JPH04106928A - Formation of oxide film on the surface of compound semiconductor - Google Patents

Abstract

PURPOSE:To obtain a stabilized oxide film with a small interface level density by vacuum-sealing a compound semiconductor and an element with a higher vapor pressure and oxygen out of the components in a quartz ampule, and then thermally oxidizing. CONSTITUTION:A wafer cut out from InP single crystal is placed into a pBN- made vessel 1. This vessel is inserted into a quartz ampule 2 with the pBN-made vessel on which red phosphorus is mounted. After the ampule 2 is evacuated, oxygen gas is introduced. Then, the ampule 2 is sealed and heated in a heating furnace 10 where an oxide film is formed on the wafer 5. Under this construction, phosphorous is supplied to the front surface of the crystal from phosphorus vapor so as to prevent the shortage of phosphorous on the surface of the oxide film where uniform oxide film grows and moreover, the original surface of the substrate is covered with the oxide film. This construction makes it possible to obtain an oxide film having a small interface level density, minimize the mixture of impurities into the oxide film to be generated and hence enhance device characteristics.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a technology for forming an insulating film of a field effect transistor (hereinafter referred to as FET) based on a compound semiconductor single crystal, and in particular to an InP single crystal and its ternary , MOSFETs, Schottky diodes, and MEs on quaternary mixed crystal substrates.
This article relates to the most effective techniques to use when forming SFETs.

[Prior Art] Compound semiconductors such as GaAs and InP have higher electron mobility than silicon, and also have excellent radiation resistance and heat resistance, and are expected to have future potential as high-frequency, high-speed electronic devices that can replace silicon. Although many studies have been conducted, MOSFETs using GaA,S single crystals or InP single crystals as substrates have not yet been put into practical use because stable oxide films with low interface state density have not been obtained. not present. Therefore, MESFETs using Schottky electrodes have been put into practical use as devices using GaAs single crystals, and have been used as discrete high-frequency FETs and small-scale digital I/O devices.
C has been put into practical use. However, GaAs MES
Since FET has a small Schottky barrier potential,
It has the disadvantage that when an IC is constructed, a large logic amplitude cannot be obtained, and a large-scale digital IC cannot be manufactured at a high yield.

On the other hand, as a device using InP single crystal as a substrate, Q
MESFE with smaller Schottky barrier potential than aAs
It was thought that only TL could do it.

Therefore, a metal layer (
Efforts have been made to create MOSFETs by forming electrodes), but in all cases, the composition of the oxide film is non-uniform, the insulation is poor, and good MOSFETs cannot be realized, so they have not been put into practical use. As an alternative to such MOSFETs, S i O,, S i Nx, AQ, 03.
Many studies have been conducted on MISFETs in which a metal layer is formed on an insulating film such as PN deposited on an InP substrate at a low temperature by CVD, plasma CVD, photo-enhanced CVD, sputtering, vapor deposition, spin-on, etc. Ta.

[Problem to be solved by the invention] However, the MISFET manufactured by the above method
All of these have a fatal drawback as an electronic device in that the drain current drifts, and so they have not been put into practical use.

By the way, as mentioned earlier, MOSFETs have not been put to practical use in compound semiconductors, and the reason for this is that the composition of the oxide film becomes non-uniform.

For example, in the case of InP, when it is thermally oxidized in oxygen, initially I
nP○ grows for about 20 minutes, but after that, because the diffusion rate of P is slower than that of In, In
, 03 film, and P is known to precipitate at the interface between InP and 1nP◯4. This phenomenon occurs regardless of the method used, such as anodic oxidation or plasma oxidation, and is the reason why a uniform and high-quality oxide film cannot be obtained.

In addition, when forming an oxide film in P,01° gas, if P, O, (solid) is used as the vapor pressure source, p, o,
(Solid) has a vapor pressure that changes depending on its crystal structure, and also easily absorbs moisture from the air, bringing moisture into the sealed tube, making it difficult to control the vapor pressure and making it difficult to consistently obtain a high-quality oxide film. .

As described above, it is difficult to form a high-quality insulating film by thermal oxidation, so various low-temperature deposition methods as mentioned above are being researched. In addition to the lattice mismatch that occurs at the interface between the insulating film and the compound semiconductor substrate due to the deposition of a system of There is a problem that current drift occurs.

An object of the present invention is to provide a technique for forming an oxide film having a low interface state density, stable, and containing few impurities using an InP-based compound semiconductor substrate.

