JPH0331677B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field] The present invention relates to a crystal growth film thickness control method in which growth is performed by counting each layer in units of monoatomic layers to control the thickness of a grown film when growing a crystal in a vacuum. . [Prior Art] Conventionally, in crystal growth in a vacuum (hereinafter referred to as crystal growth), the thickness of the grown film was controlled by the time the shutter of the crystal growth apparatus was open. Furthermore, the mixed crystal composition ratio (hereinafter referred to as composition ratio) of the mixed crystal has been determined using X-rays, a composition analyzer, etc. after the crystal has grown. These will be explained below with reference to the drawings. FIG. 1a is a schematic diagram of the crystal growth apparatus,
Here, 1 is a crystal substrate (hereinafter referred to as a substrate), 2 is a heater that heats the substrate 1, and 3 is a material necessary for crystal growth (hereinafter referred to as a source). In the case of GaAs, it is Ga, As, or These are dopants Si and Be. If it is desired to obtain the source as ions, it is necessary to provide an ion gun 4. Furthermore, if the source is a high melting point material, an electron gun or sputtering device 5 is required. Moreover, when the source is in a gas state, the cylinder 6 and the flow rate controller 7
is necessary. Each source is provided with a cell shutter (hereinafter referred to as shutter) 8 that opens and closes the source vapor (hereinafter referred to as flux). 9 is a main shutter that opens and closes the flux of all sources to and from the substrate 1; Each shutter is equipped with a shutter drive device 10 for driving it. Reference numeral 11 is a vacuum gauge, ie, a flux monitor, for determining the amount of flux in each source. 12 is a gun portion of a reflection electron beam diffraction device (hereinafter referred to as RHEED), and electrons 13 emitted from this gun 12 pass through the surface of the substrate 1 and reach a fluorescent screen (hereinafter referred to as screen) 14 of the RHEED. At this time, the electrons 13 are diffracted by the substrate 1, and an image of the diffracted electron beam is displayed on the screen. This is called the RHEED image. FIG. 1b is a reproduction of one example. All of the above parts are contained in the vacuum chamber 15. In order to grow crystals on the substrate 1, the substrate 1 is heated appropriately with the heater 2, and the shutter 8 and main shutter 9 of the necessary source 3 and other sources are opened.On the other hand, in order to stop the growth, These shutters 8 and 9 are closed. GaAs,
To grow crystals like AlGaAs, typically As
The flux is always kept open, and only the Ga and Al shutters are opened and closed to start and stop growth. In the case of mixed crystals, Ga and Al shutters are opened and closed simultaneously to start and stop growth. Hereinafter, explanation will be given using GaAs and AlAs as crystals and AlGaAs as a mixed crystal. Conventionally, in order to know the film thickness of a grown crystal, first, the fluxes of Ga, Al, and As were measured with a flux monitor 11 before growth, and then the film was grown for a long time (about 1 hour) using the flux. The thickness (approximately 5000 Å to 2 μm) is measured using an optical interference microscope or a thin film thickness meter, and a growth rate calibration table showing the relationship between the growth rate of film thickness and flux is created, and from the second time onwards, the flux value and growth are measured. The method used was to estimate the film thickness from the growth rate using only time. From a few angstroms to several 100 angstroms, which is close to a monoatomic layer obtained by short-time growth
In the case of Å, estimations were made in exactly the same way from the flux value and growth time based on the growth rate calibration table obtained from this long-term growth. However, with this method, it is not possible to control the film thickness to the level of a monoatomic layer, since it is possible to take into account changes in the growth rate due to changes in flux over time immediately after the shutter is opened. Figure 2 shows an example of the change in Ga flux over time immediately after opening the shutter. The flux changes over several minutes after the shutter is opened, and the growth rate is not constant during this time.
Therefore, the conventional method of using a growth rate calibration table cannot be applied in a time period where the growth rate is not constant. In particular, when creating a device such as a superlattice, it is necessary to open and close the shutter over a period of several seconds to several minutes. This means that the shutter must be opened and closed at times when the growth rate is not constant, resulting in a decrease in film thickness. It will be uncertain. FIGS. 3a and 3b schematically show two examples of the arrangement of monoatomic layers when two types of crystals A and B are successively grown. FIG. 3a shows an example of growth using the conventional method as described above, in which the interface is unstable in units of monoatomic layers. FIG. 3b shows an example of growth using the method of the present invention, and shows that the interface is defined in units of monoatomic layers and the film thickness is correctly controlled. Furthermore, when a mixed crystal such as Al _x Ga _1-x As is produced, it is necessary to determine the mixed crystal composition ratio x. Conventionally, the following method has been used to determine this x. That is, before the growth, the fluxes of Ga, Al, and As are measured using the flux monitor 11 shown in FIG.
