JPH0753632B2 - Semiconductor epitaxial growth method - Google Patents

Description

The present invention relates to an epitaxial growth technique for III-V group compound semiconductors such as GaAs and InP and II-VI group compound semiconductors such as ZnTe and ZnSe. It is a thing.

[Prior Art] As a technique for epitaxially growing a III-V group compound semiconductor and a II-VI group compound semiconductor with high film thickness controllability, molecular beam epitaxy (a practical technique used as a growth technique for semiconductor lasers and FET materials ( MBE method and metal organic chemical vapor deposition method (MOCVD method). Although these methods have excellent characteristics such as a heterojunction structure having a steep boundary surface and precise film thickness control, they have the following two problems. One of them is that the heterojunction interface produced by these methods has an infinite number of steps at the atomic layer thickness level, and the second problem is that the growth temperature is high. Regarding these problems, GaAs, AlAs,
Description will be made using an example of a typical III-V group compound semiconductor such as AlGaAs.

Fig. 11 schematically shows the state of the hetero interface produced by the conventional method. In the figure, 1 is a GaAs layer and 2 is an AlAs layer, and the hetero interface 3 has steps as shown in the figure.
The height h of this step is the thickness of one atomic layer. In order to reduce such steps, for example, FIG.
A method as shown in (C) has been considered. Ie Ga
After the As film 1 is grown to a constant thickness (Fig. 12 (A)), "islands" of 1-3 atomic layer thickness generated on the surface are thermally moved and flattened (Fig. 12 (B)). )), An AlAs layer or a GaAlAs layer is grown thereon (FIG. 12 (C)). However, with this method, although it becomes flat in the range of several μm to 100 μm, as seen from the figure, the nonuniformity becomes stronger when viewed in a larger range.

The second problem that the growth temperature is high will be described.
For example, in order to grow high-quality AlGaAs, the MBE method requires a temperature of 650 ° C or higher and the MOCVD method requires a temperature of 700 ° C or higher. At such a high temperature, impurities are diffused, and it is not possible to expect sufficient steepness in the impurity distribution. Especially p
With regard to form impurities, at the above temperature, several tens of
The diffusion of 0Å limits the design of semiconductor devices composed of GaAs and AlGaAs. If the above two problems can be solved, the characteristics of the semiconductor device made of these materials are improved, that is, the threshold value of the semiconductor laser is lowered,
In addition to speeding up FETs and bipolar transistors, it is also possible to design devices with new functions. However, as described above, it was impossible to achieve this by the conventional MBE method and MOCVD method.

As an improved method of MBE, a method of alternately irradiating a Ga molecular beam and an As molecular beam on a GaAs substrate has been proposed in JP-A-60-112692. However, this method uses Ga after As molecular beam irradiation.
Since the irradiation stop period of a certain time is provided until the molecular beam irradiation, there is a risk that As may come off from the grown crystal, and it takes a long time for crystal growth.

Recently, atomic layer epitaxy (ALE) / (T.Sun has been proposed as an epitaxial growth technology that has the potential to solve these problems.
tola et al., SID Digest, (1980), 128; A. Usui et al., Jpn.J.App
l.Phys., 25 (1986), L212), or molecular layer epitaxy (MLE) (J. Nishizawa et al., J. Electrochem. Soc., 132 ,
(1985), 1197) was proposed.

The ALE method is a method of growing one atomic layer on a substrate, but is applied only to II-VI group compound semiconductors containing a group II element having a high vapor pressure, and III-V containing a group III element having a low vapor pressure.
Not applicable to group compound semiconductors.

