Tài liệu Metal Organic Chemical Vapor Deposition: Technology and Equipment docx

151

1.0 INTRODUCTION

The growth of thin layers of compound semiconducting materials by

the co-pyrolysis of various combinations of organometallic compounds and

hydrides, known generically as metal-organic chemical vapor deposition

(MOCVD), has assumed a great deal of technological importance in the

fabrication of a number of opto-electronic and high speed electronic devices.

The initial demonstration of compound semiconductor film growth was first

reported in 1968 and was initially directed toward becoming a compound

semiconductor equivalent of “Silicon on Sapphire” growth technology.[1][2]

Since then, both commercial and scientific interest has been largely directed

toward epitaxial growth on semiconductor rather than insulator substrates.

State of the art performance has been demonstrated for a number of

categories of devices, including lasers,[3] PIN photodetectors,[4] solar cells,[5]

phototransistors,[6] photocathodes,[7] field effect transistors,[8] and modula￾tion doped field effect transistors.[9] The efficient operation of these devices

requires the grown films to have a number of excellent materials properties,

including purity, high luminescence efficiency, and/or abrupt interfaces. In

4

Metal Organic Chemical

Vapor Deposition:

Technology and

Equipment

John L. Zilko

152 Thin-Film Deposition Processes and Technologies

addition, this technique has been used to deposit virtually all III-V and II￾VI semiconducting compounds and alloys in support of materials studies.

The III-V materials that are lattice matched to GaAs (i.e., AlGaAs, InGaAlP)

and InP (i.e., InGaAsP) have been the most extensively studied due to their

technological importance for lasers, light emitting diodes, and photodetec￾tors in the visible and infrared wavelengths. The II-VI materials HgCdTe[10]

and ZnSSe[11][12] have also been studied for far-infrared detectors and blue

visible emitters, respectively. Finally, improved equipment and process

understanding over the past several years has led to demonstrations of

excellent materials uniformity across 50 mm, 75 mm, and 100 mm wafers.

Much of the appeal of MOCVD lies in the fact that readily transport￾able, high purity organometallic compounds can be made for most of the

elements that are of interest in the epitaxial deposition of doped and

undoped compound semiconductors. In addition, a large driving force

(i.e., a large free energy change) exists for the pyrolysis of the source

chemicals. This means that a wide variety of materials can be grown using

this technique that are difficult to grow by other epitaxial techniques. The

growth of Al-bearing alloys (difficult by chloride vapor phase epitaxy due

to thermodynamic constraints)[13] and P-bearing compounds (difficult in

conventional solid source molecular beam epitaxy, MBE, due to the high

vapor pressure of P)[14] are especially noteworthy. In fact, the growth of P￾containing materials using MBE technology has been addressed by using P

sources and source configurations that are similar to those used in MOCVD

in an MBE-like growth chamber. The result is called the “metal-organic

MBE”—MOMBE—(also known as “chemical beam epitaxy” tech￾nique).[15][16] As mentioned in the first paragraph, the large free energy

change also allows the growth of single crystal semiconductors on non￾semiconductor (sapphire, for example) substrates (heteroepitaxy) as well

as semiconductor substrates.

The versatility of MOCVD has resulted in it becoming the epitaxial

growth technique of choice for commercially useful light emitting devices

in the 540 nm to 1600 nm range and, to a somewhat lesser extent, detectors

in the 950 nm to 1600 nm range. These are devices that use GaAs or InP

substrates, require thin (sometimes as thin as 30 Å, i.e., quantum wells),

doped epitaxial alloy layers that consist of various combinations of In, Ga,

Al, As, and P, and which are sold in quantities significantly larger than

laboratory scale. Of course, there are other compound semiconductor

applications that continue to use other epitaxial techniques because of

some of the remaining present and historical limitations of MOCVD. For

Chapter 4: MOCVD Technology and Equipment 153

example, the importance of purity in the efficient operation of detectors

and microwave devices, and the relative ease of producing high purity InP,

GaAs, and their associated alloys,[17] has resulted in the continued impor￾tance of the chloride vapor phase epitaxy technique for these applications.

In addition, several advanced photonic array devices that are only recently

becoming commercially viable such as surface emitting lasers (SEL’s)[18]

and self electro-optic effect devices (SEED’s)[19] have generally been

produced by MBE rather than MOCVD because of the extreme precision,

control, and uniformity required by these devices (precise thicknesses for

layers in reflector stacks, for example) and the ability of MBE to satisfy

these requirements. In order for MOCVD to become dominant in these

applications, advances in in-situ characterization will need to be made.

More will be said about this subject in the final section of this chapter.

Finally, the emerging GaN and ZnSSe blue/green light emitting technolo￾gies have used MBE for initial device demonstrations, although consider￾able work is presently being performed to make MOCVD useful for the

fabrication of these devices, also.

