WO2000068470A1 - Magnesium-doped iii-v nitrides & methods - Google Patents

Abstract

Magnesium-doped high quality III-V nitride layers and methods for making the same. A p-type gallium nitride, indium nitride or aluminum nitride layer (12') may be produced on a sapphire substrate (5) by a hydride vapor-phase epitaxy (HVPE) process using a metal supply mixture which includes magnesium and a group III metal (Ga, In, Al) (11). The gallium nitride, indium nitride or aluminum nitride layer may be removed from the sapphire substrate to provide a Mg-dope III-V nitride substrate having low dislocation densities and being suitable for use in fabrication of, e.g. light-emitting devices.

Description

MAGNESIUM-DOPED III-V NITRIDES & METHODS

For

Tetsuzo Ueda, Glenn S. Solomon and David J. Miller

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to magnesium-doped metal

nitrides. The invention also relates to methods for growing

magnesium-doped p-type III-V nitrides. The invention further

relates to magnesium-doped group III metal nitride substrates

grown by HVPE.

2. Background of the Related Art

Due to the nature of their band-gaps, III-V nitrides (nitrides

of gallium, indium, and aluminum, and their alloys) show much

promise in fabrication of light emitting devices for short

wavelengths of the visible spectrum. For example, gallium

nitride (GaN) is currently used in the manufacture of blue

light emitting diodes, while nitride blue-violet lasers have

been demonstrated as prototypes. By the term "III nitrides" is

meant compounds consisting of one or more group III metals

(including aluminum, gallium, and indium) as an electropositive atom ligated by nitrogen atoms as some or all of the

electronegative ligands (other electronegative ligands include

phosphorus, arsenic or antimony). A typical formula for a

group III metal nitride is Ga_]___x_yAl_xInyN_]___a_ι_D__cP_aAs]_DSb_c

[0<(x,x,a,b,c)< 1] .

However, the growth of low resistive p-type GaN (p-GaN) has

proven to be problematic, using prior art methods and

materials. Even magnesium having the shallowest acceptor level

doped into GaN has resulted in highly resistive material.

During the past decade, post annealing and electron beam

irradiation techniques have enabled the production of magnesium

doped p-GaN of relatively low resistivity, by desorbing

hydrogen atoms from the doped magnesium. However, the carrier

concentration level is still around 10¹⁸ cm^"3, and ohmic contact

resistance is not low enough to even match the level of

conventional III-V semiconductor (e.g. GaAs) devices. This

high ohmic contact resistance results in high series resistance

of a pn junction-based light emitting diode or laser diode.

Thus, low voltage operation of these devices has been very

difficult. Particularly in the case of laser diodes, this

leads to higher operating current and shorter lifetime of

operation. According to prior art methods, p-type GaN has typically been

grown by metal organic chemical vapor deposition (MOCVD) on

sapphire substrates, in which bis-cyclopentadienylmagnesium

(Cp₂Mg) acts as a source of magnesium dopant. Since the

magnesium dopant in the resultant p-GaN is passivated with

hydrogen atoms, post-annealing of the p-GaN in a nitrogen gas

atmosphere is necessary to desorb the passivating hydrogen in

order to achieve a carrier concentration of about 10¹⁸ cm^"3. A

further drawback to prior art MOCVD techniques is that carbon

atoms from the metal organic sources may be incorporated in the

p-GaN film, resulting in carbon related deep level tending to

decrease the p-type carrier concentration. In addition, prior

art p-GaN layers grown by MOCVD have a high dislocation density

(about 10⁹ cm^"2 on sapphire substrates) . Furthermore, prior art

MOCVD systems are complicated and expensive, due in part to the

complex gas handling system, the high costs associated with

metal-organics , and the use of a costly fixed temperature bath

for the metal-organic source.

Another prior art technique used for growing p-GaN is hydride

vapor phase epitaxy (HVPE) . HVPE has certain advantages over

MOCVD, in that lower dislocation densities (about 10⁷ cm^"2) can be achieved using a relatively simple, low cost system by

"bulk- like growth" with high growth rates. The lower

dislocation density enables more reliable and higher

performance devices to be fabricated, such as low threshold

current laser diodes having a longer lifetime. Another

advantage of HVPE is the absence of carbon in the source

materials, with the result that higher activation efficiency of

the dopants is expected, especially in the case of p-GaN.

