EXHIBIT 1

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US007250360B2 

(12) United States Patent (10) Patent No.: US 7.250,360 B2 

Shealy et al. (45) Date of Patent: Jul. 31, 2007 

(54) SINGLE STEP, HIGH TEMPERATURE 5,679,152 A * 10/1997 Tischler et al. ............... 11797 

NUCLEATION PROCESS FOR A LATTICE 5,834,379 A 1 1/1998 Shealy 

MISMATCHED SUBSTRATE 5,909,036 A * 6/1999 Tanaka et al. ................ 257/94 

(75) Inventors: James R. Shealy, Ithaca, NY (US); 6,177.292 B1* 1/2001 Hong et al. ................... 438/46 

Joseph A. Smart, Mooresville, NC 6,218,280 B1 * 4/2001 Kryliouk et al. ............ 438/6O7 

(US) 6,478,871 B1 11/2002 Shealy 

(73) Assignee: Cornell Research Foundation, Inc., 

Ithaca, NY (US) - 0 * cited by examiner (*) Notice: Subject to any disclaimer, the term of this 

patent is extended or adjusted under 35 Primary Examiner David Nhu 

U.S.C. 154(b) by 124 days. (74) Attorney, Agent, or Firm—Jones, Tullar & Cooper PC 

(21) Appl. No.: 11/069,040 (57) ABSTRACT 

(22) Filed: Mar. 2, 2005 

(65) Prior Publication Data A single step process for nucleation and subsequent epitaxial 

growth on a lattice mismatched Substrate is achieved by 

US 2006/O199364 A1 Sep. 7, 2006 pre-treating the Substrate surface with at least one group III 

(51) Int. Cl reactant or at least one group II reactant prior to the 

iotL I/28 (2006.01) introduction of a group V reactant or a group VI reactant. 

HOIL 2L/3205 (2006.01) The group III reactant or the group II reactant is introduced 

(52) U.S. Cl. ...................... 438/603: 438/604:438/607, into a growth chamber at an elevated growth temperature to 

257/E21.097; 257/E21.108: 257/E21.112: wet a Substrate Surface prior to any actual crystal growth. 

257/E21.118: 257/E21.126 Once the pre-treatment of the Surface is complete, a group 

(58) Field of Classification Search ................ 438/603, V reactant or a group VI reactant is introduced to the growth 438/604, 606, 608, 688, 660, 663, 341, 363, chamber to commence the deposition of a nucleation layer. 

438/416,502,913, 931,933 A buffer layer is then grown on the nucleation layer pro 

See application file for complete search history. viding a Surface upon which the epitaxial layer is grown 

(56) References Cited preferably without changing the temperature within the 

chamber. 

U.S. PATENT DOCUMENTS 

5,625,202 A 4, 1997 Chai ........................... 257/94 24 Claims, 1 Drawing Sheet 

AlN/AGaN 

surface nucleation transition 

treatment layer region 

Time 

SiC 

SiC 

Sapphire 

 

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U.S. Patent Jul. 31, 2007 US 7.250,360 B2 

AlN/AGaN 

surface nucleation transition 

treatment layer region GaN growth 

NH, 

TMA SC 

TEG ::::: 

NH, SiC 

TMA site 

TEG 

Time 

FIG. 1 

 

 

 

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US 7,250,360 B2 

1. 

SINGLE STEP, HIGH TEMPERATURE 

NUCLEATION PROCESS FOR A LATTICE 

MISMATCHED SUBSTRATE 

STATEMENT REGARDING FEDERALLY 

SPONSORED RESEARCH OR DEVELOPMENT 

This invention was made with Governmental support 

from the Office of Naval Research under Contract No. 

N00014-96-1-1223 and DARPA under Contract No. 

DABT63-C95-0121. The Government has certain rights in 

the invention. 

BACKGROUND ON THE INVENTION 

1. Field of the Invention 

The present invention relates to a single step process for 

the nucleation and Subsequent epitaxial growth on a lattice 

mismatched Substrate. 