In the MLE method, a material (compound) containing A _III atoms of group III elements
It was fed to the growth substrate material (compound) alternately including B _V group V element to form the A _III -B _V compound semiconductor with. The characteristic of this method is that no matter how much A _III atomic material is supplied over a certain amount, only one molecular layer is adsorbed on the substrate crystal, and therefore the growth film thickness is automatically controlled. That is, the growth layer thickness per cycle is kept constant regardless of the supply amount. However, this method has the following serious problems. The first is that it takes a long time to grow. In these methods, it is necessary to stop the adsorption process by adsorption of a monolayer, so instead of adsorbing in the atomic state,
Adsorb in the molecular state. An example of growing a GaAs layer on a GaAs substrate 10 is shown in FIGS. 13 (A) and 13 (B). In this case, Ga is trimethylgallium (TMG) 14 composed of Ga atoms 11, carbon atoms 12, and hydrogen atoms 13 as shown in FIG. 13 (A).
In the form of, it is supplied onto the substrate 10 and adsorbed. Of the trimethylgallium adsorbed on the substrate, two methyl groups are not shown for simplicity (Ga is triethylgallium,
May be supplied in the form of gallium chloride). In this case, the adsorbed molecules hardly move on the surface of the substrate and form “islands”. "Islands" on the substrate surface with delayed adsorption
Have to wait for the molecules to be stochastically supplied to the place, and complete monolayer adsorption takes time. In the next process, As is adsorbed, but As is shown in FIG.
Is supplied in the form of H ₃₎ molecule 16, a simple Ga-As molecules react with adsorbed molecules containing Ga underlying. Excess C and H become methane (CH ₄ ) molecule 17 and are removed from the system. However, as in the case of adsorption of molecules containing Ga as described above, when unreacted "islands" occur, As in the Ga-As molecules after the reaction is removed.
Since it rarely moves on the Ga plane, the reaction of the island part reacts stochastically waiting for arsine to fly to that place, so it takes an extremely long time to complete the reaction, Not a practical means.

The second problem is that impurities are easily incorporated into the growing compound semiconductor. Since it is adsorbed in the form of a molecule as described above, a small amount of unreacted molecules such as alkyl remains in the growth process, and carbon atoms are incorporated as impurities in the crystal. This problem is a very serious problem, and thus high-purity crystals have not been obtained by these methods.

[Problems to be Solved by the Invention] The present invention has the above-mentioned conventional drawbacks (1) that minute irregularities having a thickness of 1 to several atomic layers occur at the heterojunction interface,
(2) High epitaxial growth temperature, (3) Long growth time, (4) Easy incorporation of impurity atoms, and a technique capable of realizing a heterojunction interface flat in atomic plane are provided. The purpose is to

[Means for Solving Problems] In order to achieve such an object, in the present invention, a group III element or a group II element and a group V element or a group VI element are alternately supplied onto a semiconductor single crystal substrate. III-V or II
In the method of epitaxially growing a group VI compound semiconductor on a substrate, while irradiating the substrate with a beam of a group V element or a group VI element in an amount that does not form a compound layer by combining with a group III element or a group II element, 1 A controlled number of III to 90% to 110% of the number of atoms required to form the atomic layer III
The first step of supplying and depositing a group atom or a group II atom on a substrate, and the number of group V atoms or VI adjusted to 1 to 50 times the number of atoms required to form one atomic layer The second process of supplying the group atoms to the substrate to attach them,
It is characterized by repeating alternately.

Root cause which may innumerable atomic layer steps on the growth surface and the hetero-interface in the work for] a conventional method, the conventional MBE method, the growth in the MOCVD method and the like, with respect to A _III B _V compound semiconductor growth, B _This is because it was done under _V- stabilization conditions. In other words, this is because the growth of GaAs and AlGaAs was carried out under As-stabilizing conditions. This growth mode is, so to speak, a method of supplying Ga or Al onto a substrate crystal placed in an As atmosphere, and such a growth mode has been considered to be the most basic condition for growing a good quality crystal. . In such growth, for example, for GaAs, Ga
As soon as is attached, As is adsorbed on it and thus the substance that migrates on the growth surface is the Ga-As molecule.
In order to grow a high quality and flat atomic surface, the surface migration (migratio of Ga-As molecules in this case)
n) must be activated. However, even at relatively high temperatures (about 600 ° C or higher for GaAs growth), the movement of Ga-As molecules on the surface is extremely small. Therefore, the growth surface is not sufficiently flattened during growth, which is the atomic layer thickness level. Is the cause of many irregularities. This is shown in Fig. 1 (A).
~ (C). In the figure, 10 is a substrate, 11 is a Ga atom, and 15
Is the As atom. The GaAs molecules formed on the substrate 10 increase sequentially in the order of (A) → (B) → (C) in the figure. For example, as shown in FIG. The GaAs molecules of the second layer are formed before the GaAs is completely covered. Since the moving speed of GaAs molecules is low, the GaAs molecules of the second layer move to the second layer before moving to fill the voids of the first layer.
Layers, a Ga-As molecular layer of the third layer is formed, and a part of them happens to be adsorbed to the vacant space on the substrate to form the unevenness as shown in FIG.

