larger bandgap than that of a light emitting layer formed of InGaN or the like is realized. Therefore, according to the present invention, a nitride semiconductor light emitting device with a high light emitting efficiency can be manufactured with no contact layer or the like being provided between the p type nitride semiconductor layer and the electrode.

(99) In the p type nitride semiconductor layer formed of carbon-doped Al.sub.xGa.sub.1-xN according to the present invention, a 20 mA current flows at a bias voltage of about 9V as is shown in FIG. 9. As can be seen from this, the p type nitride semiconductor layer formed of carbon-doped Al.sub.xGa.sub.1-xN according to the present invention can have low resistivity when the effective ionized acceptor concentration thereof is appropriately controlled. Therefore, the p type nitride semiconductor layer formed of carbon-doped Al.sub.xGa.sub.1-xN according to the present invention is applicable to a semiconductor laser in which the thickness of the p type nitride semiconductor layer needs to be 0.2 m or more, preferably at least 1 to 3 times the oscillating wavelength (e.g., 0.5 m or more when the wavelength is 250 nm) in order to locate the p type electrode far from the active layer, eliminate the optical absorption loss and thus reduce the oscillation threshold.

(100) In other words, when a p type nitride semiconductor formed of carbon-doped Al.sub.xGa.sub.1-xN having an effective ionized acceptor concentration controlled to, for example, (8.0 to 9.3)E+18/cm.sup.3 is applied to a cladding layer of a semiconductor laser, the thickness of the cladding layer does not need to be minimized to a level that is about the same as the oscillating wavelength in order to decrease the device resistivity of the semiconductor laser. Specifically, the thickness of the cladding layer does not need to be as small as 0.1 m as in the conventional art, but may be 0.2 m or more. Preferably, the thickness of the cladding layer is at least 1 to 3 times the oscillating wavelength (e.g., 0.5 m or more when the wavelength is 250 nm) as required in order to locate the p type electrode far from the active layer, eliminate the optical absorption loss and thus reduce the oscillation threshold.

(101) This raises the degree of freedom in designing the structure of semiconductor lasers, and allows the active layer to have such a thickness as to minimize the oscillation threshold. As a result, a short wavelength semiconductor laser (for emitting light of a deep ultraviolet range of 300 nm or less), which has not been realized so far, can be manufactured. In addition, the operating life is significantly extended and the performance thereof is greatly improved.

(102) An ultraviolet or deep-ultraviolet semiconductor laser is now strongly desired. A p type nitride semiconductor layer formed of carbon-doped Al.sub.xGa.sub.1-xN according to the present invention is optimal for a cladding layer of such an ultraviolet semiconductor laser. Since the p type nitride semiconductor layer formed of carbon-doped Al.sub.xGa.sub.1-xN according to the present invention allows a p type electrode to be provided thereon with no current diffusion layer or contact layer being provided therebetween, the ultraviolet light is output without being absorbed by the current diffusion layer or the contact layer. Thus, an ultraviolet semiconductor laser having a high light emitting efficiency is realized.

DESCRIPTION OF THE REFERENCE SIGNS

(103) 1 Substrate 2 Buffer layer 3 n Type nitride semiconductor layer 4 Light emitting layer 5 p Type nitride semiconductor layer 6 p Type electrode 7 n Type electrode