ELSEWER

Nuclear Instruments and Methods in Physics Research A 395 (1997) 125-128 NUCLEAR

INSTRUMENTS & METHODS IN PHYSiCS RESEARCH

Sectton A

Growth of high purity GaAs using low-pr~ssur~ vapour-phase epitaxy

R.L. Adams

5234 East Hatcher, Paradise Valley, AZ 85253, USA

Abstract The growth of high purity films of gallium arsenide using growth rates of greater than 1 pm/minute has been demon-

strated using Low-Pressure Vapour-Phase Epitaxy. These films have been found to be suitable for high energy particle detectors and high voltage Schottky diodes.

Keywords: Low-pressure vapour-phase epitaxy; LPVPE; GaAs

1. Introduction

Compound semiconductors are commonly used in a va-

riety of military, consumer and commercial applications. This well-established technology is used to manufacture light-emitting diodes, high-frequency transistors, diode lasers, solar cells and a variety of related electronic and optoelectronic devices. As evidence of this extensive use, one needs only to step out of the train station in Tokyo, Japan, to see the multitude of indoor and outdoor LED displays that dot the buildings in this fast-paced city. By using various epitaxial combinations of the compound

semiconductors, diodes emitting colours from visible red (660 nm) to green (555 nm) have been mass-produced and

utilized in these displays. In addition, the same materials systems can be used in the fabrication of high-frequency

transistors for various microwave devices commonly found in the new wireless communication markets. As the com- pound semiconductor technology has matured, additional uses for the various compounds have been realized. The most common of the compound semiconductor materials, edAs, is now being studied as a high-energy particle de- tector for various scientific and commercial needs. The key to this technology is the availabili~ of very high

purity GaAs in very thick, single crystal films. It is this need for high purity and thick films that has highlighted the use of Low-Pressure Vapour-Phase Epitaxy (LP- VPE), as a uniquely qualified technology to assist in this development.

The use of GaAs as a high-energy particle detector has been studied extensively. The critical parameters in the use of GaAs are to have a material with good structural properties, low impurity levels to minimize recombination

in the bulk material and thick films to allow the interaction

of the incoming high-energy particle to liberate sufficient electrons. Unless the material is of sufficient thickness, the

number of electrons liberated will not be great enough to have a measurable charge signal.

2. Current high-purity GaAs synthesis techniques

2.1. Bulk growth

The initial work on GaAs as a detector employed wafers Erom the bulk growth technoIogies. In one case, high-pad

arsenic and gallium are melted in a high-temperature vessel

and slowly cooled to produce a single crystal. This method is commonly known as Liquid-Encapsulated Czochralski (LEC) growth, and is one of the two major production meth- ods used in the GaAs business. This material can be made to have high resistivity but contains significant levels of carbon and has a crystal structure that is heavily populated with dis- locations. With these two inherent limits, the performance is not acceptable for the detector application.

Subsequent work was done with single crystals formed using a growth technique referred to as Vertical Gradient Freeze (VGF) technology. In this method, high purity gal- lium and arsenic are mixed in an enclosed quartz ampoule with a seed crystal of GaAs. The gallium and arsenic are liquefied and brought into contact with the seed crystal for the growth. The liquid is cooled slowly and the single crys- tal formed. Although high-purity materials are used in the growth, the resulting material is again found to be unaccept- able for detectors.

016%9002/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved

PII SO168-9002(97)00624-4 IV. DETECTOR MATERIALS

126 R.L. Ad~msl~ucl. instr. and Meth. ipr Phys. Res. A 395 (1997) 125-128

2.2. Epitaxial growth

The growth of high purity GaAs films by epitaxial meth- ods is well known. Impurity levels less than 1 x 1014 cmp2

are routinely achieved with several techniques. The thick- ness specifications of the detector type wafers, however, limit the growth methods severely. With present technolo-

gies, two potential methods can be identified. The first is Liquid-Phase Epitaxy (LPE). In this well-known method,

the single crystal layers are formed by growth from a su- persaturated solution. In this system, films can be grown in 4---8 h that would be thick enough for use in detectors. It is very difficult, however, to obtain the low doping levels re- quired. In addition, the preparation procedures to reach the low doping leveis extend the growth process to unacceptable limits.

