The present invention relates to an insulating nitride layer and a process for its forming, and to a semiconductor device having said layer and a process for its production. (The insulating nitride layer refers specifically to one which is formed from a doped insulating group III-V compound semiconductor in the form of nitride.)
The semiconductor device based on a group III-V compound semiconductor in the form of nitride conventionally have an Mg-doped insulating GaN layer (with a high resistance) for electrical isolation of elements. For example, semiconductor devices such as MISFET (Metal Insulator Semiconductor Field Effect Transistor) and HEMT (High Electron Mobility Transistor, a kind of FET) composed of GaN and AlGaN consist of an insulating sapphire substrate and those layers sequentially formed thereon which include a low-temperature buffer layer of AlxGa1-xN (0≦×≦1.0), a GaN layer (equal to or thicker than 1 μm), and GaN and AlGaN active layers forming a heterojunction interface.
A conventional practice to electrically isolate elements was to replace the underlying GaN layer with an Mg-doped GaN layer (Mg is a group IIA element), as mentioned by R. Dimitrov et al., Phys. Status Solidi A 168 (1998) R7. The disadvantage of doping GaN with Mg by MOCVD (organometallic chemical vapor deposition) is that hydrogen in the gas prevents Mg from becoming active, causing the Mg-doped GaN layer to have a high resistance, as reported by S. Nakamura et al., Jpn. J. Appl. Phys. 31 (1992) p. 1258-1266.
If Mg is supplied in the form of bis(methylcyclopentadienyl)magnesium ((MeCp)2Mg) or bis(cyclopentadienyl)magnesium (Cp2Mg), Mg enters by autodoping the active layer on the Mg-doped GaN layer, thereby decreasing the conductivity of the active layer.
A conventional HEMT that uses the AlGaN/GaN heterojunction is produced by sequentially forming the following layers on a sapphire substrate 1 as shown in
As mentioned above, an HEMT is a high-speed field effect transistor (FET) which uses a heterojunction. It is characterized in that the heterojunction 14 spatially separates the crystal region (GaN layer 4) for electron movement and the crystal region (n-AlGaN layer 6) for electron supply from each other. This separation reduces the scattering of electrons by donor impurity (due to the absence of donor impurity in the GaN layer 4), thereby increasing the electron mobility between the source and the drain.
In actuality, however, the two-dimensional electron gas that occurs at the heterojunction decreases in concentration (ns) and mobility (as shown in
It was found that the foregoing problem arises from Mg (not less than 1017/cm3) entering the undoped GaN channel layer 4 which lies on the Mg-doped GaN buffer layer 3a, as evidenced by SIMS (secondary ion mass spectroscopy) shown in
A probable reason for this is that the reactant gas for Mg has a vapor pressure as low as 0.5 mmHg and hence it takes a long time to completely purge the reactant gas which has been adsorbed into the pipe and reactor. The reactant gas remaining adsorbed into the pipe is released while the undoped GaN channel layer 4 is grown on the GaN buffer layer 3a, and the thus released Mg enters the GaN channel layer 4 by autodoping.
One way to cope with this situation is to replace the Mg-doped GaN buffer layer 3a with the undoped GaN buffer layer 3b (2.0 μm thick) and grow thereon the undoped AlGaN spacer layer 5 (3 nm thick), the n-AlGaN:Si carrier supply layer 6 (20 nm thick), and the undoped AlGaN cap layer 7 (15 nm thick), with the undoped GaN channel layer 4 omitted, as shown in
It is an object of the present invention to provide a nitride layer suitable for group III-V nitride compound semiconductor devices, the nitride layer being superior in insulating performance with high resistivity, permitting good electrical isolation of elements, without the active layer decreasing in conductivity. It is another object of the present invention to provide a process for forming the nitride layer. It is further another object of the present invention to provide an improved semiconductor device having the nitride layer.
The present invention is directed to an insulating nitride layer formed from a group III-V nitride compound semiconductor heavily doped mostly with a group IIB element. The present invention is directed also to a semiconductor device having the nitride layer.
The present invention is directed also to an improved process for forming a layer of group III-V nitride compound semiconductor by vapor deposition, wherein the improvement comprising feeding a reactant gas for the group III-V compound semiconductor together with a gas containing an impurity whose vapor pressure is equal to or higher than 10 mmHg at room temperature, thereby forming an insulating nitride layer which is heavily doped with the impurity.
