RU2582440C1 - SEMICONDUCTOR TRANSISTOR NANO-HETEROSTRUCTURE ON SUBSTRATE OF GaAs WITH MODIFIED STOP LAYER OF AlxGa1-xAs - Google Patents

Abstract

FIELD: electronic equipment.

SUBSTANCE: invention relates to electronic engineering and can be applied for making monolithic integrated circuits operating in centimetre and millimetre wavelength ranges. According to invention, semiconductor transistor heterostructure is proposed on GaAs substrate with modified Al_xGa_1-xAs stop-layer. Modified Al_xGa_1-xAs stop-layer is grown with gradient of aluminium molar ratio from x = 0.23 to x₀ = 0.26÷0.30 and with thickness of 1÷10 nm.

EFFECT: invention provides accurate control of etching depth by means of stop-layer introduction, which must not have any negative effect on parameters of field-effect transistor; negative effect can be manifested as reduction of breakdown voltage and operating currents, bad transistor control, high resistance of ohmic contacts.

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Description

Technical field

The invention relates to electronic equipment and can be used for the manufacture of monolithic integrated circuits operating in the centimeter and millimeter wavelength ranges.

State of the art

In order to etch recesses (depressions) in heterostructures for the further formation of electrodes in them, a stop layer is used - an epitaxial layer in the heterostructure, the etching rate of which with a certain etchant is significantly (100-1000 times) less than the etching rate of overlying layers that differ from the feet -layer chemical composition. Particularly important is the question of choosing a stop layer when developing the design of heterostructures for field effect transistors, since modern transistor heterostructures have active layers with a thickness of up to a monolayer and the accuracy of the etching depth becomes one of the key parameters in the shutter manufacturing technology.

In transistor heterostructures on a GaAs substrate, to form a gate gate recess, it is necessary to etch the n ⁺ -GaAs contact layer. For this purpose, phosphorus-containing stop layers AlGaP [1], InAlP [1], InGaP [2, 3, 4] are used. The presence of phosphorus in the stop layer allows the use of liquid etching, and the presence of aluminum - dry etching [1]. The disadvantage of such stop layers is the need for a source of phosphorus in the epitaxy unit, while all other layers of the heterostructure are grown only on the basis of solid solutions (In, Ga, Al) As without the use of phosphorus.

AlAs stop layers are also used [5, 6, 7, 8]. Their cultivation does not require an additional source in epitaxial installations. When using certain etchants, they have a significantly higher selectivity than stop AlGaAs layers: for example, when using the etchant C ₆ H ₈ O ₇ : H ₂ O: H ₂ O _2, the etching rates of Al _x Ga _1-x As and GaAs are 1 : 100 at x = 0.3 and 1: 1400 at x = 1 [7]. The disadvantages of AlAs stop layers are as follows: firstly, the strong oxidation of AlAs [9]; secondly, a rougher surface of the AlAs layer compared to the GaAs layer, since the migration rate of Ga atoms over the surface of the growing layer is much higher than the migration rate of A1 atoms [10]; thirdly, the AlAs stop layer located between the ohmic contacts and the channel degrades the properties of the ohmic contacts and also changes the band structure of the transistor heterostructure.

Stop layers Al _x Ga _1-x As with x≥0.3 are also used [11, 12, 13, 14]. They have the following disadvantages. In transistor heterostructures on GaAs substrates, Al _x Ga _1-x As layers with a molar fraction of aluminum x≤0.23 are doped with Si atoms. For x> 0.23, the lowest energy state for donor Si atoms is the DX centers, which become stable upon capture of two electrons. The negative effect of DX centers on the operation of field effect transistors is manifested in a decrease in breakdown voltage, the collapse of the current – voltage characteristics, and a decrease in operating currents. The introduction of individual stop layers of Al _0.30 Ga _0.70 As complicates the technology of growth of heterostructures by molecular beam epitaxy, since after growing an Al _0.23 Ga _0.77 As layer, it is necessary to interrupt the epitaxial growth process for a precise increase in Al flux in order to grow an Al _0.30 Ga stop layer _0.70 As. In addition, growing a stop layer of Al _0.30 Ga _0.70 As increases the distance between the In _y Ga _1-y As quantum well and the gate, which affects the controllability of the transistor.