The second technology is Atmospheric-Pressure Vapour-

Phase Epitaxy (AP-VPE). This very popular process is used

to grow millions of square inches of compound semiconduc- tor epitaxial films each year for light-emitting diode applica-

tions. This process is very cost-effective and produces good quality material for the chosen application. In the process, growth rates of 0.5 um/min can be obtained. A schematic

representation of an AP-VPE system can be seen in Fig. 1. In this set-up, the HCI carrier gas passes over the elemental gallium and mixes with arsine at the wafer surface to form the epitaxial films. The impurity level of the system is rou- tinely I x lOi cm-*. This method does not produce films

with sufficient growth rates or low enough impurity levels to satisfy detector needs.

2.3. Low-Pressure Vapour-Phase Epitaxy (LP-VPE)

In the last five years, work has been carried out at

the Aachen Technical Institute, (RWTH-Aachen), on the growth of GaAs using a modified VPE. The work demon- strated that as the pressure of the growth chamber was de- creased from atmospheric values to reduced pressures, the growth rate decreased, as one would expect, untii the pres- sure reached approximately 50 mbar. Below this pressure, the growth rate increased dramatically, reaching values in excess of 100 urn/h at l-50 mbar. These data can be seen

in Fig. 2. The LPE-VPE system used for this work was a large sys-

tem manufactured by the Aixtron Corporation for produc- tion applications. Fig. 3 shows a schematic representation of the growth chamber for this system. As can be seen, this horizontal unit has HCl flow over gallium metal to form the volatile GaCI. This material is then swept downs~eam where it mixes with amine. At the temperatures used, the arsine decomposes in stages, losing an attached hydrogen at each step. The simplified reactions for the system can be represented as follows:

2HCl+ 2Ga + 2GaCl-t HZ,

%I

A~~osp~e~c Pressure WE

Fig. 1.

Growth Rate vs Total Pressure

loo0 J B 100 -c ==8 2 B

p 10

t

B A

9 A l A

l-4 i 1 10 loo

Pressure (mb)

loo0

Fig. 2.

As& -+ ASH& = 0,l or 2),

ASH, -I- GaCl -+ GaAs + ASK + GaCI3 + Ha.

(The very complex series of reactions at the wafer surface are beyond the scope of this overview.) At the wafer surface, the GaAs is epitaxially formed. In the furnace system used, an intermediate mixing zone is present to allow for careful control of the cracking of the arsine gas as it enters the system. At the pressures used for this work, l-10 mbar, the linear gas velocity of 5-20 cm/s dictates very precise

RL. AdamslNucl. Instr. and Meth. in Phys. Rex A 395 (1997) 125-128

LP-VPE Growth Chamber Fig. 3.

control of the temperature in order to generate sufficient amounts of the partially decomposed material for the high growth rates to be observed. If the amine decomposes to the pure state, AS+ then the reaction rate decreases dramatically.

By adjusting the total gas flow, pressure and temperature, the residency time at the growth surface can be determined as well as the mean free path for the reaction species. To

grow on multiple wafers, the so-called “sweet spot” must extend over several centimetres in length. The control of this parameter is critical to uniformity over a single wafer

as well as from wafer to wafer. As can be seen from Fig. 3, the wafers sit in a holder that is similar in design to the diffusion sleds used for silicon wafers. With the large mean free path and sufficient spacing between wafers, epitaxial deposition can be done on wafers that are placed in a back- to-back position and separated by only a quartz disc. This arrangement is one of the reasons for the large capacity of the system and the ease of use. The computer-con~olled system

has the necessary safety features to allow for an operator to use the system for growth. Table 1 provides various data about the system and the technology.