2. Description of Related Art 

Heteroepitaxial growth of GaN and related alloys on 

severely mismatched Substrates Such as Sapphire, SiC and 

silicon requires thin nucleation layers to provide nucleation 

sites to initiate crystal growth. When optimized, these nucle 

ation layers accommodate the strain between the Substrate 

and epitaxial layers caused by lattice and thermal mis 

matches, while still maintaining crystallographic registra 

tion of the epitaxial films to the substrate lattice structure. 

The most commonly accepted practice for nucleation of 

these substrates with GaN-based material utilizes a two 

temperature process involving either one or two flow paths. 

In addition, different growth pressures are used on certain 

steps in the nucleation process. In this fashion, high struc 

tural and electrical quality epitaxial films can be realized in 

severely mismatched material systems. 

BRIEF SUMMARY OF THE INVENTION 

The present invention is a single step process for the 

nucleation and Subsequent epitaxial growth on a lattice 

mismatched substrate. A nucleation layer and a buffer layer 

are formed on the lattice mismatched substrate prior to the 

formation of an epitaxial layer. The epitaxial layer prefer 

ably includes a III-V compound or a II-VI compound. 

Crystal growth can be carried out by any conventional 

means Such as organometallic vapor phase epitaxy (OM 

VPE), molecular beam epitaxy (MBE), or atomic layer epitaxy. The techniques for crystal growth preferably 

involve flow (beam) modulation, growth at atmospheric or 

reduced pressures, as well as various precursor chemistries 

that are widely accepted in the industrial and research 

communities. 

In a preferred embodiment, a lattice mismatched substrate 

is pre-treated with a group III reactant or a group II reactant 

at an elevated growth temperature prior to the addition of a 

group V reactant or a group VI reactant and the growth of a 

nucleation layer. The group III reactant or the group II 

reactant is introduced into a growth chamber to pre-treat a 

surface of the substrate. Then, a nucleation layer is formed 

by introducing the group V reactant or the group VI reactant 

into the growth chamber. In a preferred embodiment, the 

nucleation layer is AlGaN where x varies from 0 to 1. A 

buffer layer, preferably GaN. is formed on the nucleation 

layer to provide a Surface upon which the epitaxial layer is 

grown. In a preferred embodiment, aluminum reacts with the 

Surface of the Substrate providing the needed nucleation sites 

while gallium prepares the aluminum Saturated Surface for 

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Subsequent growth of the epitaxial layer, preferably an 

epitaxial layer including AlN, AlGaN, or GaN. 

The present single step process is a single flow, single 

pressure, high temperature process. The single step process 

involves a Surface treatment prior to epitaxial growth that 

allows complete coalescence of the epitaxial film within the 

first 200 A of growth. This single step process eliminates the 

large temperature ramps and complex flow requirements of 

the commonly used two-step nucleation technologies. 

BRIEF DESCRIPTION OF THE DRAWING(S) 

The features and advantages of the present invention will 

become apparent from the following detailed description of 

a preferred embodiment thereof, taken in conjunction with 

the accompanying drawings, in which: 

FIG. 1 is a schematic representation of one embodiment 

of the single step nucleation process. 

DETAILED DESCRIPTION OF THE 

INVENTION 

The present invention is a single step process, preferably 

a single flow, single pressure, high temperature nucleation 

process. This single step process for nucleation and Subse 

quent growth of an epitaxial layer on a lattice mismatched 

substrate is achieved by pre-treating a surface of the sub 

strate with at least one group III reactant or at least one group 

II reactant prior to the introduction of a group V or a group 

VI reactant. The substrate is a mismatched substrate com 

prising a material that is lattice mismatched to an epitaxial 

layer to be grown thereon. Preferably, the mismatched 

substrate is silicon, sapphire or SiC. The reactants are 

generally selected from a group of compounds which are 

Volatile near room temperature and can be vapor transported 

in a carrier gas. The group III reactant is preferably selected 

from the group consisting of a gallium reactant, an alumi 

num reactant, a boron reactant, and an indium reactant. Most 

preferably, the gallium reactant is triethylgallium (TEG) or 

trimethylgallium (TMG). Most preferably, the aluminum 

reactant is triethylaluminum (TEA) or trimethylaluminum 

(TMA). Most preferably, the boron reactant is triethylboron 

(TEB) or trimethylboron (TMB). Most preferably, the 

indium reactant is triethylindium (TEIn) or trimethylindium 

(TMIn). The group II reactant is preferably selected from the 

group consisting of a cadmium reactant and a Zinc reactant. 