Closest to the proposed heterostructure and adopted as a prototype of the present invention is the transistor heterostructure described in the patent [15]. It provides for the presence of two thin stop layers Al _x Ga _1-x As used for double etching of recesses for gate and ohmic contacts. The prototype uses homogeneous stop layers Al _x Ga _1-x As with a constant aluminum content. At x = 0.23, the disadvantage of the prototype is the relatively low selectivity of liquid etchers at the GaAs / Al _0.23 Ga _0.77 As heterointerface, which, as a result, can lead to an uncontrolled increase in the etching depth. In addition, molecular beam epitaxy technology makes it possible to grow semiconductor layers with an accuracy of up to 2% [16], which complicates the growth of the Al _x Ga _1-x As stop layer with x = 0.23. At x> 0.23, the disadvantages of the prototype include an increase in the resistance of ohmic contacts, the negative effect of DX centers on the parameters of transistors. Also, the introduction of an additional layer, which is the stop layer, increases the distance between the channel and the gate of the transistor, which affects the controllability of the transistor.

Disclosure of invention

The objective of the invention is to accurately control the depth of etching during the recess of the gate contact of the field effect transistor by introducing a stop layer, which should not adversely affect the parameters of the field effect transistor.

The technical result that allows us to complete the task is to create a transistor heterostructure structure (see Fig. 1) with a modified Al _x Ga _1-x As stop layer (7), which should have high selectivity for liquid etchers when etching the n ⁺ contact layer -GaAs (8) for the formation of a gate contact recess of a field-effect transistor, as well as to minimize the formation of defects, which will be traps for conduction electrons. By a stop layer is meant a semiconductor layer as part of a heterostructure, the etching rate of which by a certain etchant is much lower than the etching rate of overlying layers that differ in chemical composition from the stop layer. The negative effect of the stop layer on the parameters of the transistor is manifested in a decrease in breakdown voltage and operating currents, a deterioration in the controllability of the transistor, and an increase in the resistance of ohmic contacts.

The dependence of the molar fraction of aluminum x on the thickness of the heterostructure h in the region of the modified stop layer is shown in FIG. 2. The dashed line in FIG. 2 corresponds to the traditional stop layer, the solid line corresponds to the modified stop layer. Compared with the traditional stop layer of homogeneous composition Al _x Ga _1-x As described in the prototype, the modified stop layer Al _{x (h)} Ga _{1-x (h)} As has a chemical composition gradient, and the molar fraction of aluminum increases from x = 0.23 to x ₀ = 0.26 ÷ 0.30 at a thickness of h ₂ -h ₁ = 1 ÷ 10 nm.

Brief Description of the Drawings

In FIG. 1 is a schematic diagram of a semiconductor transistor heterostructure with a modified stop layer on a GaAs substrate related to the present invention.

In FIG. Figure 2 shows the profile of the molar fraction of aluminum x in the Al _0.23 Ga _0.77 As (6) barrier layer and the modified Al _x Ga _1-x As (7) stop layer of the transistor heterostructure on the GaAs substrate. The dashed line corresponds to the traditional stop layer, the solid line corresponds to the modified stop layer, x ₀ = 0.26 ÷ 0.30.

The implementation of the invention

The layers of the transistor heterostructure are grown by molecular beam epitaxy. After conducting modulated doping (bulk doping or delta doping) of an Al _0.23 Ga _0.77 As (5) layer, an Al _0.23 Ga _0.77 As (6) 2–5 nm thick barrier layer is grown, taking into account the diffusion length and segregation of donor atoms to the heterostructure surface . The next layer is a modified stop layer Al _x Ga _1-x As (7) with a gradient of chemical composition in the direction of growth. The initial aluminum content in this layer is x = 0.23, and the aluminum content at the end of the layer growth is x ₀ = 0.26 ÷ 0.30, which is determined by the accuracy requirements for the recess depth for the chosen etchant. The upper layer of the transistor heterostructure is the body-doped n ⁺ -GaAs contact layer (8).

Information sources

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Claims (1)

A semiconductor transistor heterostructure on a GaAs substrate with a modified Al _x Ga _1-x As stop layer, characterized in that the modified Al _x Ga _1-x As stop layer is grown with a molar fraction of aluminum from x = 0.23 to x ₀ = 0.26 ÷ 0.30 and with a thickness of 1 ÷ 10 nm.

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