Supersonic jet epitaxy of III-nitride semiconductors

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GROWTH Journal of Crystal Growth 178 (1997) 134 146

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Supersonic jet epitaxy of III-nitride semiconductors B.A. Ferguson, C.B. Mullins* Department of Chemical Engineering, UniversiO, Of Texas at Austin, Austin, Texas 78712-1062, USA

Abstract In this review we report on the application of supersonic jets towards growth of the IlI-nitride semiconductors AIN, GaN, and InN. Certain properties of supersonic jets provide unique advantages for film growth. A seeded jet is able to accelerate precursors to kinetic energies which are one to two orders of magnitude higher than the average kinetic energies in a typical CVD environment, which has been shown to enhance precursor adsorption and thus growth rate. Indeed, dual seeded supersonic jets have been effectively employed to grow GaN and A1N from organometallic precursors and ammonia. Supersonic jets can also be coupled with excitation sources to provide highly reactive precursors for film growth. For example, a supersonic plasma jet has been developed to generate atomic nitrogen, and this source has been used together with an effusion cell for gallium delivery to produce epitaxial GaN films. The tightly focused jet exit stream generates a very high peak flux at the centerline which produced a film growth rate of 0.65 tam/h. However, deposition nonuniformity is quite dramatic due to this focusing and due to the point source nature of supersonic jets. Relatively few studies of llI-nitride supersonic jet epitaxy have been reported, so further work is needed to evaluate the usefulness of this growth technique.

1. Introduction There has been considerable recent interest in column III-column V nitride (III-N or III-nitride) semiconductor materials due to their suitability for optoelectronic devices. Many excellent review articles have been written on the III-nitrides [1 6], and some of the more significant recent advances have attracted considerable attention [7 11]. Except for boron nitride (BN), which is an indirect semiconductor, the III-N semiconductors have large, direct

*Corresponding author. E-mail: [email protected]. edu.

bandgaps (see Table 1). InN, GaN, and A1N have bandgap energies from 1.95 to 6.28 eV, corresponding to wavelengths from orange to the ultraviolet. This range of energies makes them applicable for short wavelength visible to UV optoelectronic devices, which is outside the range covered by conventional III V semiconductors. Of the III-N semiconductors, gallium nitride has attracted the most research interest. Many technical obstacles have been overcome in recent years which had previously impeded progress in G a N optoelectronics applications. One of the biggest problems was the inability to produce ptype G a N with in situ doping. It has since been demonstrated that hydrogen present in the growth

0022-0248/97/$17.00 Copyright ~; 1997 Elsevier Science B.V. All rights reserved PII S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 0 8 0 - 8

B.A. Ferguson, C.B. Mullins / Journal of Crystal Growth 178 (1997) 134-146

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Table 1 P r o p e r t i e s o f I I I - N i t r i d e s a n d o t h e r selected s e m i c o n d u c t o r s

B a n d g a p (eV) D i r e c t (D) o r I n d i r e c t (I) Z i n c b l e n d e (Z), W u r t z i t e (W), o r D i a m o n d (D) Lattice P a r a m e t e r s (~,)

InN

Cubic GaN

Hexagonal GaN

A1N

BN

Si

GaAs

1.95 D

3.26 D

3.45 D

6.28 D

6.4 I

1.11 I

1.4 D

W

Z

W

W

Z

D

Z

a = 3.54 c = 5.70

a = 4.54

a = 3.19 c = 5.19

a = 3.11 c = 4.98

a = 3.62

a = 5.43

a = 5.65

ambient will passivate p-type dopants, and a postgrowth annealing step is required to activate the dopants [12]. Another significant challenge for the growth of high-quality GaN films is the lack of a suitable substrate. The various crystal faces of sapphire (At203), SiC, Si, and GaAs have typically been used. Basal plane sapphire (0 0 0 1), which has a lattice mismatch of approximately 16%, has been widely used because of its high purity, availability, and past success with GaN heteroepitaxy. However, it has been difficult to produce high-quality GaN layers, with cracks forming due to thermal mismatch. While an ideal substrate for GaN epitaxy has yet to be developed, the use of a buffer layer of GaN or A1N deposited at low temperatures (400-600°C) has been shown to partially alleviate the substrate mismatch difficulty E13-21]. The buffer layer is amorphous when deposited at the low temperature, but crystallizes when heated to the growth temperature, and thus provides a template for epitaxial growth. Significant improvements in the quality of GaN films have been achieved with this two-step growth technique. These recent advances have contributed to the production of the first GaN-based blue LED by Nakamura et al. [-9]. This is a very important achievement due to the lack of an efficient blue LED which would complete the color triad (red, green, and blue) needed, for example, to produce LED color displays. In addition, Nakamura has also recently demonstrated the first nitride-based blue laser, which is another important and exciting development [7].