At least one group III reactant or at least one group II 

reactant is introduced into a growth chamber at an elevated 

growth temperature to pre-treat the surface of the substrate 

prior to any actual crystal growth. In a preferred embodi 

ment, the elevated growth temperature is 900 to 1100°C. In 

a preferred embodiment, more than one group III reactant or 

more than one group II reactant is introduced into the growth chamber. For example, two group III reactants, preferably a 

gallium reactant and an aluminum reactant, may be intro 

duced into the growth chamber. Once the pre-treatment is 

complete, the group V reactant or the group VI reactant is 

introduced to begin the growth of the nucleation layer. Preferably, the group V reactant contains a nitrogen reactant, 

an arsenic reactant or a phosphorus reactant. In one preferred embodiment, the group V reactant is a group V hydride, 

most preferably ammonia, arsine or phosphine. In a second 

preferred embodiment, the group V reactant is a group V 

organic precursor, most preferably tertiarybutylarsine 

(TBAS) or tertiarybutylphosphine (TBP). Preferably, the 

group VI reactant contains a tellurium reactant or a Sulfur 

reactant. After the nucleation layer is formed, a buffer layer 

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US 7,250,360 B2 

3 

is formed on the nucleation layer. The buffer layer provides 

a Surface upon which the epitaxial layer is grown. The 

epitaxial layer includes a III-V compound or a II-VI com 

pound. Preferably, the III-V compound or the II-VI com 

pound includes, but is not limited to, AlN. GaN. AlGaN. 

BGaN, BInN, BAIN, CdTe., and ZnS. 

In a preferred embodiment, an AlGaN nucleation layer 

where x varies from 0 to 1 is grown on a substrate. 

Generally, the aluminum content requirements increase from 

SiC. to Sapphire, then to silicon for obtaining atomically flat 

two-dimensional epitaxial growth. In order to obtain the 

desired nucleation layer, the surface of the substrate is first 

cleaned with hydrogen near a growth temperature. The 

substrate is then subjected to the desired elevated growth 

temperature, preferably 900 to 1100° C. After the growth 

temperature has stabilized, an aluminum reactant and a 

gallium reactant are introduced into the growth chamber to 

pre-treat the surface of the substrate prior to the introduction 

of a nitrogen Source and nucleation layer growth. The 

exposure time and flux of the aluminum reactant and the 

gallium reactant depend again on the Substrate material and 

the crystal qualities desired. Once the pre-treatment of the 

Surface is complete, a nitrogen Source, preferably ammonia, 

is introduced into the growth chamber to commence the 

deposition of a nucleation layer. Preferably, the nucleation 

layer is grown at the same elevated growth temperature used 

to pre-treat the surface of the substrate. This provides a 

nucleation layer of crystalline AlGaN of a specific thick ness and aluminum content. In the case of an AlN nucleation 

layer, the gallium reactant is stopped prior to the formation 

of the nucleation layer and then re-introduced after the AIN 

nucleation layer is formed. After the nucleation layer is 

formed, a buffer layer is grown on the nucleation layer. 

Preferably, the buffer layer is grown at the same elevated 

growth temperature used to pre-treat the surface of the 

substrate. In the preferred embodiment, the aluminum reac 

tant is ramped down and Switched out resulting in a pre 

ferred GaN buffer layer. The buffer layer provides a surface 

upon which the epitaxial layer is grown. In a preferred 

embodiment, a GaN buffer layer provides a surface on which 

GaN-based materials can subsequently be deposited without 

changing the temperature within the chamber. 

Numerous applications exist for a lattice mismatched 

Substrate having an epitaxial nitride based material. Such as 

(Al)GaN. in the areas of high power, high frequency elec 

tronics, short wavelength optoelectronics, biology, and 

micromachining technologies. 

EXAMPLE 1. 