The present invention is directed also to a process for producing a semiconductor device, the process comprising a step of forming a layer of group III-V nitride compound semiconductor by vapor deposition from a reactant gas for the group III-V compound semiconductor which is fed together with a gas containing an impurity whose vapor pressure is equal to or higher than 10 mmHg at room temperature, thereby forming an insulating nitride layer which is heavily doped with the impurity, and a step of growing an active layer on the insulating nitride layer by vapor deposition.
The present invention produces its effect when applied to MISFET elements or HEMT elements in which the underlying layer of the channel layer is a nitride layer which has good insulating performance owing to an impurity doped therein. The advantage of this nitride layer is its good insulating performance and its ability to isolate elements completely due to heavy doping with a group IIB element (particularly zinc). Another advantage is that the reactant gas of group IIB element used to form the nitride layer has a high vapor pressure (particularly equal to or higher than 10 mmHg). In other words, the process can use a reactant gas for impurity doping that is readily purged. As a result, the reactant gas for impurity doping is rapidly released when the active layer is formed on the insulating nitride layer by vapor deposition. This protects the active layer from autodoping with an impurity. The result is easy production of high-speed elements, without the active layer decreasing in conductivity.
As mentioned above, the insulating nitride layer heavily doped with an impurity can be readily obtained by doping the group III-V nitride compound semiconductor with an impurity (such as Zn). In this way it is possible to form a highly insulating nitride layer whose resistivity (sheet resistance) is equal to or higher than 0.3 MΩ, without causing the active layer to decrease in conductivity. This insulating nitride layer contributes to high-speed elements. See
According to the present invention to achieve the above-mentioned object, it is desirable that the insulating nitride layer should be heavily doped with a group IIB element (substantially a group IIB element alone or at least Zn) as an impurity.
The amount of the group IIB element to be added should preferably be not less than 1×1017/cm3 so that the nitride layer has a sufficiently high resistance for practical use. More preferably, it should be equal to or higher than 1×1018/cm3 so that it keeps a sufficiently high resistance regardless of the level of undoping by carriers contained in the layer.
The above-mentioned impurity is supplied from a reactant gas containing a compound of a group IIB element (at least Zn). It is essential that the reactant gas should have a vapor pressure equal to or higher than 10 mmHg at room temperature. Any reactant gas having a vapor pressure lower than specified above presents difficulties in purging and is liable to cause autodoping. The reactant gas having a high vapor pressure is exemplified by alkyl zinc such as diethyl zinc (DEZn) and dimethyl zinc (DMZn).
The amount of the above-mentioned impurity to be doped should preferably be not less than 1×1017/cm3, more preferably not less than 1×1018/cm3. For example, Zn as an impurity should be added in an amount not less than 1×1017/cm3 when the crystal of group III-V nitride compound semiconductor is grown by organometallic vapor phase epitaxy. The upper limit of the amount is determined by the saturated concentration of impurity dissolved in the matrix.
The substrate on which the insulating nitride layer is grown should preferably be an insulating one of sapphire. However, it may be replaced with a conducting one of SiC or the like.
Table 1 below shows the vapor pressure of various organometallic compounds. It is to be noted that the requirement that the reactant gas to dope the nitride layer should have a vapor pressure equal to or higher than 10 mmHg at room temperature is met by not only DEZn and DMZn but also dimethyl cadmium.
In the present invention, the group III-V nitride compound semiconductor mentioned above may be GaN, AlN, InN, or BN, or a mixture thereof. They are converted into insulating nitrides upon doping with a group IIB element, and such nitrides constitute other layers in the group III-V compound semiconductor device.
In other words, the semiconductor device according to the present invention employs the above-mentioned group III-V nitride compound semiconductor as at least part of its constituents. The insulating nitride layer is used to isolate not less than one kind of integrated elements including field effect transistor, bipolar transistor, light-emitting diode, semiconductor laser, and photodiode.
The structure of HEMT according to the present invention is shown in
The advantage of this structure is that the Zn-doped GaN buffer layer 3c under the active layer 4 has a sufficiently high resistance and hence effectively isolates other elements (not shown) formed on the common sapphire substrate 1. Moreover, the Zn-doped buffer layer 3c protects the active layer 4 from autodoping with an impurity as mentioned above and keeps its conductivity adequately.
The present invention can also be applied to MISFET (metal insulator semiconductor field effect transistor) and MESFET (metal semiconductor field effect transistor), whose structure is shown in
The present invention can be applied to any device of mesa structure or planar structure which needs element isolation. The device is not limited in structure and material.
The invention will be described with reference to the following examples.