The present invention provides an improved HVPE system for

growing magnesium-doped p-type III-V nitrides, in a more cost-

effective manner and with simpler equipment, as compared with

prior art apparatus and methods .

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a

simple and cost-effective method for growing Mg-doped p-type

III-V nitride layers or substrates . A first reagent gas

component may be provided by passing HCl source gas over a

mixture of a group III metal (gallium, Ga; indium, In; or

aluminum, Al) and magnesium (Mg) . A metallic mixture of this

type is referred to as a group III/Mg metal mixture. The

resultant reagent gas (e.g., GaCl) reacts with ammonia in a

HVPE system to form a p-type III-V nitride layer, the latter

deposited on a suitable substrate. Using this method, p-GaN

layers can be formed which have lower dislocation densities and

less incorporated carbon atoms, as compared with layers formed

using prior art methods. Lower dislocation densities are

expected to lead to higher activation efficiency of the

magnesium dopant. Furthermore, using the technique of the

invention, p-GaN substrates can be obtained by removing the p-

GaN layer after HVPE growth.

Although hydrogen or nitrogen may be used as carrier gas in the

practice of the invention, according to a currently preferred

embodiment, nitrogen is used as carrier gas. The rationale for

avoiding hydrogen as carrier gas is that hydrogen atoms from the hydrogen carrier gas may passivate magnesium in the grown

III-V nitride film, thereby resulting in a lower p-type carrier

concentration of the nitride layer. Minimizing the amount of

hydrogen gas in the HVPE reactor enables higher p-type carrier

concentration of the III-V nitride layer. This is advantageous

for III-V nitrides used in light emitting devices.

According to a preferred embodiment of the invention, the

temperature of the group III/Mg metal mixture is greater than

661 °C.

One feature of the invention is that it provides a method for

forming a Mg-doped p-type group III metal nitride layer by

hydride vapor-phase epitaxy. Another feature of the invention

is that it provides a Mg-doped p-type group III metal nitride

layer, in which the Mg dopant is derived by passing HCl over

elemental Mg.

One advantage of the invention is that it provides a simple and

cost-effective method for forming a Mg-doped p-type group III

metal nitride layer. Another advantage of the invention is

that it provides a method for forming a p-type nitride layer in

which hydrogen atom passivation of the magnesium dopant is avoided .

These and other objects, advantages and features are

accomplished by the provision of a method of making a p-type

nitride layer, including the steps of: a) providing a HVPE

system including a reactor; b) arranging a substrate in the

reactor; c) passing HCl over a metal mixture to provide a first

reagent gas component, the metal mixture including magnesium

metal; d) introducing ammonia and the first reagent gas

component into the reactor; and e) growing a magnesium-doped p-

type nitride layer on the substrate.

These and other objects, advantages and features are

accomplished by the provision of a p-type nitride layer grown

on a substrate by HVPE, the p-type nitride layer including: a

group III nitride doped with magnesium, the p-type nitride

layer formed by reacting a first reagent gas component with

ammonia, the first reagent gas component prepared by passing

HCl over a group III metal and magnesium metal .

These and other objects, advantages and features of the invention

will be set forth in part in the description which follows and in

part will become apparent to those having ordinary skill in the art upon examination of the following, or may be learned from

practice of the invention. The advantages of the invention may

be realized and attained as particularly pointed out in the

appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 schematically represents an MOCVD growth system of the

prior art;

Fig. 2 schematically represents a HVPE growth system of the

prior art;

Fig. 3A schematically represents a HVPE system suitable for

growing Mg-doped p-type nitride layers, according to one

embodiment of the invention;

Fig. 3B schematically represents a HVPE system suitable for

growing Mg-doped p-type nitride layers, according to

another embodiment of the invention; and

Fig. 4 schematically represents a series of steps involved in a

method of making a p-type metal nitride layer, according

to another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For purposes of illustration, the invention will be described

with particular emphasis on the HVPE growth of p-type GaN.