[Means for solving the problem] In a MOSFET using an S1 single crystal as a substrate, when forming a silicon oxide film, the Si/SiO interface is not the original surface of the silicon substrate, but a Since oxygen diffuses through the oxide film and reacts with Si in the substrate, an interface is formed inside the original crystal.

For this reason, Si substrates can achieve a low interface state density that is unaffected by defects on the substrate surface, and therefore MOSFE
T has been put into practical use.

Comprehensively considering the above points, compound semiconductor FET
In order to achieve this, it is considered that the best method is to use a thermal oxide film as an insulating film.

By the way, regarding InP MOS devices using thermal oxide films, in addition to thermal oxidation in oxygen, thermal oxidation in high pressure oxygen, thermal oxidation in p, o, steam, oxidation in HNO3 solution, Several methods have been studied in the past, such as depositing p, o, and then heating and oxidizing them, but all of them are insufficient, and MOSFETs have not yet been put into practical use. It was concluded that the cause of this was that the composition of the oxide film on the crystal surface became non-uniform, and oxides with poor insulation properties were formed in some areas.

Therefore, in forming an oxide film on a compound semiconductor substrate, the present invention provides that the compound semiconductor substrate placed in a semi-closed pBN container is placed in a quartz ampoule together with an element having a high vapor pressure among the constituent elements of the compound semiconductor substrate. The ampoule is inserted into the ampoule, the inside of the ampoule is evacuated, oxygen gas is introduced, the ampoule is sealed, and the ampoule is heated.

The amounts of P and O are controlled so that the phosphorus (P,) pressure remaining from the reaction of equation (1) is close to the equilibrium phosphorus pressure at the stoichiometry InP oxide film formation temperature.

P+0. →P706+P4...(1) [Operation According to the present invention, for example, when the substrate is InP, P (phosphorus) and 02 (oxygen) are placed in the ampoule in advance, so that the crystal surface side is removed from the phosphorus vapor. Herrin is replenished and the oxide film surface is prevented from forming Linnean legs, a uniform oxide film grows, and as the oxide film grows, the original substrate surface is incorporated into the oxide film, increasing the interface level. In addition to obtaining a low-density oxide film, since the substrate is placed in a pBN container and sealed in the ampoule, it is less likely that the oxide film that is formed will be absorbed by the quartz ampule compared to the case where the substrate is placed in the ampoule and sealed. The contamination of impurities is significantly reduced, and the characteristics of devices using this oxide film can be improved.

[Example] An undoped n-type InP single crystal with a diameter of 2 inches was subjected to LEC.
The wafers were grown by the method, cut in a direction perpendicular to the pulling axis, and the cut wafers were mirror-polished, washed with an organic solvent, etched with brome methanol, and then washed with HF (hydrofluoric acid) immediately before oxidation. The carrier concentration of the wafer used was (4-6) x 10"am-".

This wafer is placed in a pBN container 1 as shown in FIG.
Then, this container 1 was inserted into a quartz ampoule 2 together with a pBN vessel 3 on which red phosphorus was placed. After making the inside of this quartz ampoule 2 a vacuum, add 0.1 to 0.3 of oxygen gas.
After introducing ATM (room temperature), the ampoule was sealed and placed in a heating furnace IO.

This quartz ampoule 2 is heated to 450°C to 7°C in a heating furnace 10.
The wafer 5 was heated at 00° C. for 5 to 20 hours to form an oxide film on the wafer 5.

FIGS. 2(a) and 2(b) show an example of the structure of the container 1 made of pBN. The container 1 consists of cylindrical containers la and lb each having a bottom on one side, and is constructed so that the two cylindrical containers are fitted together to create a nearly hermetically sealed state. Further, a small hole 1c with a diameter of 0.5 mm was opened in the lower part for evacuation, and the outer diameter of the container 1 was slightly smaller than the inner diameter of the ampoule 2. The InP wafer 5 is mounted on a support stand 4 made of pBN.
It was placed on the container 1 and placed indirectly in the ampoule 2. As for the amount of phosphorus in the ampoule, the pressure during heating is 0.2 to 2. The amount to be Oatm was determined and sealed. Note that the pBN container 1 and the support stand 4 were heated for 90 minutes in air.
It was used after baking at 0°C for 2 hours.

