CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. Section 119(e) of the following co-pending and commonly-assigned application:

U.S. Provisional Application Serial No. 62/721,145, filed on August 22, 2018, by Abdullah Almogbel, Burhan Saifaddin, Chris Zollner, Michael Iza, Hamad Albraithen, Ahmed A!yamani, Abdu!rahman Albadri, Steven P. Denbaars, Shuji Nakamura and James S. Speck, entitled“METHOD FOR FABRICATING CONDUCTIVE NITRIDE LAYERS,” attorneys’ docket number G&C 30794.0689USP1 (UC 2018-766-1);

which application is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention.

This invention relates to a method for use in the preparation of improved quality semiconductor devices by fabricating semiconductor nitride layers at a temperature less than 1300 °C in order to improve the quality and the electrical conductivity of the semiconductor devices.

2. Description of the Related Art.

The usefulness of Ill-nitride lay ers, such as Gallium Nitride (GaN), and its ternary and quaternary compounds incorporating Aluminum and Indium (AlGaN, InGaN, AlInGaN), has been well established for the fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices.

Additionally, the development of AlGaN for short wavelength devices has enabled 111-nitride based light emitting diodes (LEDs) and laser diodes (LDs) to overtake many other research ventures. Consequently, AlGaN based materials and devices have become the dominant material system used for ultraviolet light semiconductor applications.

However, the growth of high quality AlGaN with high aluminum content remains a challenge. This challenge arises due to lack of a suitable substrate, which leads to buildup of strain energy during the deposition of aluminum- containing Ill-nitride layers. The increase in strain energy of the deposited layers can lead to poor material quality, such as film cracking and increased dislocation density .

SUMMARY OF THE INVENTION

The present invention discloses a method for fabricating a highly conductive semiconductor epitaxial film by depositing and doping an AlGaN film at a growth temperature less than 1300 °C. In one embodiment, the AlGaN has an aluminum content >= 50%.

The method also comprises heating an underlying substrate to a temperature greater than 1000 °C before starting the depositing and doping, while exposing the underlying substrate to an atmosphere that contains some ammonia (NFL·)

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings:

FIG. 1 is a flowchart of process steps used in one embodiment of the present invention.

FI G. 2 is a graph of Hall resistivity vs. gas density for disilane (Suile) and trimethyaluminum (TMA1) precursors for growing 63% Silicon-doped AlGaN.

FIG. 3 is a graph of Hall resistivity vs. Si flow rate illustrating the effect of Indium on growing 63% Si-doped AlGaN.

FIG. 4 is a graph of Hall resistivity vs. growth rate for growing 63% Silicon- doped AlGaN. FIG. 5 is a graph of Current (A) vs. Voltage (V) illustrating Transmission Line Measurements of n-type 66% AlGaN deposited using the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment m which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

Overview

The present invention illustrates a method to fabricate conductive AlGaN layers by controlling the AlGaN reaction temperature. Intrinsic AlGaN is highly resistive due to a low free carrier concentration. Therefore, a doping agent might be used to increase the free carrier concentration and convert the AlGaN into conductive material.

The present invention also describes a method for treating a semiconductor substrate by exposing the substrate to ammonia at a high temperature as a means of improving the characteristics of IP-nitride layers or films subsequently fabricated on or above the substrate.

Technical Description

The present invention deals with modifying AlGaN material to become a conductive material by setting the optimal reaction conditions. A doping agent is added to the AlGaN during reaction to yield a high free carrier concentration that can actively contribute to the AlGaN conductivity. The concentration of dopants is kept under certain limits to avoid over-doping the AlGaN. At the same time, the reaction temperature is set to maximize the conductivity of the AlGaN and improve the reliability and repeatability of the process.

Excessive high reaction temperature might improve the crystal quality and reduce the impurities contamination; however, it sharply deteriorates the AlGaN’ s conductivity and significantly narrows the optimal reaction window; and it also reduces the growth efficiency due to desorption of the Gallium atoms at high temperatures. Therefore, the growth reaction conditions have to be engineered to maximize the conductivity, preserve the quality and improve the reliability of the conductive AlGaN material.

The present invention also deals with treating the substrate before depositing the AlGaN by exposing the substrate to ammonia at a high temperature. Thereafter, semiconductor nitride layers are grown on or above the treated substrate, including the AlGaN material.

Process Steps

FIG. 1 illustrates generally a method for fabricating a semiconductor device, and more specifically, a method for treating a semiconductor substrate, wherein the method comprises the following steps:

(1) Loading a substrate into a reactor. The substrate may comprise a Sapphire, Silicon Carbide, AIN, or GaN substrate. In addition, the substrate may have a growth surface oriented as a c-plane, nonpolar plane, or semipolar plane.