The migration of the Ga-As molecules on the growth surface becomes smaller as the temperature decreases, and even at temperatures below 400 ° C, the surface molecules do not even move to stable lattice positions. As a result, the crystallinity deteriorates, which is the reason why the growth temperature cannot be lowered.

However, if we supply only A _III atoms to the growth surface, not A _III -B _V molecules, that is, only Ga atoms in the growth of GaAs, Ga atoms have a surface migration rate of 10% compared to Ga-As molecules.
It was found to be 0 times faster. Ga atoms supplied to the growth surface due to this phenomenon form a flat atomic layer in an extremely short time. B _V atoms (As in the case of GaAs) are supplied after the end of A _III atom supply, and are adsorbed on the flat A _III atom surface to form A _III −
The B _V molecular system is formed, and this process ends in an extremely short time if B _V is in the atomic state or the simple molecular state. Patterns in which a flat atomic plane grows in this manner are shown in FIGS. 1 (D) to (F). Since the Ga atom 11 has a high moving speed, when the required amount is supplied onto the substrate 1 in the process of (D) → (E), the substrate rapidly covers the substrate, and the Ga—Ga metal bond is Ga—As. Since it is weaker than the bond of Ga atoms on the Ga atoms, the Ga atoms on the Ga atoms are attracted to the As plane and completely cover the substrate to form the Ga atom plane. Next, when As is supplied, as shown in FIG.
A flat layer is formed. The actual growth is performed by repeating this periodically. The surface migration of A _III (Ga) atoms is extremely active even at low temperatures, so such growth is possible even at extremely low temperatures. In this growth technique, the number of A _III atoms supplied in each cycle must be the number necessary for forming one atomic layer, but according to the experiment, strict control of one atomic layer is not necessary and one atomic layer growth is required. It was found that no problem was obtained in the range of 90% to 110% of the required number of atoms. This level of control is
It can be sufficiently achieved by the growth layer thickness control technology such as MBE method and MOCVD method.

The fast migration of Group III Ga is considered to be due to the weak Ga—Ga metal bond, and since In and Al also have the same electronic state as Ga, the migration is similarly fast. Therefore, the arrangement of one atomic layer can be realized at high speed.

[Examples] The present invention will be described in detail below based on Examples.

Example 1 A GaAs crystal was grown on a GaAs substrate using a molecular beam epitaxial growth (MBE) apparatus. The ultra-high vacuum container containing the raw material elements and the substrate was evacuated to the range of 10 ^-6 to 10 ^-11 Torr,
The substrate was heated to 580 ° C. and metallic Ga and metallic As were heated to form a beam of those elements, which was supplied onto the substrate. The supply method of each element was performed according to the time chart of FIG.
That is, about 1 × 10 ¹⁴ pieces / cm ² · sec A per substrate surface area at all times
While irradiating the s beam, 6.4 × 10 ¹⁴ / cm ² · sec Ga beam and 2.5 × 10 ¹⁵ / cm ² · sec As beam each 1
The substrate was irradiated alternately every second. The amount of As beam constantly radiated does not form GaAs as a bond with Ga, but prevents As from coming out from the growing crystal. The supply amount of Ga atoms to the substrate is one amount that forms one atomic plane by one irradiation, and the supply amount of As is about four times the amount required to form one atomic surface. The supply amount of each atom is determined by the product of the beam intensity and the irradiation time, and the beam intensity can be controlled by adjusting the heating temperature of the source element. The amount of Ga supplied to form one atomic layer was determined from the period of oscillation of the intensity of the reflected electron beam (RHEED) in ordinary MBE growth.

