JPH0315333B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Technical Field of the Invention The present invention relates to a molecular beam crystal growth method for forming a semiconductor crystal having a uniform impurity concentration distribution on an uneven substrate.

(b) Background of the technology Molecular beam crystal growth (molecular beam epitaxy) is a method of chemically adsorbing a single crystal substrate by evaporating a flow of molecules consisting of the constituent elements of the crystal, or molecular beams, from an ejection cell and colliding with the substrate surface. This is a method of epitaxially growing crystals with aligned crystal axes.

Figure 1 shows the basic configuration of a molecular beam crystal growth system.It differs from a vacuum evaporation system in that it requires an ultra-high vacuum and is designed to perform epitaxial growth on the substrate with the highest efficiency. The substrate temperature is selected, a molecular beam source is provided to evaporate each component element that makes up the crystal, and the evaporation rate distribution is controlled.

In other words, in Figure 1, the degree of vacuum in chamber 1 is maintained at an ultra-high degree of vacuum of about 10 ^-10 torr, so that the mean free path of the evaporated molecules is made sufficiently large so that the effects of collisions between molecules can be ignored. There is.

Next, a plurality of ejection cells 2 and 3 that constitute the molecular beam source
Molecules of component elements (two in this figure) are evaporated at an appropriate ratio and attached to the surface of the wafer 4, thereby growing a thin film crystal epitaxially.

For example, in the case of growing gallium arsenide (GaAs), the heating temperature of the ejection cell 2 that evaporates gallium molecules (Ga) and the ejection cell 3 that evaporates arsenic molecules (As, As ₂ , As ₄ ) is
1000-1050℃ for the latter 300-330℃ and GaAs
While adjusting the vapor pressure according to the composition ratio of
In this example, the temperature of wafer 4 made of GaAs is
The temperature is maintained at 600-650°C to allow efficient growth of single crystals.

That is, the molecular beams of each element that evaporate and fly from the ejection cells 2 and 3 collide with the wafer 4, but at this time, the deposition efficiency differs depending on the component element, and some have high efficiency while others have very low efficiency. Therefore, the necessary energy is applied so that the component elements adhere in the required composition ratio and are settled in stable lattice positions.

The present invention relates to a method of epitaxially growing a crystal having a uniform impurity concentration distribution on an uneven substrate surface using such a molecular beam crystal growth method.

(c) Prior art and problems When crystal growth is performed using the molecular beam crystal growth method, ejection cells equal in number to the number of constituent elements are prepared, and molecular beams are emitted from these cells onto the substrate.The reason for this is described above. This is to grow a single crystal having a desired composition ratio by adjusting the vapor pressure of each component element.

Now, by incorporating an impurity element as a dopant into a compound semiconductor such as GaAs, an electronically conductive type (N
When epitaxially growing a hole-conducting type (type) or hole-conducting type (P-type) semiconductor by molecular beam crystal growth, conventionally, as shown in FIG. An ejection cell 5 was provided to irradiate the wafer 4 with molecular beams 6 of each element.

Here, the ejection cells 2, 3, and 5 are made of graphite or boron nitride (BN), and each element filled therein is heated by resistance heating using a heater made of tantalum (Ta) or tungsten (W). The molecular beam is ejected from the ejection hole (orifice) 7.

Wafer 4 on which epitaxial growth is performed
is configured to rotate at a low speed with respect to the molecular beam 6, so that the molecular beam 6 is uniformly projected onto the wafer 4, but the fixed position of the wafer 4 is fixed to the base body as shown in Ejection cells 2 and 3 of crystal constituent elements
In some cases, the dopant element ejection cell 5 is provided close to the dopant element ejection cell 5 .

In such a case, there would be no problem if the surface of the wafer 4 was flat, but an uneven surface is created by processing such as etching for device formation, and when a molecular beam crystal is grown on this surface, the uneven surface may cause problems. This results in shadowed areas on the surface of the wafer, and all molecular beams do not reach the wafer surface uniformly.

The portions where the dopant elements do not reach have high resistance due to the small amount of dopant elements present, which causes a decrease in the yield of semiconductor devices.