A schematic representation of a single step process for 

nucleation and subsequent growth of a preferred GaN-based 

material is shown in FIG. 1. Relative reactant fluxes and 

exposure cycles are depicted for the Surface pre-treatment, 

the AlN/A1GaN nucleation layer, and the transition to GaN 

epitaxial growth on both sapphire and SiC substrates. For 

this example, flow modulation OMVPE is used for the 

growth. 

A detailed description of the single step process depicted 

in FIG. 1 follows. 

1. The substrates are heated up to 1100° C. in hydrogen 

for 10 minutes to clean the surface of the substrate. 

2. After 10 minutes, the temperature is decreased to the 

growth temperature of 1040° C. 

3. After the temperature has stabilized, the aluminum 

reactant and the gallium reactant, preferably TEG and 

TMA, are introduced into the growth chamber. For 

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4 

Sapphire substrates the TMA/TEG ratio is set at 0.15, 

while for SiC is fixed at 0.06. The Substrates are 

exposed for 100 seconds. 

4. Ammonia is then Switched into the growth chamber, 

forming AlGaN (x=0.06 for SiC., and x=0.15 for 

Sapphire) on the metal rich Substrate Surface. Around 

20 nm of AlGaN is grown. 

5. The gallium reactant is switched into the growth 

chamber if not already present (in the case of an AlN 

nucleation layer). 

6. The aluminum reactant is then ramped down to 0.0 in 

30 seconds, then switched out of the growth chamber. 

7. This results in GaN deposition, providing a buffer layer 

on which other GaN-based epitaxial layers can subse 

quently be deposited without any change in growth 

chamber temperature. 

EXAMPLE 2 

The following is an example of a high electron mobility 

transistor (HEMT) nucleation sequence in accordance with 

the present invention. TEG and TMA flows are total flows, 

not actual organometallic flux. Temperature calibrations 

with a clean cell indicated 101.6°C. was actually more like 

1040°C. The nucleation sequence is as follows: 

1. The substrate is heated in hydrogen to a temperature of 

approximately 1060° C. It generally takes about 14 

minutes to ramp up from room temperature to approxi 

mately 1060° C. The RF is on at a lower power for the 

first 4 minutes. 

2. RF power is decreased to stabilize the substrate at the 

growth temperature of about 1016°C. It takes about 10 

minutes from turning RF down to the start of growth, typically arriving at the growth temperature about 5 

minutes before growth. 3. An organometallic pre-treatment then begins, whereby 

TEG and TMA are switched into the cell to Zone 1 for 

100 sec for sapphire, 80 sec. for SiC in the absence of 

the nitrogen source. TEG flow is at 90 sccm and TMA 

flow is adjusted based on previous runs, typically 15-25 

sccm for sapphire, 5-15 sccm for SiC. All procedures 

other than this step are the same for SiC and sapphire. 

4. TEG is switched out of the cell and ammonia is 

switched into Zone 1 at approximately 4500 sccm, and 

the balance of hydrogen flow to Zone 1 is switched out, 

all simultaneously. Immediately after (1-2 sec.), ammo 

nia is switched into the block at 1200 sccm for distri 

bution to the rest of the cell. This causes some pressure fluctuations due to the Sudden change in ammonia flow, 

affecting all flows in to the cell, but primarily the 

ammonia flow. It typically takes 20-30s to stabilize. 

5. 60 sec. after ammonia is first switched in, TMA flow is 

increased from nucleation value to full AlN flow of 

about 90 sccm. This increase occurs somewhat slowly, over about 30 sec. 

. Continue growing AlN for 20 min. 

7. Switch in TEG at full flow of approximately 90 sccm 

and simultaneously begin reducing TMA flow. Steadily 

decrease flow for 2 min. to zero and switch out. 

8.30 sec. after TMA switched out, raise ammonia flow to 

GaN buffer value of about 9000 sccm to Zone 1, and 

2600 sccm to block. GaN buffer is now grown. 

Although the present invention has been disclosed in 

terms of a preferred embodiment, it will be understood that numerous additional modifications and variations could be 

made thereto without departing from the scope of the 

invention as defined by the following claims. 