(2) Turmng on the heater of the reactor and ramping the temperature in the reactor to a set point temperature of at least about 1000 °C to about 2000 °C, preferably at least about 1200 °C, and more preferably at least about 1250 °C, while nitrogen (N?.) and/or hydrogen (Hz) flow over the substrate.

(3) After the set point temperature is achieved in the reactor, exposing the substrate to an atmosphere that contains at least some ammonia (Nil·,), in addition to the nitrogen and/or hydrogen, at the set point temperature. The semiconductor nitride layers subsequently deposited on or above the substrate have an enhanced material quality as compared to the semiconductor nitride layers deposited on or above a substrate that has not been so treated.

(4) After a desired time period is achieved in (3), growing one or more initial layers on or above the substrate. Specifically, turning on the flow of trimethyaluminum (TMA1) for the growth of at least one Aluminum Nitride (AIN) layer on or above the substrate.

(5) After the AIN layer reaches a desired thickness, shutting off the flow' of TMA1 and cooling the reactor down to a set point temperature of less than about 1300 °C, preferably about 1175 °C or lower, or more preferably about 1050 °C or lower, while flowing ammonia to preserve the planarity of the AIN layer.

(6) After the temperature in the reactor reaches the set point temperature, turning on the flow of trimethygallium (TMGa) and disilane (SbHe) gas, in addition to the flow of trimethyaluminum, for the depositing of one or more semiconductor nitride layers containing at least some Aluminum on or above the substrate at a growth temperature less than about 1300 °C, while doping the semiconductor nitride layers. In addition, an Indium precursor may be flowed during the depositing of the semiconductor nitride layers.

In one embodiment, the semiconductor nitride layers comprise at least one AlGaN layer deposited on or above the AIN layer, wherein the AlGaN has an Aluminum content greater than or equal to about 50%, i.e., AlxGai-xN where x >= 0.50, and preferably of about 63% where x = ^: 0.63. In one embodiment, the A!xGai-xN layers have a Hall resistivity less than about 50 hiW-em, and more preferably, the AlxGai-xN layers have a Hall resistivity less than about 30 hiW-em. In one embodiment, the AlxGai-xN layers have a contact resistivity' less than about 10 ⁴ ohm. cm ² . In one embodiment, the AlxGai- xN layers have a sheet resistance less than about 170 W/sq. In one embodiment, the AlGaN layer is doped with Silicon (Si) at a concentration greater than or equal to about 10 ¹⁶ /cm’. However, in other embodiments, the semiconductor nitride layers may be doped with Germanium (Ge), Carbon (C) and/or Oxygen (O), as well as Silicon.

In one embodiment, the growth temperature is the temperature at the substrate’s surface, the growth temperature is less than about 1300 °C, and the semiconductor nitride layers have a higher doping level and higher conductivity than the semiconductor nitride layers deposited at a growth temperature of about 1300 °C or more. In another embodiment, the growth temperature is about 1175 °C or less, and the semiconductor nitride layers deposited at the growth temperature of about 1 175 °C or less have a higher doping level and higher conductivity than the semiconductor nitride layers deposited at a growth temperature of more than about 1 175 °C. In still another embodiment, the growth temperature is about 1050 °C or less, and the semiconductor nitride layers deposited at the growth temperature of about 1050 °C or less have a higher doping level and higher conductivity than the semiconductor nitride layers deposited at a growth temperature of more than about 1050 °C.

In one embodiment, the semiconductor nitride layers have a surface oriented as a c-plane, nonpolar plane, or sermpolar plane.

(7) After a desired time period is achieved in (6), depositing subsequent layers on or above the semiconductor nitride layers.

(8) After the subsequent layers are deposited, turning off the heater and the flow of ammonia, trimethyaluminum, trimethygallium, disilane, hydrogen and nitrogen.

(9) The end result of the method comprises a semiconductor device including one or more initial layers, semiconductor nitride layers and subsequent layers, fabricated using the above steps. The semiconductor nitride layers deposited on or above the substrate may be comprised of a nitride alloy which contains some aluminum such as AIN, AlInN, AlGaN or AlInGaN. Moreover, the semiconductor nitride layers deposited on or above the substrate may comprise multiple layers having varying or graded compositions. In addition, the semiconductor nitride layers deposited on or above the substrate may comprise a heterostructure containing layers of dissimilar or similar (Al,Ga,in,B)N compositions. Also, the semiconductor nitride layers deposited on or above the substrate may be comprised of various thicknesses.

