JPH07115988B2 - Epitaxial growth method - Google Patents

Description

The present invention relates to a molecular beam epitaxy technique, for example, a technique suitable for use in a method for controlling a substrate temperature in a molecular beam epitaxial growth process.

[Prior Art] In molecular beam epitaxial growth, it is necessary to maintain a constant substrate temperature during epitaxial growth in order to uniformize the characteristics in the growth layer and reduce in-plane defects to improve crystallinity. Conventionally, the method of controlling the substrate temperature in the molecular beam epitaxial growth is generally to keep the temperature of the substrate heater constant, and even when forming the heterojunction growth layer, the substrate temperature is only changed stepwise. .

[Problems to be Solved by the Invention] However, when an attempt is made to grow a semiconductor layer having a small bandgap on a semiconductor substrate having a large bandgap, such as growth of an InGaAs layer on an InP substrate, a small bandgap is required. It has been found that the semiconductor has a larger infrared ray absorptivity than a semiconductor having a large band gap, so that the substrate temperature gradually rises even if the heater temperature is kept constant.

In order to solve this problem, a method of controlling the substrate heater so that the substrate temperature becomes constant while measuring the substrate temperature with a temperature sensor that detects light radiant energy such as an infrared pyrometer can be considered.

However, in this method, since the infrared emissivity of the substrate and the epitaxial layer are not the same, it is necessary to correct the detection output, but the correction is troublesome and the measurement error due to the contamination of the observation peep window by the infrared pyrometer. Occurs. Moreover, it has been found that there is a defect that the stain increases when the growth is repeated, and the detection temperature is not reproducible.

The present invention has been made to solve the above problems, and epitaxially grows on a semiconductor substrate heated to a predetermined temperature by a heater, a semiconductor layer having a smaller bandgap than that of the semiconductor substrate and having a higher infrared absorption rate than the semiconductor substrate. In this case, it is an object of the present invention to provide an epitaxial growth technique capable of maintaining a constant substrate temperature with good reproducibility and obtaining an epitaxial layer having good crystallinity.

[Detailed Description of the Invention] It is known that a semiconductor absorbs light having an energy higher than its band gap and almost transmits light having a lower energy. For example, the band gap of InP at room temperature is 1.423 eV (871.3 nm), and the band gap of In _0.53 Ga _0.47 As lattice-matched to InP is 0.742 eV (167
0 nm). In other words, the InP substrate transmits almost all infrared rays with a wavelength of 871 nm or more, whereas In _0.53 Ga
_0.47 As absorbs infrared rays up to 1670 nm.

Therefore, when a semiconductor layer having a larger infrared absorption rate than the semiconductor substrate is epitaxially grown on the substrate surface while heating with a heater from the back surface of the semiconductor substrate, the infrared radiation emitted from the heater is used. Some of the wavelengths transmitted through the substrate are
It is absorbed as the epitaxial growth layer on the surface grows.

By the way, the energy distribution of the infrared radiation from the substrate heater is expressed by the equation (1) from Planck's law.

(C ₁ = 3.7402 × 10 ⁻¹² Wcm ² , C ₂ = 1.38448 cm deg) FIG. 1 shows the energy distribution of the substrate heating heater near 0 to 2000 nm of this distribution. Considering the case where the temperature of the substrate heating heater is 600 ° C., the temperature of the substrate is about 500 ° C. at this time. In Fig. 1, point A is In at about 500 ° C.
The band gap of P is shown, and the point B shows the band gap of In _0.53 Ga _0.47 As. Therefore, the InP substrate absorbs only the energy in the region C on the shorter wavelength side than the point A. When an In _0.53 Ga _0.47 As epitaxial layer is grown on this, it absorbs energy in the region D + C on the shorter wavelength side than the point B, and if the temperature of the substrate heater is kept constant, In _0.53 Ga _0.47 As absorbs extra energy by the amount of energy in the region D of the middle hatched line, and the substrate temperature rises. Further, this difference in energy absorption becomes more remarkable as the epitaxial layer becomes thicker, and has a great influence on the actual substrate temperature.