3. Commercial applications

The LP-VPE technology has demonstrated the ability to grow high-quality GaAs epitaxial films that are suitable for some markets requiring large volumes. In particular, work has been done to produce high speed switching diodes with these wafers. These rectifying diodes were 1-3x IO” crnw3 for doping and 10 urn in thickness. The measured diodes

had reverse breakdown voltages of 200-220 V. The real opportunity for this technology comes as the market for even higher breakdown voltage devices starts in 1997. The 400 V rectifier requires 40 urn thick films with doping levels

of 1-3 x 1014 cmm3. More importantly, for these devices to be accepted in the commercial market place, cost will be a major factor. This LP-VPE technology is the only system

that has been publicly defined that can grow these types of wafers with the necessary volume and the potential for low cost.

Much work has been done with the technology to demon- strate the growth of GaAsP on GaAs to form light-emitting diodes (740 nm). Bv adjusting the alloy ratio, diodes emit- ting in the visible can also be formed. The technology has the potential to produce these wafers very cost effectively. Although not mentioned in earlier parts of this work, the LP- VPE technology can be extended to work on larger wafer

diameters as well. As GaAs substrates increase in diameter from 3 in. for optoele~~oni~s applications to 4 or 6 in. wafer

sizes, this technology will be able to be adapted to these geo- metries. One final consideration for the technology in the LED market is to grow very thick GaP films sequentially followed by GaAsP on silicon wafers. If this technology can be reduced to practice, then one can see the day where the LED cost can be reduced even lower by using lower price and more commonly available silicon wafers as the start- ing substrate. If this hetero-epitaxy concept can be imple- mented, other ideas such as the growth of GaAs on silicon could also be considered as a way to produce very low cost but high-quality GaAs films that could compete directly with the silicon device technology used today.

IV. DETECTOR MATERIALS

128 R.L. Adams1 Nucl. Ins@. and Me& in P&w. Rex A 395 ji997J U-128

Table 1 LP-VPE system data

Materials grown

Maximum no. of wafers/run demonstrated for GaAs

Typical thickness uniformity on single wafer of GaAs

GaAs on GaAs. GaAs on GaP

GaP on Gap, GaAsP on GaAs

GaN on Sapphire, GaN on silicon

GaAs on Silicon, GaP on silicon

20 wafers of 2 in. diameter

2 in. wafer f 5%

3 in. wafer * 5%

Typical thickness variation on multiple wafers of GaAs

Typical background doping levels for GaAs

Typical mobilities for background doping for GaAs

2 in. wafers f 15%

1-3 x 1014 cm-3

6000 cm2jV s at 300 K

120000cm2/Vs at 77K

Typical compensation ratios for GaAs

Lowest doping level for GaAs

Highest mobilities for GaAs

1.1-1.3

6 x 1013 cmp3

8000 cm2/V s at 300 K

170000cm2/V s at 77K

Typical GaAs growth rates

Highest GaAs growth rates

Highest GaP on GaP growth rate

Highest GaN on sapphire growth rate

Typical alloy uniformity for GaAsP on GaAs

Typical total gas flow in system

Typical dopants

Typical growth pressure

Typical system temperatures for GaAs on GaAs, GaAsP on GaAs, GaP on GaP, GaAs on Si, GaE’ on Si

60-100 pm/h

200 Fmjh

100 pm/h

60 pm/h (FWHM of X-ray 7 arcmin

and PL value FWHM - 6 meV at 2 K)

Single wafer f 1%

Multiple wafers * 3%

2 l/min

n-type Te; p-type Zn

l-3 mbar

Gallium - 700°C Mixing zone ~ 720°C Substrate - 6SO”~7000C

4. Summary

LP-VPE technology has been demonstrated as a viable deposition method to grow several compound semiconduc- tor films. Additional development work focused on specific device markets will accelerate the use of this new technol- ogy at a very rapid pace. The use of the LP-VPE technol-

ogy in the high-volume rectifier business will offer great

opportunities for the application of this material in various detector markets. As the rectifier markets move to higher

breakdown values, the doping levels and thickness of the films become almost identical to those required for the high energy particle detectors. If this happens, the cost of such

detectors can become low enough to be considered as com- mercial market opportunities.