Determine the growth rates of both AlAs crystals and create a calibration table for each. From the growth rates of GaAs and AlAs, we determined the growth rate and composition ratio x when they were grown simultaneously, and after the growth, we measured the lattice constant using X-rays and analyzed the composition using compositional spectroscopy. The value of the composition ratio x is determined. Even in the conventional determination of the composition ratio, it is based on the growth rate during long-term growth, and therefore the composition ratio cannot be determined during a time period when the flux does not reach a constant value. In this way, in determining the growth rate required for conventional film thickness control and composition ratio determination,
In practice, there are considerable difficulties in measuring the most important film thickness due to long-term growth, and it cannot be determined very accurately, with an accuracy of one to two significant digits.
Therefore, the accuracy of film thickness and composition ratio by conventional methods is at most two digits. Moreover, in the case of short-term growth, the accuracy is even lower than this. If the composition ratio can be determined with high precision in principle, a standard crystal with a fixed composition ratio can be produced. In conventional film thickness control methods, the film thickness is not determined as described above, and the interface of the heterojunction is uncertain.
Moreover, since the composition ratio of the mixed crystal is not determined, it is actually impossible to manufacture the device according to the design. Therefore, the laser wavelength of a quantum well laser or a multi-quantum well laser may be broadened or split into several wavelengths, or the laser wavelength itself may not be realized as designed. In LEDs, the emission wavelength may deviate from the design value or the efficiency may deteriorate. Also,
In HEMTs (high electron mobility transistors) and devices that use two-dimensional electron conduction, disturbances at the interface,
The deviation of the potential step from the designed value causes scattering due to the interface and deviation of the two-dimensional electron concentration from the designed value, which impedes high-speed operation. Furthermore, in vertical superlattice devices such as one-barrier diode, two-barrier diode, chirp superlattice, superlattice gate FET, etc.
Characteristics will vary greatly due to interface uncertainty and deviations in design values. As described above, it is strongly expected that the above-mentioned problems will be solved and improved in the production of each element. [Object of the Invention] Therefore, the object of the present invention is to eliminate the above-mentioned drawbacks and to improve the performance of quantum well lasers, multi-quantum well lasers, and
In LEDs, wavelength narrowing, wavelength stabilization and high efficiency are achieved, HEMTs and two-dimensional electronic devices are faster and more reliable, and superlattice devices have more stable characteristics, higher precision, and higher reliability. It is an object of the present invention to provide a crystal growth film thickness control method that allows growth of crystals with well-defined interfaces and very well-controlled film thicknesses and composition ratios. [Structure of the Invention] In order to achieve the above object, the present invention forms a backscattered electron beam diffraction image using a backscattered electron beam diffraction device during crystal growth in vacuum, and synchronizes the intensity change of the diffraction image with time. It is characterized by controlling the start and stop of crystal growth. [Example] The present invention will be described in detail below with reference to the drawings. FIG. 4 shows a schematic configuration diagram of a crystal growth apparatus applied to the present invention. Here, the same parts as in FIG. 1 are given the same reference numerals, and detailed explanation thereof will be omitted. In the figure, 16 is a camera lens, 17 is an optical fiber, and 18 is an X that moves the optical fiber vertically and horizontally.
-Y stage, 19 is a light receiver, 20 is a recorder,
21 is an A-D converter, 22 is a computer, 23
is a shutter opening/closing drive control device. The RHEED image currently projected on the screen 14 is formed on the X-Y stage 18 by the camera lens 16, and the light from a suitable point of the image is transmitted to the fiber 1.
7 to a photoreceiver 19, and its output signal is supplied to a computer 22 through an A-D converter 21.