A semiconductor sample was prepared which consists of thin layers formed on the (0001) C plane of a sapphire substrate. The substrate was heated under normal pressure in a horizontal furnace for metal organic vapor phase epitaxy. The furnace was supplied with a reactant gas composed of trimethyl gallium (TMGa), ammonia (NH3), bis(methylcylcopentadienyl)magnesium ((MeCp)2Mg), and diethyl zinc (DEZn). The ratio of group V to group III is from about 2,400 to 12,000.
The resulting sample has the layer structure as shown in
The foregoing suggests that it is possible to form an insulating GaN layer having a steep profile if diethyl zinc (DEZn) is used as a dopant.
The sample in this example has the layer structure as shown in
The fact that the C concentration is considerably low suggests that Zn is the major dopant. A probable reason for this that NH3 as the reactant gas suppresses the doping with carbon. Moreover, the effect of suppressing the doping with carbon is enhanced when TMGa is replaced by triethyl gallium (TEGa) which readily decomposes and permits liberated carbon to be discharged easily.
A sample of high electron mobility transistor (HEMT) was prepared which consists of thin layers formed on the (0001) C plane of a sapphire substrate. The substrate was heated under normal pressure in a horizontal furnace for metal organic vapor phase epitaxy (MOVPE). The furnace was supplied with a reactant gas composed of trimethyl gallium (TMGa), ammonia (NH3), trimethyl aluminum (TMAl), and monomethylsilane (CH3SiH3). The ratio of group V to group III is from about 2,400 to 12,000.
The resulting sample has the layer structure as shown in
The sample was tested for the distribution of carrier concentrations in the depth direction by the C-V method. The results are shown in
It is noted from
The sample with a gate electrode 12 (whose gate length (d) is 1.0 μm) gave a maximum cut-off frequency of 10 GHz. This value is better than 9 GHz achieved by the sample (shown in
A sample of transistor (MISFET) was prepared which consists of thin layers formed on the (1120) A plane of a sapphire substrate. The substrate was heated under normal pressure in a horizontal furnace for metal organic vapor phase epitaxy. The furnace was supplied with a reactant gas composed of trimethyl gallium (TMGa), ammonia (NH3), trimethyl aluminum (TMAl), and monomethylsilane (CH3SiH3). The ratio of group V to group III is from about 2,400 to 12,000.
The resulting sample has the layer structure as shown in
Vapor phase epitaxy is continued to sequentially form the Zn:Mg-codoped GaN layer 9 (about 1 μm thick), the Zn-doped insulating GaN buffer layer 3c′ (equal to or thicker than 300 nm), the GaN channel layer 4 (200 nm thick), and the undoped AlGaN insulating layer 7 (40 nm thick).
The Zn:Mg-codoped GaN layer 9 has an Mg concentration equal to or higher than 1×1019/cm3, and it is subsequently activated by electron beam irradiation to form a p-type conducting layer. The buffer layer 3c′ of the same composition as the buffer layer 3c has a Zn concentration equal to or higher than 1×1018/cm3.
Subsequently, the undoped AlGaN insulating layer 7 undergoes reactive ion etching (RIE) through a mask of SiO2. On the etched part is grown the Si-doped GaN layer 6 for source and drain contact.
Masking and etching are carried out to fabricate the Zn-doped GaN layer 3c′, to surface the Zn:Mg-codoped GaN layer 9, to isolate elements by the GaN layers 3c′, 9, and 3c, and to form the electrodes 11, 12, 13, and 15.
The FET obtained as mentioned above has its channel frequency characteristics controlled by the lead electrode 15 (which is the fourth electrode).
In this example, too, it was confirmed that the two-dimensional electron gas that occurs at the heterojunction is identical in concentration and mobility with that in the example in which the undoped GaN buffer layer is used. It was also confirmed that the active layer in this example dose not decrease in conductivity.
This example demonstrates a GaN MESFET to which the present invention is applied. Its layer structure is shown in
The present invention is characterized in that the insulating nitride layer is formed by heavily doping with a group IIB element (particularly Zn) as an impurity. The thus doped nitride layer has good insulating properties necessary for complete element isolation. Another advantage is that the reactant gas for the group IIB element has such a high vapor pressure that it can be readily purged when the nitride layer is formed. Thus the reactant gas for impurity is completely released when the active layer is formed by vapor phase epitaxy on the insulating nitride layer. In this way the active layer is protected from autodoping with impurity, with the result that the active layer keeps its conductivity high and the completely isolated elements work at high speeds.
Number | Date | Country | Kind |
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P2000-241581 | Aug 2000 | JP | national |
Number | Date | Country | |
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Parent | 09925153 | Aug 2001 | US |
Child | 10990116 | Nov 2004 | US |