While growing the GaAs crystal on the substrate in this way, its surface was irradiated with an electron beam accelerated to about 10 keV, and RHE
The ED intensity was observed. Figure 3 shows the time variation of the RHEED intensity obtained. As shown in the figure, the RHEED intensity decreases with the start of Ga supply and shows a minimum due to Ga surface formation.
It increases with the start of As supply and shows a maximum with the formation of As surface. That is, the RHEED intensity oscillates corresponding to the supply cycle of the raw material elements. In the case of the present embodiment, the RHEED oscillation continues without any deterioration even after the growth of several thousand atomic layers, indicating that the flatness of the growth surface at the atomic level is not deteriorated at all.

For comparison, Fig. 4 shows the oscillation of RHEED intensity when a GaAs crystal is grown by the ordinary MBE method (JHNeave
Appl. Phys., A31 , (1983, 1). The substrate temperature is 580
℃. In the ordinary MBE method, As molecules are supplied to the substrate surface before growth, and the substrate surface is relatively flat at the atomic level due to long-term treatment. The intensity of RHEED before the start of growth, that is, at t ≦ 0 reflects this flatness. A
When the supply of Ga atoms in addition to s molecules is started (that is, t
> 0), the RHEED intensity decreases sharply and reaches a minimum. This is just half the monolayer growth completed,
As shown in FIG. 6B, it is considered that the intensity of the irregularities on the growth surface reaches the extreme level, which reduces the electron beam reflectance and the RHEED intensity. With growth, the RHEED intensity now reaches its maximum. This means that the growth of just one molecular layer has been completed, but what is very important here is that the maximum value of each period is significantly lower than the maximum intensity of the previous period. This is because the movement of Ga-As molecules on the growth surface is insufficient as described with reference to FIGS. 1 (A) to 1 (C), so that the original flat atomic plane is reproduced after the growth of one molecular layer. This is because it is not possible and steps of one to several atomic layers are generated. This tendency becomes more and more intense with the growth, and as shown in Fig. 4, the oscillation of the RHEED intensity disappears after the growth of several tens of molecular layers.
This is because the growth surface becomes severely uneven as shown in FIG. 1 (C).

As described above, according to the method of the present invention, the crystal growth is significantly improved as compared with the conventional method, and the crystal growth maintaining the atomic level flatness is performed.

Next, the supply amount of Ga for forming one atomic layer was examined.
While irradiating a small amount of As onto the substrate, various amounts of Ga atoms were supplied onto the substrate, and As for forming a compound after a certain time.
The supply of atoms was restarted and the change of the RHEED signal at that time was observed. The results are shown in FIGS. 5 (A) to (C). FIG. 6A is a time chart of the beam intensity of As and Ga for forming a compound, and the supply amount of Ga was changed by changing the beam intensity or the irradiation time τ. In the figure (B), Ga and
The change in RHEED intensity due to As irradiation is shown, and FIG. 6C shows the state of recovery of RHEED intensity depending on the amount of Ga atoms supplied. In FIG. 6C, the supply amount of Ga atoms is standardized with the amount of forming one atomic layer being 1. This amount is 6.4 × 10 when growing on a GaAs substrate (100) surface.
^{It is 14} / cm ² . As the recovery amount, the initial value of RHEED intensity and Ga
The ratio b / a of the rapid recovery amount b immediately after As supply to the difference a at the time of surface formation and the recovery degree 10 seconds after As supply (a-
c) / a. The flatness of the surface deteriorates with the supply of Ga, and the intensity of the RHEED reflected beam deteriorates sharply. Although the RHEED signal recovers after the supply of As atoms is restarted after a certain period of time, it is clear that the speed depends strongly on the Ga supply amount. The recovery rate is fastest when the amount of Ga corresponds to just one atomic layer. However, this optimum value is not extremely narrow, and as shown in the figure, 90% to 110% of the amount corresponding to one atomic layer.
If it is between%, there is almost no problem.