Experimental Data

FIG. 2 is a graph of Hall resistivity (mQ.cm) vs. gas density for disilane (SEHe) and trimethyalummum (TMAl) precursors, for 63% Si-doped AJGaN (i.e.,

Alo.53Gao.37N: Si), as a function of the AlGaN growth temperature, showing that a higher temperature of 1175 °C results in a higher Hail resistivity, lower Si doping and lower conductivity as compared to a lower temperature of 1050 °C, which results in a lower Hall resistivity, higher Si doping and higher conductivity, across different disilane / TMAl gas densities.

FIG. 3 is a graph of Hall resistivity (mQ.cm) vs. Si flow rate (seem), illustrating the effect of Indium on 63% Si-doped AlGaN, at a temperature of 1050 °C, showing that “without Indium” results in a higher Hall resistivity as compared to“with Indium” that results in a lower Hall resistivity, across different Si flo w rates.

FIG 4 is a graph of Hall resistivity (mQ.cm) vs. growth rate (A/s) that plots Hall resistivity measurements as a function of the growth rate for 63% Si-doped AlGaN.

FIG 5 is a graph of Current (A) vs. Voltage (V) illustrating Transmission Line Measurements (TLMs) of n-type 66% AlGaN deposited using the present invention, and showing a contact resistivity of less than I0 ^~4 O.ciri ² and a sheet resistance of less than 170 W/sq, wherein the pads used for the TLMs have a uniform area of 94 urn x 202 pm. With this invention, the contact resistivity for n-type 66% AlGaN dropped to below 10 ⁴ O.cm ² , which reduces the resistance at the metal/semiconductor interface in the n-contact (area = 0.04 mm ² ) of an UV LED to less than 0.25 W. The sheet resistance of the 66% n- AlGaN was 169.8 W/sq, significantly reduces the heat generation at higher current operation and increases the lifetime of the UV LED.

Advantages and Improvements

AlGaN is a highly resistive material at high Aluminum compositions, and to convert AlGaN from a resistive to a conductive material is a nontrivial task. Certain growth conditions have to be met in order to produce conductive AlGaN with a high Aluminum composition. The present invention illustrates a method that simplifies this task and improves the conductivity of the AlGaN material

In addition, the present in vention reduces the resistance at the

metal/semiconductor interface in the n-contaet of an UV LED, and the sheet resistance of the 66% n- AlGaN significantly reduces the heat generation at higher current operation and increases the lifetime of the UV LED

The present invention also discloses the treatment conditions for a substrate needed to ensure a crack-free epi layer surface of the semiconductor nitride layers grown on or above the substrate.

Nomenclature

This invention relates to the fabrication of devices using semiconductor nitride layers. The semiconductor nitride layers may be comprised of Gallium (Ga), Aluminum (AG), Indium (In) and/or Boron (B), combined with Nitrogen (N). As used herein, the term“nitride” or“Ill-nitride,” refers to any alloy composition of the (Ga,Al,In,B)N semiconductors having the formula Ga _w AlxIn _y B _z N where:

0 < w < 1, 0 < X < 1, 0 < y < 1, 0 < z < 1, and w + x + y + z = 1.

The underlying substrate may be comprised of Sapphire (AI2O3), Silicon Carbide

(SiC), Aluminum Nitride (AIN), or Gallium Nitride (GaN). The underlying substrate may- have a growth surface oriented as a conventional c-plane, or a nonpolar plane such as a- plane or m-plane, or any semipoiar plane such as {20-21 } , or { 11 -22} , or {10- 11 } . The underlying substrate and IP-nitride layers may have a Ga, Al, In, B or N terminated surface. When SiC is used, the underlying substrate may have a Silicon (Si) or Carbon (C) terminated surface.

The semiconductor nitride layers may be comprised of a single or multiple layers having varying thicknesses or graded compositions, including layers of dissimilar (Al,Ga,In,B)N composition. The semiconductor nitride layers may be grown m any crystallographic direction such as on a conventional polar c-p!ane or on a nonpolar plane, such as an a-plane or m-plane, or on any semipoiar plane, such as {20-21 }, {11-22} or {10-1 1 } . Moreover, the semiconductor nitride layers may also be doped with elements such as silicon (Si), germanium (Ge), magnesium (Mg), boron (B), iron (Fe), oxygen (O), and zinc (Zn).

The semiconductor nitride layers may be grow'n using deposition methods comprising metalorganic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE) or molecular beam epitaxy (MBE).

Conclusion

This concludes the description of the preferred embodiment of the present invention. The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.