However, the invention is also applicable to the deposition of

other III-V nitrides by HVPE.

Referring now to the drawings, FIG. 1 schematically represents

an MOCVD epitaxial growth system 20 of the prior art. System

20 includes furnace coils 22 situated around a reactor or

growth tube 24, and a reactor inlet 26. A substrate 5, e.g.,

sapphire, is arranged within reactor 24. Reagent and carrier

gases are supplied to reactor 24 via a complex arrangement of

tubing, as follows. Gallium is supplied from a gallium

containing organo-metallic compound 7, such as trimethylgallium

(TMGa) , present in a first bubbler 28a, using a carrier gas

such as hydrogen 3. Ammonia 2 is supplied as a reagent gas via

a lower sub-inlet 26b. Hydrogen 3 is also used as a carrier

gas for a magnesium containing compound 8 , such as bis-

cyclopentadienylmagnesium (Cp2Mg), contained in a second

bubbler 28b. Compound 8 provides the Mg required for magnesium

doping. It is also noted that hydrogen 3 is also supplied to

reactor 24 via an upper sub-inlet 26a. Mass flow meters are

used to control the gas flow rates. As a result of supplying TMGa 7, ammonia 2 , and Cp₂Mg 8 to reactor 24, p-GaN 12 is grown

as a wafer on substrate 5. Subsequently, the wafer is annealed

in a nitrogen atmosphere to desorb the hydrogen atoms from the

doped magnesium. The resultant hole concentration in the p-GaN

layer (12) is up to 10¹⁸ cm^"3.

Another prior art method for forming p-type III-V nitride

layers is Mg-doping using a HVPE system. Fig. 2 schematically

represents a HVPE growth system 30 of the prior art. Briefly,

system 30 includes a first furnace 32a surrounding a reactor or

growth tube 34. Reactor 34 has first and second reactor

inlets, 36a, 36b, respectively, and a production chamber 35.

Production chamber 35 houses a supply of liquid group III metal

9 (Ga, In, or Al, or alloys thereof) . Reagent gas (ammonia 2)

and carrier gas (hydrogen 3) are supplied to reactor 34 via

second inlet 36b. HCl (precursor or source gas) 4 is supplied

via first inlet 36a to chamber 35, where HCl 4 reacts with

metal 9 to form reagent gas, such as GaCl .

In system 30, a source of magnesium dopant is in the form of

magnesium metal 10 housed within a dopant chamber 38. Dopant

chamber 38 and magnesium 10 are heated by a second furnace 32b.

System 30 uses a separate furnace for magnesium 10; therefore the temperature of dopant chamber 38, on the one hand, and the

temperature of reactor 34 and production chamber 35, on the

other hand, can be controlled independently. However, as a

result of this arrangement, system 30 becomes more complicated

as well as more costly.

In view of the above, it can be readily appreciated that an

improved system and method for HVPE growth of P-type III-V

nitride layers is required. Fig. 3A schematically represents a

HVPE system 40 suitable for growing Mg-doped p-type nitride

layers, according to one embodiment of the invention. System

40 includes a furnace 42 surrounding a reactor 44 having first

and second inlets 46a, 46b, respectively. First inlet 46a

leads to production chamber 48. A substrate 5, such as

sapphire, is arranged within reactor 44.

Production chamber 48 houses a supply of a group III metal (Ga,

In, or Al) together with elemental (metallic) magnesium, but

may also be a chloride of magnesium. Preferably, the group III

metal and the magnesium source are combined to form a group

III/Mg metal mixture 11. The magnesium component of mixture 11

serves as the source of the Mg dopant in system 40. Preferably

the magnesium is a relatively minor component of mixture 11; more preferably the magnesium component of mixture 11 is in the

range of 10 pp to 10,000 ppm. Mixture 11 is heated by furnace

42 to a temperature in the range of from 500 to 1000 °C; more

preferably in the range of from 600 to 900 °C; and most

preferably to a temperature of from 650 to 750 °C.