The exact mechanism is unknown, but it is supposed that the Ga deficiency is supplemented by the next Ga period and the Ga excess is replaced by the excess As. This result is for a substrate temperature of 580 ° C, but the tendency is almost the same at other temperatures. It was found that the amplitude of RHEED oscillation was extremely large when Ga and As were alternately supplied onto the substrate with the amount of Ga set in this range, and the oscillation lasted as long as the growth continued.

The time from the stop of the Ga supply to the start of the As supply in FIG. 5 (A) shows that the mobility of Ga atoms is fast regardless of the compound formation and the flatness of the Ga surface.

Regarding the amount of As supplied, a flat As surface can be formed by supplying 1 to 50 times the amount required to form an As1 atomic surface. Since the vapor pressure of As is high, excess As that does not contribute to the Ga-As bond vaporizes and leaves the substrate surface, forming a flat As surface.

In Fig. 6, the temperature of the GaAs substrate is 100 ° C, and 1 x 10 ¹⁴ /
While irradiating As beam of cm ² · sec, 3 × 10 ¹⁴ pieces / cm ² ·
GaAs by alternately irradiating the substrate with a sec Ga beam and a 6 × 10 ¹⁴ / cm ² · sec As beam for 2.2 seconds and 4 seconds, respectively.
The RHEED signal when a crystal is grown is shown. In this case RHE
The ED intensity increases with the supply of Ga and exhibits a maximum, and decreases with the supply of As, and exhibits an oscillation corresponding to the supply cycle of the raw material elements. This vibration continued even after several thousand cycles of growth (several microns in film thickness). It is considered that this is because the flatness of the atomic plane is maintained by supplying Ga alone, despite the surprisingly low substrate temperature of 100 ° C.

FIG. 7 is a photoluminescence spectrum at 4.2 k observed in 1.1 μm-thick GaAs grown at a substrate temperature of 200 ° C. Reference numeral 21 in the figure indicates light emission by excitons at the band edge, which indicates that this crystal is of sufficiently high quality.

As a method of supplying Ga and As onto the substrate, the beam intensity of one As source is set to be strong or weak as shown in FIG. 8 instead of using two As sources as shown in FIG. Alternatively, Ga and As may be alternately irradiated while constantly irradiating a small amount of As.

Example 2 The substrate temperature was 580 ° C. and the conventional MBE growth mode, that is,
The same structure is obtained by the method of simultaneously supplying Ga (or Al) and As and the growth method of the present invention, that is, the method of alternately supplying Ga (or Al) and As while irradiating a small amount of As onto the substrate. An AlAs-GaAs single quantum well structure was fabricated and the photoluminescence spectra were compared. The structure is shown in FIG. Both quantum well widths are about 60Å
(20 atomic layers). In FIG. 9, 31 is a GaAs substrate,
32 is a GaAs buffer layer, 33 is an AlAs barrier layer (600Å), 34
Is a GaAs quantum well (60Å), 35 is a GaAs cap layer (50Å)
Is. FIG. 10 shows spectra at 4.2k, where 41 is the spectrum of the quantum well structure manufactured by the present invention and 42 is the spectrum of the structure manufactured by the conventional method. The width of the spectrum 41 according to the present invention is as narrow as 40 Å, indicating that the hetero-interface forming the quantum well is extremely flat.

Like GaAs, AlAs can be grown with good crystal quality on low temperature substrates. By combining AlAs-GaAs heterojunctions, good quality quantum wells could be grown at a substrate temperature of 200-300 ℃. From a single well with a 30 Å well width (GaAs width), a strong emission of 7200 Å, which corresponds to the transition between quantum levels, was obtained, and it was revealed that the quality of AlAs was also sufficiently good.

