MOCVD technology and material growth

1.Introduction2.The MOVD technique and growth system3.Metalorganic compound4.Gas phase and surface reaction5.Materials Characterization6.MOCVD growth of GaN

Most of the advances in semiconductor processing have centered on the ability to decrease the physical dimensions of the electronic device structure.

 • Lateral dimension:

Photolithographic, DepositionEtching techniques…

• Vertical dimension:Epitaxial deposition

1.Introduction

Compare of epitaxial methods

Growth method time features limit

LPE

(Liquid phase epitaxy)

1963 Growth form supersaturated solution onto substrate

Limited substrate areas and poor control over the growth of very thin layers

VPE

(Vapor phase epitaxy

1958 Use metal halide as transport agents to grow

No Al contained compound, thick layer

MBE

(Molecular Beam Epitaxy)

1958

1967

Deposit epilayer at ultrahigh vacuum

Hard to grow materials with high vapor pressure

MOCVD

(Metal-Organic Chemical Vapor Deposition)

1968 Use metalorganic compounds as the sources

Some of the sources like AsH3 are very toxic.

Some about the name of MOCVD

In the reference, MOCVD also have some other names. Different people prefer different name. All the names refer to the same growth method.MOCVD (Metalorganic chemical vapor deposition)OMCVD(Organometallic CVD)MOVPE (MO vapor phase epitaxy)OMVPE

AP-MOCVD (Atmosphere MOCVD)LP-MOCVD (Low pressure MOCVD)

2. The MOVD growth system

Vacuum andExhaust system

Gas handle system

ComputerControl

Reactor

MOCVD Growth System

Gas handling system

The function of gas handling system is mixing and metering of the gas that will enter the reactor. Timing and composition of the gas entering the reactor will determine the epilayer structure.

 Leak-tight of the gas panel is essential, because the oxygen contamination will degrade the growing films’ properties.

Fast switch of valve system is very important for thin film and abrupt interface structure growth,

 Accurate control of flow rate, pressure and temperature can ensure the stable and repeat.

Reactor-1

Reactor-2

Aixtron Model-2400 reactor

Exhaust system

•Pump and pressure controller

For low pressure growth, we use mechanic pump and pressure controller to control the growth pressure. The pump should be designed to handle large gas load.

•waste gas treatment system

The treatment of exhaust gas is a matter of safety concern.

The MOCVD system for GaAs and InP use toxic materials like AsH3 and PH3. The exhaust gases still contain some not reacted AsH3 and PH3, Normally, the toxic gas need to be removed by using chemical scrubber.

For GaN system, it is not a problem.

3. Metalorganic compound

The vapor pressure of the MO source is an important consideration in MOCVD, since it determines the concentration of source material in the reactor and the deposition rate. Too low a vapor pressure makes it difficult to transport the source into the deposition zone and to achieve reasonable growth rates. Too high a vapor pressure may raise safety concerns if the compound is toxic. Further more, it is easier to control the delivery from a liquid than from a solid.

Vapor pressures of Metalorganic compounds are calculated in terms of the expression 

Log[p(torr)]=B-A/T

Vapor pressure of most common MO compounds

Compound P at 298 K

(torr)

A B Melt point

(oC)

(Al(CH3)3)2 TMAl 14.2 2780 10.48 15

Al(C2H5)3 TEAl 0.041 3625 10.78 -52.5

Ga(CH3)3 TMGa 238 1825 8.50 -15.8

Ga(C2H5)3 TEGa 4.79 2530 9.19 -82.5

In(CH3)3 TMIn 1.75 2830 9.74 88

In(C2H5)3 TEIn 0.31 2815 8.94 -32

Zn(C2H5)2 DEZn 8.53 2190 8.28 -28

Mg(C5H5)2 Cp2Mg 0.05 3556 10.56 175Log[p(torr)]=B-A/T

Calculate the mole flow rate of MO sources

When we read some reference, we often see people use mol/min to indicate the flow rate. Normally, we use the formula to calculate it.  

F (mol/min)=p MO/p Bubbler*[flow rate (ml/min)]/22400 (mol/ml)

 We need to calculate the mole flow rate before we determine the growth condition. If we want to grow alloys, we can use the mole flow rate to estimate the alloys’ composition.

For example, if we grow AlGaN, we can estimate the Al concentration use the following formula if we assume the efficiency of Al and Ga sources is the same.

x Al=F Al/(F Al+ F Ga)

The basic reaction describe GaN growth can simply write asGa(CH3)3+NH3 GaN+3CH4

The growth procedure as follows:

1. MO sources and hydrides inject to the reactor.2. The sources are mixed inside the reactor and transfer to the deposition area3. At the deposition area, high temperature result in the decomposition of

sources and other gas-phase reaction, forming the film precursors which are useful for film growth and by-products.

4. The film precursors transport to the growth surface5. The film precursors absorb on the growth surface6. The film precursors diffuse to the growth site7. At the surface, film atoms incorporate into the growing film through

surface reaction8. The by-products of the surface reactions absorb from surface9. The by-products transport to the main gas flow region away from the

deposition area towards the reactor exit

4. Gas phase and surface reaction

reaction

•Physical characterizationX-ray diffraction (XDS)Transmission electron microscopy (TEM)Optical microscopyScanning electron microscopy (SEM)Atom force microscopy (AFM)Secondary ion mass spectroscopy (SIMS)

•Electrical MeasurementsVan der Pauw HallCapacitance-voltage (C-V)

•Optical measurements Photoluminescence (PL)

5. Materials Characterization

Two Step MOCVD Growth procedure

High temperaturetreatment

Bufferlayer

EpilayerGrowth

TMGa

NH3

Temperature

1150oC

550oC1050oC

Ga(CH3)3+NH3 GaN+CH4

6. MOCVD grow GaN and related materials

Some basic problem related to GaN growth

MOCVD and other epitaxial techniques have developed more than 30 years, but high quality GaN and related compound only available in recent years. There are some special problems for GaN and related materials.

•No suitable substrate

•Difficult to obtain p-type epilayer