HCl 4 is introduced into chamber 48 via first inlet 46a, where

the HCl reacts with mixture 11 to provide a first reagent gas

component which is carried into reactor 44. Preferably, the

first reagent gas component is composed primarily of a chloride

of Ga, In, or Al , such as GaCl, with lesser amounts of Mg .

According to the embodiment represented in Fig. 3A, a second

reagent gas component, ammonia 2, is supplied to reactor 44 via

second inlet 46b using hydrogen as carrier gas. The first and

second reagent gas components react in reactor 44 to form a p-

type nitride layer 12', e.g., of GaN, which is deposited on

substrate 5.

After the growth of layer 12' as a relatively thick film, e.g.,

as a film of p-GaN, layer 12 ' may be removed from sapphire

substrate 5 to provide a p-GaN substrate. Layer 12' may be

grown to a thickness in the range of from 5.0 micron to 500

micron; more preferably to a thickness of 100 micron. According to the invention, system 40 may be used for the cost-

effective production of Mg-doped p-type group III nitride

(e.g., p-GaN) layers. Such layers are formed in the absence of

organic compounds, so that there is no carbon incorporated into

the nitride layer. Carbon related deep level traps reduce the

carrier concentration. Thus, the absence of carbon represents

a significant advantage over prior art MOCVD techniques, in

that it allows for higher carrier concentration.

Fig. 3B schematically represents a HVPE system 40' suitable for

growing Mg-doped p-type III-V nitride layers, according to

another embodiment of the invention. System 40' is

substantially similar to system 40, described hereinabove with

reference to Fig. 3A. However, in system 40" nitrogen gas 6 is

used as a carrier gas for ammonia 2 at inlet 46b, instead of

hydrogen 3 (Fig. 3A) . By using nitrogen 6 as a carrier gas to

the exclusion of hydrogen 3, passivation of magnesium atoms in

p-GaN film 12'' by hydrogen atoms is greatly decreased. Thus,

in the absence of hydrogen 3 carrier gas, passivation is

limited to that caused by relatively trace quantities of

hydrogen produced during decomposition of ammonia and HCl. The

resultant p-type film 12 ' ' shows lower resistivity than films 12, 12' grown according to systems and methods which use

hydrogen as carrier gas (Figs. 1, 2, 3A) .

Fig. 4 schematically represents a series of steps involved in a

method of making a Mg-doped p-type metal nitride layer,

according to another embodiment of the invention, in which step

50 involves providing a HVPE system. A HVPE system provided in

step 50 may be, for example, either of the systems 40, 40'

described hereinabove with reference to Figs. 3A and 3B,

respectively. Step 52 involves arranging a substrate in the

reactor of the HVPE system. The substrate arranged in the

reactor in step 52 is preferably a sapphire substrate.

Step 54 involves passing a source gas including HCl over liquid

metal. The liquid metal over which the HCl is passed in step

54 includes magnesium or a magnesium source (such as a

magnesium chloride) , and a group III metal selected from the

group consisting of Ga, In, Al , and alloys of Ga, In, and Al .

According to a currently preferred embodiment, elemental

(metal) magnesium and the group III metal are combined to form

a group III/Mg metal mixture, and the HCl is passed over the

group III/Mg metal mixture. Typically, the magnesium is

present in the group III/Mg metal mixture in relatively trace amounts, e.g., 100 pmm, but may be in any range between 1 ppb

(parts-per-billion) and to 10,000 ppm (parts per million).

Preferably, the group III/Mg metal mixture is heated to a

temperature in the range of 650 °C to 900 °C. Step 54 results

in the formation of a first reagent gas. This first reagent

gas component includes magnesium and a chloride of a group III

metal, e.g., GaCl or InCl .