In the above embodiments, the MBE apparatus was used and the III-V group compound semiconductor was mainly described, but it goes without saying that the present invention can be realized by other thin film growth apparatuses and can be applied to the II-VI compound semiconductor.

[Advantages of the Invention] As described above, in the III-V group compound semiconductor, the A _III element and the B _V element are added while constantly supplying a small amount of the B _V element.
'While constantly supplied _VI element A' trace B in group II-VI compound semiconductor growth 'alternately supplying _VI elements on a growth substrate, A _III atoms or A' _II element and B 1 cycle of _II atoms By setting the supply amount per 90% to 110% of the amount required for forming one atomic layer, it is possible to fully utilize the property that A _III and A ′ _II atoms can move extremely fast on the substrate surface. As a result, many atomic level irregularities generated at the hetero interface, which was a problem of conventional MBE growth, disappeared,
It has become possible to obtain a very good hetero interface.

[Brief description of drawings]

FIGS. 1 (A) to (F) are schematic diagrams for comparing and explaining the operation of the present invention and the conventional method, and FIGS. 1 (A) to (C) show the conventional method.
FIGS. 3D to 3F are schematic diagrams of crystal growth according to the present invention, FIG. 2 is a time chart of an embodiment of a method for supplying a raw material element according to the present invention, and FIGS. Of the present invention and the RHEED intensity oscillation of the GaAs crystal grown by the conventional method, FIG. 5 (A) is a supply time chart of Ga and As in the embodiment of the present invention, and FIG. 5 (B). , (C) are diagrams showing the recovery of the RHEED intensity during the growth process, and FIG. 6 shows GaAs grown at a substrate temperature of 100 ° C. according to the present invention.
Fig. 7 is a diagram showing the RHEED intensity of the grown layer, Fig. 7 is a photoluminescence spectrum of GaAs grown at a substrate temperature of 200 ° C at 4.2K, and Fig. 8 is a time chart of another embodiment of the method for supplying raw material elements. FIG. 9 is a diagram showing a quantum well structure, FIG. 10 is a photoluminescence spectrum of a quantum well structure formed by the present invention and the conventional method, FIG. 11 is a schematic diagram showing unevenness of a hetero interface by the conventional method, and FIG. 12 ( (A), (B) and (C) are schematic diagrams showing a conventional growth method, and FIGS. 13 (A) and (B) are schematic diagrams showing a conventional growth by molecular layer epitaxy. 1 ... GaAs layer, 2 ... AlAs layer, 3 ... Hetero interface, 10 ... GaAs substrate, 11 ... Ga atom, 12 ... C atom, 13 ... H atom, 14 ... Trimethylgallium, 15 ... … As atoms, 16 …… arsine, 17 …… methane, 31 …… GaAs substrate, 32 …… GaAs buffer layer, 33 …… AlAs barrier layer, 34 …… GaAs quantum well, 35 …… GaAs cap layer.

Claims (2)

[Claims] 1. A group III element or II on a semiconductor single crystal substrate.
Alternately supplying group elements and group V elements or group VI elements III
In the method of epitaxially growing a group-V or group II-VI compound semiconductor on the substrate, a beam of the group-V element or group-VI element in an amount that does not form a compound layer by combining with the group-III element or group-II element is applied to the substrate. While irradiating the top, the number of group III atoms adjusted to 90% to 110% of the number of atoms necessary to form one atomic layer or
First step of supplying and depositing group II atoms on the substrate, and the number of group V atoms or group VI atoms adjusted to 1 to 50 times the number of atoms required to form one atomic layer A semiconductor epitaxial growth method, characterized in that the second step of supplying and adhering to the substrate is alternately repeated. 2. The semiconductor epitaxial growth method according to claim 1, wherein the temperature of the substrate is 100 ° C. to 700 ° C.