It is possible to calculate the actual substrate temperature from this difference in energy absorption capacity using the infrared absorption coefficient, heat conductivity, etc., but the coefficient changes depending on the substrate layout, the distance between the substrate and the heater, etc. Becomes very complicated.

Therefore, the present inventors consider this difference in absorbed energy, continuously change the temperature of the substrate heating heater according to the thickness d of the epitaxial layer, and keep the substrate temperature constant. The change of the substrate temperature with respect to the film thickness was investigated.

FIG. 2 shows the film thickness dependence of the substrate temperature measured by a pyrometer when the temperature of the substrate heater is constant. A simple function that matches this measurement result as much as possible was sought, and it was found that it was a logarithmic function, although the rationale for it was unknown. Moreover, conveniently, the growth rate can be regarded as almost constant in the normal growth temperature range of 300 to 600 ° C., so that the growth film thickness is proportional to the time. Therefore, it is concluded that the substrate temperature can be kept constant by decreasing the temperature of the substrate heating heater logarithmically.

[Means for Solving the Problem] The present invention has been made based on the above-mentioned findings, and while heating one main surface of a semiconductor substrate by radiation from a heating unit, the other main surface of the semiconductor substrate is heated. In the case of epitaxially growing a semiconductor layer having a larger infrared absorption rate than the semiconductor substrate due to the smaller band gap than the semiconductor substrate, the temperature of the heating means is logarithmically changed to keep the semiconductor substrate at a predetermined temperature. To do so.

In the above case, the grown film thickness also relates to the growth rate, so to be precise, the growth rate is calculated from the temperature of the heating means, the growth rate is calculated from the growth time, and the heating means is controlled accordingly. However, since the growth rate is almost constant over a wide temperature range as described above, it is considered that the film thickness changes almost linearly with time. Therefore, approximately, if the temperature of the heating means is changed logarithmically with respect to time as described above, the substrate temperature can be kept sufficiently constant in a practical range.

[Operation] According to the above-described means, the temperature of the heater is controlled internally by controlling the heater temperature instead of detecting the substrate temperature from the outside through the viewing window. And the change over time can be avoided, and the cumbersome process of correcting the detection output due to the difference in the infrared emissivity of the growth layer of the substrate becomes unnecessary.

[Example] In _0.53 Ga _0.47 As was grown on an InP substrate using an MBE (Molecular Beam Epitaxial Growth) apparatus. InP substrate is
A plane orientation (001) with a diameter of 2 inches was used.

As shown in FIG. 3, the back plate 2 made of PBN was joined to the back surface of the InP substrate 1 and attached to the Mo block 4 by the metal fittings 3 so that the back surface of the substrate faces the heater 5. It was installed in the growth chamber of the MBE system. A temperature sensor 6 composed of a thermocouple was attached to the heater 5. Before the start of growth, the molecular beam intensity ratio of In and Ga was adjusted so that the composition was In _0.53 Ga _0.47 As. Then, while irradiating the InP substrate 1 with the molecular beam of As, the heater 5 is powered to raise the temperature of the substrate 1 to remove the oxide film on the surface, and then the shutter of the evaporation source cell of In and Ga is opened to grow the substrate. It started. The growth conditions are growth rate 1.
The molecular intensity ratio of the group V element (As) and the group III element (In, Ga) was set to 40 at 7 μm / hr.

During the first growth, the substrate temperature was detected from the observation window using a pyrometer while controlling the heater power so that the temperature of the substrate heating heater was constant, and the temporal change was examined. From this measurement result, the temperature change of the substrate
It was calculated as a function of [minutes]. As a result, the substrate temperature Tsub
It was found that (t) [° C] is represented by the equation (2).