Step 56 involves introducing reagent gases into the HVPE

reactor. Reagent gases introduced into the reactor include the

first reagent gas component formed as a result of step 54, and

a second reagent gas component. Preferably, the second reagent

gas component is ammonia. The second reagent gas component is

introduced into the reactor by means of a carrier gas. The

carrier gas for introduction of ammonia is preferably nitrogen,

although other gases such as hydrogen may also be used. As

mentioned hereinabove, nitrogen is preferred over hydrogen as

carrier gas, since hydrogen gas leads to passivation of Mg

dopant in the nitride layer.

Step 58 involves growing the group III nitride layer on the

substrate. The group III nitride layer grown in step 58

results from the vapor phase reaction between ammonia gas and the first reagent gas component. As an example, the group III

nitride layer may be a Mg-doped GaN layer formed by reaction of

ammonia with GaCl in the presence of minor quantities of

magnesium. Step 58 may involve growing the group III nitride

layer to a thickness of up to 300 micron or more. The desired

thickness of the group III nitride layer grown in step 58 will

depend on factors such as the intended application of the

layer. After the group III nitride layer has been grown to the

desired thickness, it may be removed from the sapphire

substrate in step 60. For example, by polishing the backside

of the structure, the sapphire substrate may be removed.

The method of Fig. 4 provides a Mg-doped p-type group III

nitride substrate having low dislocation density and high

carrier concentration. The preceding steps described with

reference to Fig. 4 provide a relatively simple and cost-

effective method for forming Mg-doped p-type group III

nitrides. No organic source materials are used in the method

of Fig. 4. Therefore, as compared with similar materials

formed by MOCVD, higher activation efficiency of the dopant is

expected, especially in the case of p-GaN. It is preferable that the temperature of the group-III/Mg

mixture 11 of systems 40 and 40* be maintained at a temperature

above about 660.45 °C. The rationale for stipulating this

temperature is as follows. The commonly used group-Ill metals

for III-V nitride semiconductors are gallium, aluminum, and

indium, having melting points of 29.8 °C, 660.45 °C, 156.6 °C,

respectively. Since the melting point of magnesium is 650 °C,

the common group-III/Mg mixture 11 is in the liquid phase at a

temperature above 660.45 °C. At this temperature (660.45 °C) ,

sufficient magnesium can be incorporated in film 12 '/12' ' to

achieve low resistive p-type III-V nitrides, because no

undesirable alloys (such as Mg₃Ga₂) is present in the group-

III/Mg metal supply.

The foregoing embodiments are merely exemplary and are not to be

construed as limiting the present invention. The present

teaching may be applied to other types of apparatuses and

methods . The description of the present invention is intended to

be illustrative, and not to limit the scope of the appended

claims. Many alternatives, modifications, and variations will be

apparent to those skilled in the art.

Claims

WHAT IS CLAIMED IS:

1. A method of making a p-type nitride layer, comprising the

steps of:

a) providing a HVPE system including a reactor;

b) arranging a substrate in the reactor; and

c) introducing reagent gases into the reactor, wherein the

reagent gases include a first reagent gas component, the

first reagent gas component prepared by passing HCl over

a group III metal and Mg.

2. The method of claim 1, wherein the group III metal

comprises Ga, In, Al, or one of their alloys.

3. The method of claim 1, wherein the group III metal and the

Mg are combined to form a group III/Mg metal mixture prior

to passing HCl over the group III metal and the Mg.

4. The method of claim 3, wherein the Mg of the group III/Mg

metal mixture is elemental Mg.

5. The method of claim 3, wherein the group III/Mg metal

mixture consists essentially of at least one group III

metal and elemental magnesium.

6. The method of claim 3, wherein the group III/Mg metal

mixture comprises from 1 parts-per-billion to 10,000

parts-per-million of Mg metal.

7. The method of claim 1, wherein the reagent gases further

include ammonia, and the ammonia is carried to the reactor

via a carrier gas.

8. The method of claim 7, wherein the carrier gas is free

from hydrogen.

The method of claim 7, wherein the carrier gas is a gas

selected from the group consisting of nitrogen and helium.

10. The method of claim 1, wherein the first reagent gas

component comprises a chloride of a group III metal and

magnesium.

11. The method of claim 10, wherein the first reagent gas

component further comprises a chloride of magnesium.