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US 7,250,360 B2 

5 

What is claimed is: 

1. A process for growing an epitaxial layer on a lattice 

mismatched Substrate comprising the steps of 

a) providing a substrate; 

b) pre-treating a surface of the substrate with at least one 

group III reactant or at least one group II reactant at an 

elevated growth temperature prior to introducing a 

group V reactant or a group VI reactant; 

c) introducing a group V reactant or a group VI reactant 

to grow a nucleation layer on the Surface of the Sub 

strate; and 

d) growing a buffer layer on said nucleation layer, said 

buffer layer providing a surface upon which said epi taxial layer is grown. 

2. The process of claim 1, wherein said substrate is 

selected from the group consisting of silicon, Sapphire and 

SiC. 

3. The process of claim 1, wherein said group III reactant 

is selected from the group consisting of a gallium reactant, 

an aluminum reactant, a boron reactant, and an indium 

reactant. 

4. The process of claim 3, wherein said group III reactant 

is selected from the group consisting of triethyl gallium, trimethyl gallium, triethylaluminum, trimethyl aluminum, 

triethylboron, trimethylboron, triethylindium and trimeth 

ylindium. 

5. The process of claim 1, wherein said group II reactant 

is a cadmium reactant or a Zinc reactant. 

6. The process of claim 1, wherein said step of pre treating includes pre-treating the Surface of the Substrate 

with an aluminum reactant and a gallium reactant. 

7. The process of claim 6, wherein said nucleation layer 

is ATN or AlGaN. 

8. The process of claim 1, wherein said buffer layer is 

GaN. 

9. The process of claim 1, wherein said epitaxial layer includes a compound selected from the group comprising 

AlN, GaN, AlGaN, BGaN, BInN, BAIN, CdTe., and ZnS. 

10. The process of claim 1, further comprising the step of 

preheating said Substrate in hydrogen prior to pre-treating 

the surface of the substrate with at least one group III 

reactant or at least one group II reactant. 

11. The process of claim 1, wherein said elevated growth 

temperature is 900 to 1100° C. 

12. The process of claim 11, wherein the step of intro 

ducing a group V reactant or a group VI reactant includes 

growing a nucleation layer at a growth temperature of 900 

to 1100° C. and the step of growing a buffer layer includes 

growing a buffer layer at a growth temperature of 900 to 

1100° C. 

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13. The process of claim 1, wherein said group V reactant 

is a group V hydride. 

14. The process of claim 13, wherein said group V hydride 

is selected from the group consisting of ammonia, arsine, 

and phosphine. 

15. The process of claim 1, wherein said group V reactant 

is a group V organic precursor. 

16. The process of claim 15, wherein said group V organic 

precursor is tertiarybutylarsine or tertiarybutylphosphine. 

17. The process of claim 1, wherein said group VI reactant 

is a tellurium reactant or a Sulfur reactant. 

18. A process for growing an epitaxial layer on a lattice 

mismatched Substrate comprising the steps of 

a) providing a Substrate; 

b) pre-treating a surface of the substrate with at least a first 

group III reactant and a second group III reactant at a 

growth temperature of 900 to 1100° C. prior to intro 

ducing a nitrogen source; 

c) stopping a flow of the first group III reactant; 

d) introducing a nitrogen source to grow a nucleation 

layer on the surface of the substrate; 

e) resuming the flow of the first group III reactant and 

reducing a flow of the second group III reactant; 

f) stopping the flow of the second group III reactant; and 

g) growing a buffer layer on said nucleation layer, said 

buffer layer providing a surface upon which said epi taxial layer is grown. 

19. The process of claim 18, wherein said first group III 

reactant is a gallium reactant and said second group III 

reactant is an aluminum reactant. 

20. The process of claim 19, wherein said gallium reactant 

is triethylgallium and said aluminum reactant is trimethyla 

luminum. 

21. The process of claim 18, wherein said nucleation layer 

is ATN or AlGaN. 

22. The process of claim 18, wherein said buffer layer is 

GaN. 

23. The process of claim 18, wherein said epitaxial layer includes a compound selected from the group comprising 

AlN, GaN, AlGaN, BGaN, BInN, and BAIN. 

24. The process of claim 18, further comprising the step of preheating said Substrate in hydrogen prior to pre-treating 

the surface of the substrate with the first group III reactant 

and the second group III reactant. 

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