Tsub (t) = 497 + 30log (t) (2) Next, the growth conditions are the same as above, and the heater temperature T _H
The growth was performed while controlling the power of the heater 5 so that the temperature changes according to the following equation (3).

T _H (t) = T ₀ −30log (t) (3) Here, T ₀ is the initial temperature of the heater for bringing the substrate to a desired temperature, for example, the temperature measured by the pyrometer for the InP substrate is 4
It is the temperature (about 660 ° C) of the substrate heater when it reaches 90 ° C. Further, the coefficient of log (t) varies depending on the substrate thickness, the arrangement of the heater, etc., and is experimentally determined by the measurement of the equation (2).

Thus in Figure 4 the temporal change b of the substrate temperature measured by temporal changes a and pyrometers substrate heater temperature measured by the thermocouple when changing according to the above formula the temperature T _H of the substrate heater 5 Show. Here, the time is 0 when the growth starts
Therefore, the time corresponds to the grown film thickness.

As is clear from FIG. 4, the substrate temperature could be kept constant for a long time by continuously changing the temperature of the substrate heater by a logarithmic function.

In this way, the substrate temperature can be kept constant even if the temperature of the heater is changed logarithmically with respect to the growth film thickness instead of the growth film thickness. This is because the growth rate can be made almost constant if the supply amount of the group III element is kept constant, depending on the amount.

If it is desired to change the growth temperature during the epitaxial growth, the initial value T ₀ in the above equation may be changed at the time point tc at which the growth temperature is desired to be changed.

FIG. 5 shows a temporal change in the temperature of the heater when T ₀ is increased on the way. With this configuration, after the switching time tc, the substrate temperature can be kept constant at a temperature different from the previous temperature.

[Effect of the Invention] As described above, according to the present invention, while heating one main surface of a semiconductor substrate by radiation from a heating means, the band gap on the other main surface of the semiconductor substrate is smaller than that of the semiconductor substrate. Thus, in the case of epitaxially growing a semiconductor layer having a larger infrared ray absorptivity than the semiconductor substrate, the heating temperature is logarithmically changed to keep the semiconductor substrate at a predetermined temperature. In addition to avoiding detection errors and changes over time due to the use of a substrate, the complicated process of correcting the detection output due to the difference in infrared emissivity between the substrate and the growth layer is not necessary, and the substrate temperature can be kept constant with good reproducibility, and thus epitaxial with good crystallinity can be obtained. The effect is that layers can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing the radiant energy distribution of the substrate heating heater and the difference in the amount of energy absorbed when the epitaxial layer is grown only on the substrate and on the substrate. FIG. 2 shows the temperature of the substrate heating heater constant. FIG. 3 is a graph showing the relationship between the substrate temperature and the film thickness, FIG. 3 is a cross-sectional view showing the mounting state of the heater and the substrate in the growth chamber of the MBE apparatus, and FIG. FIG. 5 is a graph showing changes in the heater temperature and the substrate temperature when epitaxial growth was performed, and FIG. 5 is a graph showing changes in the heater temperature and the substrate temperature when the initial heater temperature was changed during the epitaxial growth. 1 ... semiconductor substrate, 2 ... back plate, 3 ... holding metal, 5 ... heating heater, 6 ... temperature sensor.

Claims (1)

[Claims] 1. The semiconductor substrate has one main surface heated by radiation from a heating means, and has a band gap smaller than that of the semiconductor substrate on the other main surface of the semiconductor substrate, thereby having an infrared absorption rate of the semiconductor. In the case of epitaxially growing a semiconductor layer larger than the substrate, when the initial value of the temperature of the heating means is T ₀ , the temperature T of the heating means is T = T ₀ −Alog d with respect to the film thickness d of the semiconductor layer. (However, A is a constant) The epitaxial growth method is characterized in that the semiconductor substrate is maintained at a predetermined temperature according to the following formula.