12. The method of claim 3, wherein the group III/Mg metal

mixture is heated to a temperature of 660 °C to 900 °C.

13. The method of claim 3 , wherein the HVPE system further

includes a production chamber, the group III/Mg metal

mixture is housed within the production chamber, and the

HCl is passed into the production chamber and over the

group III/Mg metal mixture.

14. The method of claim 1, wherein the p-type nitride layer is

doped with magnesium, and the magnesium component of the

p-type nitride layer is minimally passivated with hydrogen

atoms .

15. The method of claim 1, wherein said step c) comprises

introducing ammonia and the first reagent gas component

into the reactor, wherein the ammonia and the first reagent

gas component react to form a Mg-doped p-type group III

metal nitride on the substrate.

16. The method of claim 1, wherein the p-type nitride layer

has a dislocation density is less than IQ /_cm2 ^.o and a

resistivity value less than 0.1 ohms cm.

17. The method of claim 1, wherein the p-type nitride layer is

formed on the substrate to a thickness of from 50 micron

to 500 micron

18. The method of claim 1, wherein the p-type nitride layer

comprises Mg-doped p-GaN.

19. The method of claim 1, further comprising the step of:

d) after said step c) , removing the p-type nitride layer

from the substrate arranged in said step b) .

20. A p-type nitride layer grown on a substrate by HVPE, said

p-type nitride layer comprising: a group III nitride doped

with magnesium, the p-type nitride layer formed by

reacting a first reagent gas component with ammonia, the

first reagent gas component prepared by passing HCl over a

group III metal and magnesium metal.

21. The p-type nitride layer of claim 20, wherein the group

III metal comprises Ga, In, Al, or one of their alloys.

22. The p-type nitride layer of claim 20, wherein the group

III metal and the Mg comprise a group III/Mg metal

mixture .

23. The p-type nitride layer of claim 22, wherein the group

III/Mg metal mixture comprises from 1 parts-per-billion to

10,000 parts-per-million of Mg metal.

24. The p-type nitride layer of claim 20, wherein the group

III/Mg metal mixture comprises from 1 parts-per-billion to

10,000 parts-per-million of Mg metal.

25. The p-type nitride layer of claim 20, wherein the group

III/Mg metal mixture consists essentially of at least one

group III metal and elemental magnesium.

26. The p-type nitride layer of claim 20, wherein the first

reagent gas component comprises a chloride of a group III

metal, and Mg.

27. The p-type nitride layer of claim 26, wherein the first

reagent gas further comprises a chloride of Mg.

28. The p-type nitride layer of claim 22, wherein the group

III/Mg metal mixture is heated to a temperature of 660 °C

to 900 °C.

29. The p-type nitride layer of claim 20, wherein the

magnesium component of the p-type nitride layer is

minimally passivated with hydrogen atoms.

30. The p-type nitride layer of claim 20, wherein the p-type

nitride layer has a dislocation density is less than

10^ /cm^ to and a resistivity value less than 0.1 ohms cm.

31. The p-type nitride layer of claim 20, wherein the p-type

nitride layer is formed on the substrate to a thickness in

the range of 50 micron to 500 micron.

32. The p-type nitride layer of claim 20, wherein the p-type

nitride layer comprises Mg-doped p-GaN.

33. A method of making a p-type nitride layer, comprising the

steps of :

a) providing a HVPE system including a reactor;

b) arranging a substrate in the reactor; c) passing HCl over a metal mixture to provide a first

reagent gas component, the metal mixture including

magnesium metal;

d) introducing ammonia and the first reagent gas component

into the reactor; and

e) growing a magnesium-doped p-type nitride layer on the

substrate .

34. The method of claim 33, wherein the metal mixture comprises

elemental magnesium and a group III metal, the group III

metal selected from the group consisting of Ga, In, and Al .

35. The method of claim 33, wherein the group III/Mg metal

mixture comprises from 1 parts-per-billion to 10,000 parts-

per-million of Mg metal.

36. The method of claim 33, further comprising the step of:

f) after said step e) , removing the p-type nitride layer

from the substrate .