Workshop on diamond thin films 777

Factorz Influencing the Propertia of Diamond Grown from the Vapor Phase

Yoichiro Sat0

National Institute for Research in Inorganic Materials l-1 Namiki, Tsukuba, Ibaraki 305, Japan

A number of different methods for growing diamond from the vapor phase on non-diamond substrates are known and practiced today. Each method has its own characteristics, or merit8 and demerit8 as viewed from a specific standpoint, e.g., for basic study of reaction mechanism or of fabrication of diamond8 for various uses.

The important common feature of all these diamond CVD methods is that the structures of the deposit8 depend on the rowth conditions, most critically on gas composition and substrate temperature. If condition8 ! eviate from the optimum, structural defect8 inVOlving spa carbon8 increase and further deviation leads to the formation of graphite structure. Such than es obvious1 nature of nucleation and growth of diamond from the vapor phase in which the ormation o ! 9

reflect the spJ and spa

bonding8 compete.

Detailed studies of the structures are, therefore, of great importance in understanding the properties of CVD diamond8 as well as the growth mechanism.

The present work ha8 been initiated with the intention to develop CVD as a new means of diamond synthesis which enables us to prepare diamond of reasonably high crystal quality appropriate for the fundamental studies of diamond as well as for optical and electronic applications. It would be plausible to assume that CVD has an advantage in impurity control over the static high pressure techniques. An apparent disadvantage at the present stage is the presence of defect8 unique to CVD diamond, which the author presumes to come predominantly from the residual Spl bonding. In the present study, attention is focused on the changes in the structure and properties as function8 of gas composition. The observations have been made with single crystals and polycrystalline films.

Individual crystals and polycrystalline film8 have been prepared by microwave plazma;lssisted CVD from gaseous mixtures of methane (0.36 ~01%) and hydro hydrogen and water vapor (ca.0.2 ~01%) at the total pressure of 40 !

en, and of methane (0.5-5 vol%), orr and at a substrate temperature

of 85O’C. The plasma etching was conducted in the same apparatus used for the growth, introducin oxygen gas or air in place of methane-hydrogen mixtures. The film8 and crystals have been studle .8 mainly by optical and electron microscopy, Flaman spectroscopy, cathodoluminescence and observation of patterns produced by reactive plasma etching.

SINGLE CRYSTALS: Individual crystals prepared under lower methane concentrations were found to consist predominantly of sin have been prepared in the presence o B

le crystals or twinned crystals. Single crystals of up to 100 ~UU water vapor at methane concentrations of 0.5 and 1%. Though

there is scatter in morphology and growth features, it ha8 been found that at higher methane concentrations, secondary nucleation take8 place predominantly on (111) face8, resulting in polycrystalline particle8 before they grow to a size of 100 p. The single crystals show Raman spectra in which a broad Ftaman band centered at 1450-1550 cm-1 due to spa structure is unappreciably weak and the diamond line is as narrow as that observed with high pressure synthetic diamond8 (1.8-2.Ocm-1). The peak position agrees with high pressure and natural diamond8 within 0.2 cm-l. Ftaman microprobe measurements showed a small but appreciable line width dependence on crystallographic face, i.e., line width of (111) face is slightly wider than that of (100) face.

The cathodoluminescence study showed that the single crystals exhibit the intrinsic emission (edge emission diamond, it is k

which is characteristic of high crystal quality diamonds. In the case of natural nown that the emission is observed only with type II diamonds.. The edge emission

proved to be undetectably weak for the individual crystals prepared at methane concentrations of 2% or higher.

Reactive plasma etching using oxygen or air proved to be a highly sensitive means for detecting defect8 in these particles.

It has shown that defect density and strain depend not only on gas composition but also on the crystallographic face8, i.e., (111) or (100).

POLYCRYSTALLINE FILMS: Polycrystalline films prepared under the same conditions employed for the single crystal growth proved to have Raman spectra entirely different from those of

778 Workshop on diamond thin films

the single crystals. The spectra consist of appreciable amount of non+liamond structures at 1450-1550 cm-r and the diamond line is about three times &B wide as that of the single crystals. Besides, the position is found to be shifted to the higher Haman~hift b l-2 cm-i even after removal o substrate, indicating residual stress of a compressive nature. (’

p”k the The line width decreased only by 20%

and the peak position wa8 little changed when a Haman microprobe with a focal spot size of about 1 p was used.)

The intrinsic emission has been hardly detected for the polycrystalline specimens.

Plasma etchin are: Etch pits have %ee

was found to be useful in identifying where the defects and disordered graphite n found mainly at grain boundaries, but they were also found invariabl

grains, except on whiskers described below. Stacking faults are commonly observed with the gi on the amond

grains in the films. It treema that etch pits are related to the etacking faults.

Structure and texture of the films have been observed to depend not only on gas composition but also on nucleation density and distance from the substrate. texture should also be noted.

Structural anisotropy due to columuar Compared with single crystals grown under the same conditions,

polycrystalline films proved to have much higher defect density aud an appreciable amount of strain.

DIAMOND WHISKERS: Besides lower methane conceutrations, another condition haa been found where high quality cryetals grow: At 3-4% methane concentrations, typical columnar growth of needle crystals or diamond whisker8 of l-3 jmr in diameter extendi been observed. The intergrain re

f ‘on8 consist mainly of fine-grained x

along the <lOO> direction has amend and dieordered

that can be readily removed by p asma etching. &cking faults.

The whisker8 were found to be gubat~ti~y aphltes

!L! from The Baman spectra of the whiskers have low non-diamond ~m~nents and a narrow

diamond line comparable to those of high pressure diamonds.

SUMMARY: The properties of CVD diamond depend critically on the gas composition and substrate temperature. In this work, changes in some of the properties have been measured with si le crystals and polycrystalline films grown under varying gas corn

P o&ions. Results and some of t “pi, e

conclusions derived from the present work may be summarized as ollows:

1)

2)

3)

4)

Sin hig g”

e crystals prepared at lower methane concentration and whiskers prepared at relatively er methane concentration were shown to be of high crystal quality aa defined by Haman and

emission spectra and etch patterns.

Polycrystalline films are subject to hi quality single crystals prepared under t ‘6

her defect density and hi e same conditions. This s

her strain compared with hi %

h OWE that mertsurements on t % e

single crystals are essential for unde~t~~~ the properties of CVD diamond and also for the correct diagnosie ofthe deposition conditions.

Haman spectroscopy and oxidative etching are simple and useful tools for the evaluation of the crystal quality of CVD diamonds.

It 8eem8 highly probable that defects unique to diamond are related to spl carbons. Attainment of higher control of chemical bandings, i.e., ap3 over spa, will permit ua to grow higher quality diamonds and larger single crystals.