Much of the effort of the last few years has centered around improv￾ing the quality of materials that can be grown by MOCVD while maintain￾ing and improving inter- and intrawafer uniformity on increasingly large

substrates. This effort has lead to great improvements in MOCVD equip￾ment design and construction, particularly on the part of equipment ven￾dors. Early MOCVD equipment was designed to optimize either wafer

uniformity, interfacial abruptness, or wafer area, depending on the device

application intended. For example, solar cells based on GaAs/AlGaAs did

not required state-of-art uniformity or interfacial abruptness, but, for

economic viability, did require large area growth.[20] During the 1970s and

early to mid-1980s there were few demonstrations of all three attributes—

uniformity, abrupt interfaces, and large areas—in the same apparatus and

no consensus on how MOCVD systems, particularly reaction chambers,

should be designed. A greater understanding of hydrodynamics, signifi￾cant advancements by commercial equipment vendors, and a changing

market that demanded excellence in all three areas, however, has resulted

in the routine and simultaneous achievement of uniformity, interfacial

abruptness, and large area growth that is good enough for most present

applications.

In this chapter, we will review MOCVD technology and equipment as

it relates to compound semiconductor film growth, with an emphasis on

providing a body of knowledge and understanding that will enable the reader

154 Thin-Film Deposition Processes and Technologies

to gain practical insight into the various technological processes and

options. MOCVD as it applies to other applications such as the deposition of metals,

high critical temperature superconductors, and dielectrics, will not be dis￾cussed here.

We assume that the reader has some knowledge of compound

semiconductors and devices and of epitaxial growth. Material and device

results will not be discussed in this chapter because of space limitations

except to illustrate equipment design and technology principles. For a more

detailed discussion of materials and devices, the reader is referred to a

rather comprehensive book by Stringfellow.[21] An older, but still excellent

review of the MOCVD process technology is also recommended.[22]

Although most of the discussions are applicable to growth of compound

semiconductors on both semiconductor and insulator substrates, we will be

concerned primarily with the technologically useful semiconductor sub￾strate growth. We will use abbreviations for sources throughout this

chapter. Table 2 in Sec. 3.1 provides the abbreviation, chemical name, and

chemical formula for most of the commercially available and useful

organometallics.

This chapter is organized into five main sections. We first motivate

the discussion of MOCVD technology and provide a “customer focus” by

briefly describing some of the most important applications of MOCVD.

We then discuss some of the physical and chemical properties of the

sources that are used in MOCVD. Because the sources used in MOCVD

have rather unique physical properties, are generally very toxic and/or

pyrophoric, and are chemically very reactive, knowledge of source proper￾ties is necessary to understand MOCVD technology and system design. The

discussion of sources will focus on the physical properties of sources used in

MOCVD and source packaging.

The next section deals with deposition conditions and chemistry.

Because MOCVD uses sources that are introduced into a reaction chamber at

temperatures around room temperature and are then thermally decom￾posed at elevated temperatures in a cold wall reactor, large temperature

and concentration gradients and nonequilibrium reactant and product

concentrations are present during film growth.[23] Thus, materials growth

takes place far from thermodynamic equilibrium, and system design and

growth procedures have a large effect on the film results that are obtained.

In addition, different effects are important for the growth of materials

from different alloy systems because growth is carried out in different

growth regimes. For these reasons, it is impossible to write an “equation of

Chapter 4: MOCVD Technology and Equipment 155

state” that describes the MOCVD process. We will, however, give a general

framework to the chemistry of deposition for several classes of materials. In

addition, we will give a general overview of deposition conditions that have

been found to be useful for various alloy systems.

In the next section, we consider system design and construction. A

schematic of a simple low pressure MOCVD system that might be used to

grow AlGaAs is shown in Fig. 1. An MOCVD system is composed of

several functional subsystems. The subsystems are reactant storage, gas

handling manifold, reaction chamber, and pump/exhaust (which includes a

scrubber). This section is organized into several subsections that deal with

the generic issues of leak integrity and cleanliness and the gas manifold,

reaction chamber, and pump/exhaust. Reactant storage is touched upon

briefly, although this is generally a local safety issue with equipment and

use obtainable from a variety of suppliers.

The last section is a discussion of research directions for MOCVD.

The field has reached sufficient maturity so that the emphasis of much

present research is on manufacturability, for example, the development of

optical or acoustic monitors for MOCVD for real-time growth rate control

and the achievement of still better uniformity over still larger wafers. In

addition, work continues to make MOCVD the epitaxial growth technique

of choice for some newer applications, for example, InGaAlN and ZnSSe.

Figure 1. Schematic of a simple MOCVD system.