In order to evaluate the oxide film of the wafer on which the oxide film was formed by the above method, a resist was applied to one side of the wafer.
After removing the oxide film on the other side by lapping and using a bromine-based etchant to form an ohmic electrode, the Au
A layered metal layer consisting of -Ge was deposited. Next, after removing the above resist, it was heated at 350'C in N gas for 50 minutes.
An ohmic electrode having an Au-Ge laminated structure was formed on the surface from which the oxide film had been removed by annealing for a minute. Furthermore, a metal mask was used as a mask to coat the surface of the wafer on which the oxide film remained with a diameter of 0.
AQ electrode layers were deposited with a thickness of 3 mm and an interval of 1 mm to create a MOS capacitor.

FIG. 3 shows the CV characteristics measured for the produced MOS capacitor.

According to the conventional method, the shift amount of the C-■ characteristic in the voltage axis direction was 2V or more, but in this embodiment, as is clear from FIG. 3, the shift amount of the hysteresis in the voltage axis direction is 0.2V.
An excellent insulating film with a voltage of 5V or less was formed. This shows that the method shown in the present invention is extremely effective in reducing the drain current drift phenomenon of MOSFETs using an InP single crystal as a substrate. In addition, the interface state density measured by the Terman method is l x 10"cm-' e V-
1 or less, and a high-quality insulating film with a low interface state density was formed by the present invention. Note that the thickness of the formed oxide film was about 45 OA.

Figure 4 shows approximately 110 mm from the surface of the oxide film according to SIMS analysis.
It represents the distribution of the amount of impurities (Si, Zn) up to a depth of 0n. FIG. 5(a) shows the case where the oxide film was formed using the pBN container 1, and FIG. 2(b) shows the case where the oxide film was formed under the same conditions except that the pBN container 1 was not used. Note that the vertical axis shows the amount of impurities as a ratio of spectral intensity to In. From the figure, it can be seen that by applying the above embodiment, 81 and Zn in the oxide film are significantly reduced.

From this, it is considered that Sl contained in the oxide film when the pBN container 1 is not used is mixed in from the quartz ampoule.

In the above embodiment, two cylindrical containers are fitted together to constitute the semi-closed container 1, but a cylindrical container and a lid covering the top may constitute a semi-closed container. .

[Effects of the Invention] As explained above, the present invention provides a method for thermally oxidizing an element having a higher vapor pressure among the constituent elements of a compound semiconductor and its oxygen after vacuum-sealing it together with the compound semiconductor substrate in a quartz ampoule. Among the constituent elements of the compound semiconductor, an element with a high vapor pressure is placed in an ampoule, and the compound semiconductor substrate is placed in a pBN container and sealed in the ampoule so as not to directly face the inner wall of the quartz ampoule. For example, if the substrate is InP, P (phosphorus) and 02 (oxygen) are added to the ampoule in advance, so phosphorus is supplied from the phosphorus vapor to the crystal surface side, and the oxide film surface becomes phosphorus feet. This prevents the growth of a uniform oxide film, and as the oxide film grows, the original substrate surface is incorporated into the oxide film, resulting in an oxide film with a small interface state density. Since the oxide film is placed in a pBN container and sealed in the ampoule, the amount of impurities mixed into the oxide film produced is extremely low compared to when the oxide film is placed directly in the ampoule, which improves the characteristics of devices using this oxide film. It has the effect of being able to improve the

[Brief explanation of drawings]

FIG. 1 is a sectional view showing a configuration example of an oxide film forming apparatus according to the present invention, FIG. 2(a) is a sectional front view of a pBN container used therein, and FIG. 2(b) is a sectional view taken along line BB'. 3 is a graph showing the capacitance-voltage characteristics of a MOS capacitor obtained by applying the embodiment of the present invention, and FIG. 4(a) is a cross-sectional side view taken along the line. A graph showing the distribution of the amount of impurities in the depth direction in the obtained oxide film. FIG. 4(b) is a graph showing the distribution in the depth direction of the amount of impurities in the oxide film formed without using a pBN container. . 1... pBN container, 2... quartz ampoule, 4
...Support stand, 5...Wafer. Figure 1 Figure 2 (0) (b) Day 5 Ku-,be ＼1゜・IC ４ Woman's grace ＼Raga chin (nm) (b) Surface ra no 1 (nm) Heisei 2 November 7th

Claims (1)

[Claims] (1) When forming an oxide film on a compound semiconductor substrate, the above compound semiconductor substrate placed in a semi-closed pBN container is inserted into a quartz ampoule together with an element having a high vapor pressure among the constituent elements of the compound semiconductor substrate. A compound semiconductor characterized in that the ampoule is evacuated, oxygen gas is introduced, the ampoule is sealed, and the ampoule is heated to form an oxide film on the compound semiconductor substrate. Method of forming an oxide film on the surface.