On the other hand, the output signal of the light receiver 19 is also output to a recorder 20 for monitoring. The shutter opening/closing drive control device 23 operates based on commands from the computer 22, and controls the extraction voltage of the shutter 8, main shutter 9, and ion gun 4 of each source, the gas flow rate control, the electron current of the electron gun 5, etc. to open/close, half open, and turn on. ,
It is possible to control such as OFF. Figure 5a shows an actual measurement example in which the intensity of a specularly reflected electron beam image (hereinafter referred to as specular), which is particularly strong among RHEED images, was recorded on a recorder at the same time as crystal growth started. Since we are growing a GaAs crystal, the point in Figure 5a is when the shutter of the Ga source is opened and crystal growth begins. From this point on, the crystal growth begins, and as time passes, the intensity of this specular becomes... ,...it vibrates. Vibrations also occur at points other than specular. The reason why this RHEED image exhibits oscillations is currently interpreted to be that the crystal growth mechanism is as shown in Figure 5b. That is, as the growth progresses, microcrystals spread out in dots and islands over one surface, and when the coverage is half (θ=0.5), the surface becomes most uneven. The growth progresses further, and at the time of , the further growth is completed, most of it is covered with a new growth layer, and the surface becomes flat again. To repeat this below,
The intensity of the RHEED image oscillates. Furthermore, it is known that this frequency is proportional to the growth rate. By the way, as shown later, the inventor
We confirmed that one period of vibration in the RHEED image corresponds exactly to one monoatomic layer. Below is one of the RHEED images.
The period will be explained assuming that it corresponds to one monoatomic layer. Therefore, one monoatomic layer between the parentheses in Figure 5a,
This means that one monoatomic layer has grown from .
By counting the number of oscillations at the same time as growth begins, it is possible to determine which layer of growth is occurring. The monoatomic layer of GaAs and AlAs crystals is
The paired layers of Ga and As and the paired layers of Al and As are each monoatomic layers. In the case of mixed crystals like Al _x Ga _1-x As,
Both (AlAs) _x and (GaAs) _1-x form one monoatomic layer. The initial phase at which the vibration of this RHEED image starts is
It is currently quite difficult to accurately control the initial phase as it depends on the image position of the RHEED image, crystal orientation, incident angle of the electron beam, incident direction of the electron beam, energy of the electron beam, substrate temperature, and As flux intensity. It is. Actual measurement examples when the initial phase changes are shown in Figure 6 a~
Shown in e. Among the numbers ~ in FIGS. 6a to 6e, ~ corresponds to one monoatomic layer, and ~ corresponds to the next monoatomic layer, no matter at which initial phase the vibration starts. Therefore, in FIG.
2, and the computer 22 measures the vibration state of this RHEED image from the time of the start of growth.
During the growth, the computer 22 can determine at which layer and at which phase point the growth is being performed. Therefore, in order to be able to stop the growth in synchronization with the phase that is consistent with the initial phase at which the growth has just started in FIGS. can send signals. The shutter opening/closing drive control device 23 starts the shutter drive device 10 in response to a signal from a computer. Furthermore, this control device 23 can control the gas flow rate controller 7, the extraction voltage of the ion gun 4, and the electron current of the electron gun 5 to stop crystal growth. In the manner described above, growth can be stopped at any number of monoatomic layers. The vibrations in the RHEED image show damped vibrations as shown in Figures 5 and 6, and after a long period of time, the vibrations are no longer visible and the number of monoatomic layers cannot be counted.
However, even if the oscillations of the RHEED images with various initial phases are attenuated after the shutter is opened and growth begins, as shown in Figures 7a to 7e, the growth is temporarily interrupted when the shutter is closed. , after a period of recovery, open the shutter again and start growing.
The vibrations appear again. At this time, if the recovery time is short, the amplitude of the generated vibration is also small, and if the recovery time is sufficiently long, the vibration is restarted with exactly the same amplitude. 7th
The black areas in Figures a to e are the areas that are vibrating. In the embodiment shown in FIG. 8, the growth of the GaAs crystal is stopped before the vibrations completely disappear, and the growth is resumed after a period of recovery time.
37 GaAs layers are stacked, and 30 layers are grown on top of that, and 18 layers are grown on top of that, making a total of 85 layers, with a film thickness of 240.55 Å (one monoatomic layer is GaAs's lattice constant a ₀ = 5.65 Å) (calculated as half a ₀ /2 = 2.83 Å). If the vibration damping is so severe that the peaks and troughs of the vibration become a noise level, the computer determines this and resumes growth after the above-mentioned recovery period. ,
Growth can be performed while digitally counting the number of growth layers. Therefore, the thickness of the grown film can also be calculated as (number of layers) x (thickness of monoatomic layer) depending on how many layers have grown in terms of monoatomic layers. The example shown in FIG. 9 is an example of the vibration of the RHEED image when a heterojunction of different crystals and a mixed crystal are grown. In this example, we will first open the Ga shutter and expose the GaAs.