(d) Object of the Invention The object of the present invention is to provide a molecular beam crystal growth method capable of growing a semiconductor crystal with a uniform impurity distribution even if the surface of a semiconductor wafer is uneven.

(e) Structure of the Invention An object of the present invention is to add impurities to a molecular beam material, evaporate the molecular beam material, irradiate the substrate with the molecular beam, and thereby apply a predetermined amount of the molecular beam onto the substrate. This can be achieved by forming a grown film containing impurities.

(f) Embodiments of the Invention Figures 3 and 4 show wafers with a heterojunction structure that are mesa-etched to expose the interface between GaAs and N ^- AlGaAs. This is an example in which the crystal structure is formed using a molecular beam crystal growth method.

In the figure, N-type AlGaAs 9 is epitaxially grown and bonded onto a high-resistance GaAs wafer 8, but since the forbidden band widths of the two are different, a two-dimensional electron gas 11 is generated near this interface 10. Existing.

It is necessary to mesa-etch such a wafer and form a low-resistance N ⁺ -GaAs layer 12 using molecular beams in order to establish ohmic contact from the exposed portion of this interface 10. In this case, it is necessary to grow an N ⁺ -GaAs layer with uniform properties at the mesa-etched device formation locations.

However, as shown in FIG. 2, the dopant ejection cell 5 is located apart from the other ejection cells 2 and 3,
Furthermore, if the slope of the mesa-etched wafer 8 is large, a shadowed area is created that is difficult for the molecular beam to reach. Third
The figure shows this in an exaggerated manner. Because the irradiation angles and directions of the Ga and As molecular beams 13 and the dopant tin (Sn) molecular beams 14 are different, even if the wafer 8 is rotating, N ⁺ -GaAs The shaded portion 16 of the layer 12 has a non-uniform Sn concentration distribution and is partially high in resistance.

In the present invention, a dopant is mixed at a certain ratio in a Ga ejection cell, and in this example where N ⁺ -GaAs is to be obtained, the molar ratio of Sn to Ga is 10 ⁵ :1. mixed with.

Here, the Ga ejection cell was heated to 1050°C and the As ejection cell was heated to 330°C to generate respective molecular beams and form an N ⁺ -GaAs layer at a rate of 1 μm/hour.

In this case, the GaAs wafer 8 was heated to 560° C. and rotated at a speed of 5γpm.

Figure 4 shows molecular beam 15 containing Ga, As and Sn.
shows the irradiation status.

As described above, when carrying out the present invention, the molecular beam for the host crystal and the molecular beam for the dopant are generated from the same position, so even if the wafer surface has significant unevenness and a shadow is created that is difficult for the molecular beam to reach, impurities can be detected. A crystal film with uniform distribution can be obtained.

(g) Effects of the invention By carrying out the present invention, it becomes possible to perform epitaxial growth with uniform dopant distribution using the molecular beam crystallization method even on wafers that have irregularities due to processing such as mesa etching, thereby improving manufacturing yield. be able to.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram of the molecular beam crystal growth apparatus, Figure 2 is an explanatory diagram of the positional relationship between the ejection cell and the wafer, and Figure 3 is an illustration of the positional relationship between the ejection cell and the wafer.
This figure and FIG. 4 are explanatory views of the growth state of molecular beam crystals on the mesa etched plane. In the figure, 2, 3, and 5 are ejection cells, 4 is a wafer, 6, 13, 14, and 15 are molecular beams, and 8 is GaAs.
Wafer 12 is an N ⁺ -GaAs layer.

Claims (1)

[Scope of Claims] 1. A molecular beam material containing a crystal component element is introduced into an ejection cell, and the molecular beam obtained by evaporating the molecular beam material is irradiated onto the substrate to cause crystal growth on the substrate. In the molecular beam crystal growth method, impurities are added to the molecular beam material, and the substrate is irradiated with a molecular beam obtained by evaporating the impurity-added molecular beam material, thereby forming a predetermined pattern on the substrate. A molecular beam crystal growth method characterized by forming a grown film containing an amount of impurities.