JPS6214093B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming an insulating film on an InP substrate. Passivation of the InP surface by forming an insulating film on the InP substrate is
It has extremely important meaning in forming gate insulating films of MIS field effect transistors and protecting the surfaces of various electronic devices and light-emitting and light-receiving devices. The insulating film formation methods that have been reported so far include CVD
Reported methods include methods of forming Al ₂ O ₃ and SiO ₂ by a method such as a method, an electron beam evaporation method, and a sputtering method, and a method of directly anodizing a part of InP in an electrolytic solution. However, the reality is that these insulation techniques have not been able to provide materials with sufficiently good characteristics for practical use.

The present invention provides a method for forming an insulating film with extremely low density of interface states on an InP substrate, in which Al is vacuum-deposited on an InP substrate, and this is anodized until the oxidation of Al is completed. It is characterized in that it includes a step of subjecting it to heat treatment.

This will be explained in detail below using examples.

First, InP substrate (n of carrier density 3.4×10 ¹⁵ cm ^-3
After degreasing and cleaning and etching the (type InP) using commonly used methods, 800 Å of Al was vacuum deposited, and this was deposited using a mixture of tartaric acid, ethylene glycol, and water until the oxidation of the Al was completed. Anodic oxidation is performed to convert Al into Al oxide, which is an insulator. A constant current source was used for this anodization, and the anodization was carried out at a current density of 5 mA/cm ² . Next, this was heat-treated at 350° C. for 30 minutes in a N ₂ atmosphere. Using a metal mask on this insulating film
Al electrodes were deposited. Made in this way
Figure 1 shows the capacitance-voltage characteristics (hereinafter abbreviated as CV characteristics) of a MOS diode. (The measurement frequency is 5
Hz, 20Hz, 50Hz, 500Hz, 5KHz, 50KHz. ) This CV characteristic has small hysteresis, and the capacitance increases to the insulating film capacitance at low frequencies in response to an applied voltage in the opposite direction, clearly showing an inversion characteristic. In order to obtain good characteristics, it is important to know where to stop the anodic oxidation, and if the oxidation is carried out until the end of the oxidation of Al, an insulating film with a low interface state density can be obtained. It should be noted that it is impossible as a practical matter to terminate the oxidation of Al at the interface between Al and InP;
Although a very small amount of InP is oxidized after completion of the process, a sufficiently small value for the interface state density is obtained. Al
Since the oxidation rates of Al and InP are different, by using a recorder to record the change in anode voltage over time, it is possible to tell when the oxidation of Al has finished from the change in the slope of the graph. In this example, FIG. 2 is a graph showing the time change of this anode voltage. In the figure, the slope changes rapidly at the position indicated by B, indicating that the oxidation of Al has ended at this point. 1st
In the MOS diode according to this embodiment whose CV characteristics are shown in the figure, anodic oxidation is stopped at the position indicated by C in FIG. In the sample in which the anodic oxidation was stopped at the position indicated by A in Figure 2, that is, in the sample in which the anodic oxidation was stopped before the completion of the oxidation of Al, the structure had a metal sandwiched between the insulating film and the semiconductor. Therefore, the capacitance of the diode's CV characteristic does not change. In addition, in a sample in which anodic oxidation was stopped at the position indicated by D in Figure 2, that is, in a sample in which InP was continued to be oxidized even after Al oxidation was completed, the hysteresis of the CV characteristics of the diode was large, the interface state density became large, and the oxidation The insulation resistance of the membrane is lowered.

In addition to the above-mentioned method of recording the change in anode voltage over time, optical detection can also be used as a method of determining the end of Al oxidation. This is because, due to the difference in reflectance between the surfaces of Al and InP, no interference color is observed with the Al oxide film, but interference colors are observed with the InP oxide film. Therefore, when oxidation is performed and interference colors begin to appear, it can be seen that the oxidation of Al has been completed. In this way, the end point of Al oxidation can be determined relatively easily.

The essence of the present invention is to anodize Al deposited on InP at a low temperature just to the interface.
In addition to the anodic oxidation method described in this example,
Other techniques are also possible, such as anodization in plasma oxygen.

As can be seen from the fact that the method of forming an insulating film on an InP substrate according to the present invention has an inversion characteristic, the interface state density is extremely small, and it is suitable for gate insulating films of MOS type field effect transistors and the surfaces of various other devices. It can be used for protection, and its industrial value is extremely large.

[Brief explanation of the drawing]

FIG. 1 shows a method for forming an insulating film according to the present invention.
It is a graph of CV characteristics of an InPMOS diode.
FIG. 2 is a graph of changes in anode voltage over time in anodization shown in Examples.

Claims (1)

[Claims] 1 InP is vacuum-deposited with Al and anodized, including the step of anodizing and heat treatment until the oxidation of Al is completed.
A method of forming an insulating film on a substrate.