10 layers are grown, then 10 layers of AlGaAs are grown by opening the Al shutter while leaving Ga open, then 10 layers of AlGaAs are grown with the Ga shutter closed, and then
Open the Ga shutter and grow 10 layers of AlGaAs.
Finally, 10 layers of GaAs alone were grown.
Through the above steps, a total of 50 layers (approximately 142.5 Å) were grown. As is clear from Fig. 9, one vibration in the intensity of the RHEED image corresponds to one monoatomic layer, so the composition ratio x of the mixed crystal Al×Ga _1-x As can be directly determined from this frequency. can. That is, the frequency of GaAs is f
(GaAs) If the frequency of AlAs is f(AlAs), the composition ratio x is given by x=f(AlAs)/f(GaAs)+f(AlAs) (1). On the other hand, the frequency of Al _x Ga _1-x As is f
(AlGaAs), the composition ratio x is x = f (AlGaAs) - f (GaAs) / f (AlGaAs)
(2) is given by The values of x given by equations (1) and (2) should essentially match. Equation (1) is the composition ratio _x of _the mixed crystal Al
It gives Formula (2) is AlGaAs, GaAs
The frequency of each vibration is known, and this is a method of determining the concentration of Al that will be growing. Ga cell temperature,
In other words, by keeping the flux of Ga that comes out constant,
Table 1 below shows how equations (1) and (2) match while changing the Al cell temperature, that is, the Al flux. As can be seen from Table 1, the agreement between equations (1) and (2) is very good.

〔effect〕

As is clear from the above, according to the present invention,
By turning growth on and off in synchronization with changes in the intensity of the RHEED image, growth can be performed in units of monoatomic layer thickness, greatly improving the accuracy of film thickness control. Therefore, the thickness of the grown film can be accurately measured. In addition, even when a heterojunction is formed, the interface is flat on an atomic layer basis. For this reason, quantum well laser LEDs can achieve narrower wavelengths and higher reliability, while HEMTs can achieve higher speeds and higher reliability.
Superlattice devices can stabilize their characteristics.

[Brief explanation of the drawing]

Figure 1a is a schematic diagram of a conventional crystal growth apparatus;
Figure 1b appears on the screen of the device in Figure 1a.
A copy of the RHEED image, Figure 2 is a diagram showing an actual measurement example of the change in Ga flux over time immediately after opening the shutter, and Figures 3a and b are graphs showing the results of the sequential growth of two types of crystals. FIG. 4 is a schematic diagram showing an example of the arrangement of atomic layers, FIG. 4 is a schematic configuration diagram showing an example of a crystal growth apparatus to which the present invention is applied, and FIG. , FIG. 5b is a diagram showing the state of the growing crystal surface corresponding to the vibration in FIG. Figures 7a to 7e are waveform diagrams showing an example of the relationship between the vibration of the RHEED image during the crystal growth process and its recovery time, and Figure 8 is a waveform diagram showing an example of the vibration of the RHEED image during the crystal growth process. Waveform diagram, Figure 9 is a waveform diagram showing an example of vibration of the RHEED image when growing a crystal heterojunction and a mixed crystal.
Figure 0 is a schematic diagram showing an example of a cross section of a crystal in which a predetermined layer of AlAs is arranged in a GaAs crystal, along with a photograph showing the crystal structure. ) ₃ (AlAs) A waveform diagram showing an example of the vibration waveform of the RHEED image when a ₃ superlattice is formed, and FIGS. 12 and 13 respectively show other configurations of the crystal growth apparatus applied to carry out the present invention. It is a schematic block diagram which shows two examples. 1...Crystal substrate (substrate), 2...Heater, 3...
...Materials (source) necessary for crystal growth, 4...Ion gun, 5...Sputtering device or electron gun,
6...Cylinder, 7...Flow controller, 8...
Cell shutter, 9... Main shutter, 10...
Shutter drive device, 11... Vacuum gauge (flux monitor), 12... Gun of reflection electron beam diffraction device, 1
3...electronic, 14...fluorescent screen, 15...
Vacuum chamber, 16... Camera lens, 17... Optical fiber, 18... X-Y stage, 19... Light receiver, 20... Recorder, 21... A-D converter,
22... Computer, 23... Shutter opening/closing drive control device, 24... AC magnetic field generating coil.

Claims (1)

[Claims] 1. A crystal characterized in that during crystal growth in vacuum, a reflected electron beam diffraction image is formed using a reflected electron beam diffraction device, and the start and stop of crystal growth is controlled in synchronization with the time change in the intensity of the diffraction image. Growth film thickness control method.