Growth of high purity GaAs using low-pressure vapour-phase epitaxy

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

NUCLEAR INSTRUMENTS & METHODS IN PHYSiCS RESEARCH Sectton A

ELSEWER

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 demonstrated 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 variety 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 compound 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 detector 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 (LPVPE), 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 methods 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 dislocations. 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 gallium 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 crystal formed. Although high-purity materials are used in the growth, the resulting material is again found to be unacceptable 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

%I

The growth of high purity GaAs films by epitaxial methods is well known. Impurity levels less than 1 x 1014cmp2 are routinely achieved with several techniques. The thickness specifications of the detector type wafers, however, limit the growth methods severely. With present technologies, 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 supersaturated 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 required. In addition, the preparation procedures to reach the low doping leveis extend the growth process to unacceptable limits. The second technology is Atmospheric-Pressure VapourPhase Epitaxy (AP-VPE). This very popular process is used to grow millions of square inches of compound semiconductor epitaxial films each year for light-emitting diode applications. 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 routinely I x lOi cm-*. This method does not produce films with sufficient growth rates or low enough impurity levels to satisfy detector needs.

A~~osp~e~c Pressure WE Fig. 1.

Growth Rate vs Total Pressure loo0

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

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 demonstrated that as the pressure of the growth chamber was decreased from atmospheric values to reduced pressures, the growth rate decreased, as one would expect, untii the pressure 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 system manufactured by the Aixtron Corporation for production 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,

2

100

==8 -c J

B

B

p

9

A

10 l

t

l-4 1

10

A

A loo

i loo0

Pressure (mb) 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 backto-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 demonstrate the growth of GaAsP on GaAs to form light-emitting diodes (740 nm). Bv adjusting the alloy ratio, diodes emitting 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 LPVPE 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 geometries. 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 starting substrate. If this hetero-epitaxy concept can be implemented, 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

R.L. Adams1 Nucl. Ins@. and Me&

128

in P&w. Rex A 395

ji997J

U-128

Table 1 LP-VPE system data Materials grown

Maximum

GaAs on GaAs. GaAs on GaP GaP on Gap, GaAsP on GaAs GaN on Sapphire, GaN on silicon GaAs on Silicon, GaP on silicon

no. of wafers/run

Typical thickness uniformity

Typical thickness variation Typical background Typical mobilities

demonstrated

for GaAs

20 wafers of 2 in. diameter

on single wafer of GaAs

2 in. wafer f 5% 3 in. wafer * 5%

on multiple wafers of GaAs

2 in. wafers f

doping levels for GaAs for background

Typical compensation

15%

1-3 x 1014 cm-3 6000 cm2jV s at 300 K 120000cm2/Vs at 77K

doping for GaAs

ratios for GaAs

1.1-1.3

Lowest doping level for GaAs

6 x 1013 cmp3

Highest mobilities

8000 cm2/V s at 300 K 170000cm2/V s at 77K

for GaAs

Typical GaAs growth rates

60-100

Highest GaAs growth rates

200 Fmjh

Highest GaP on GaP growth rate

100 pm/h

Highest GaN on sapphire growth rate

60 pm/h (FWHM of X-ray 7 arcmin and PL value FWHM - 6 meV at 2 K)

Typical alloy uniformity

Single wafer f 1% Multiple wafers * 3%

for GaAsP on GaAs

pm/h

Typical total gas flow in system

2 l/min

Typical dopants

n-type Te; p-type Zn

Typical growth pressure

l-3 mbar

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

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 semiconductor films. Additional development work focused on specific device markets will accelerate the use of this new technology at a very rapid pace. The use of the LP-VPE technology

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 commercial

market

opportunities.