Although AIN and InN have been the subject of considerably less study than GaN, they are very important for device applications which require alloying with GaN for obtaining desired film properties. Among the III-nitride semiconductors, A1N has the highest direct bandgap energy (6.28 eV). This large gap makes A1N suitable for high-energy optoelectronic devices operating in the UV, such as a solar blind UV photodetector. With such a large bandgap, A1N could also be used as a barrier material in heterostructure devices. Indium nitride has been studied the least of the III-V nitrides. This material decomposes at 500°C in N 2 , and at 300°C in air, necessitating care in selecting the growth environment. With its comparatively low, direct bandgap of 1.95 eV, InN offers little that cannot be achieved with the well-established III-V arsenides and phosphides. However, its value lies primarily in its ability to form a ternary alloy with GaN, where the composition of such an alloy determines its bandgap and lattice parameter. The bandgap energy range which is covered by Inl-xGaxN includes the blue to violet region of the visible spectrum which, as mentioned above, is a very important region for the development of optoelectronic devices. Much progress has been achieved in the growth of the III-nitrides using metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Supersonic jet epitaxy, which is a relatively new technique, has also generated much interest in the growth of epitaxial semiconductor thin films [22]. A jet is an interesting tool with

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regard to film growth due to the directional, highintensity flux of precursors that can be produced [23]. In addition, by seeding growth precursor molecules in a lighter carrier gas, the precursors can be accelerated to kinetic energies well above thermal levels [23-25]. This acceleration has been shown in many chemical systems to enhance the reaction probability of precursor molecules by accessing the "direct" dissociative chemisorption reaction pathway, wherein the incident kinetic energy is used to overcome the activation barrier to chemisorption.

2. Review of the use of supersonic jets for film growth 2.1. Review of supersonic expansions Kantrowitz and Grey first suggested in 1951 the use of a supersonic expansion for a high intensity beam source [26]. Ever since, there has been considerable interest in the properties and applications of supersonic expansions. A supersonic jet inherently provides a higher directed intensity than an effusive beam source. However, due to its point source nature a supersonic expansion yields nonuniform deposition when applied to film growth. A supersonic jet also generates a much tighter velocity distribution of the exiting gas molecules compared to effusive sources. In addition, high kinetic energies ( > 1 eV) can be imparted to molecules by using the seeding technique. Put simply, a supersonic jet is a stream of gas molecules created by expanding a gas from high pressure ( > 1 0 0 T o r r ) to low pressure ( < 10 -3 Torr) through a small (20-1000 gm) aperture. During the expansion, the stagnation enthalpy of the gas is converted into directed kinetic energy. Assuming a constant heat capacity ratio (~ = Cp/Cv), the maximum stream velocity Uo~ achievable from a pure jet is /2yR To, u~ = X/m(y -- 1)

velocity a jet can achieve, there are several experimental parameters which may prevent the supersonic jet from achieving the terminal velocity given by Eq. (1). During an expansion, gas molecules experience a number of collisions with other molecules as they exit the small aperture. These collisions direct the molecules out of the aperture, primarily along the centerline. At some point in the expansion, the intermolecular collisions cease as the flow transforms from the viscous regime to the molecular regime. The collisions in the expansion also serve to efficiently convert rotational energy into directed kinetic energy, while the vibrational state population remains essentially the same as in the nozzle [25, 27]. Once these collisions cease, the molecules in a single component jet are all traveling at approximately the same velocity. Since the molecules exiting the nozzle orifice are highly directed, the flux distribution incident on a flat surface will be nonuniform. In fact, it has been shown analytically and verified experimentally that a single component supersonic jet will have a flux distribution proportional to cos 4 0, where 0 is the angle from the nozzle axis [25]. For comparison, an effusive point source (one in which the source flow occurs by molecular flow) yields a flux distribution proportional to cos 0, so the uniformity from an effusive source is somewhat improved. A technique for increasing the kinetic energy of growth molecules is to "seed" the precursors in a carrier gas with a smaller molecular or atomic weight (e.g. He or H2). During expansion out of the nozzle, the lighter, faster-moving carrier gas will accelerate the more massive precursor molecules through intermolecular collisions, thereby increasing the kinetic energy of the precursor. The maximum precursor kinetic energy Ep~ that may be achieved in this manner occurs when the precursor and carrier gas velocities are equal after the expansion. mp

(1)

where To is the nozzle temperature, R is the gas constant, and m is the molecular mass [25]. While this equation is useful for predicting the maximum

Ep~ = 7 2 7 (Cp)To, \rn2

(2)

where mp is the precursor molecular weight, ( m ) is the molar average gas mixture molecular weight, and (Cp) is the molar average constant pressure

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heat capacity (assumed to be constant in the expansion) [25]. Kinetic energies obtained with this technique can easily exceed 1 eV (which is 1-2 orders of magnitude greater than average thermal kinetic energies), but may be limited by experimental conditions. One might think that simply increasing the mass ratio (mp/{m)) will increase the precursor kinetic energy, but as the difference in masses increases, so does the difference in final velocities, limiting the kinetic energy. This effect is referred to as velocity slip, and has been quantified in many studies [28]. The quantity Pod (where Po is the nozzle stagnation pressure and d is the aperture diameter) is very important in determining the velocity slip and thus the terminal velocity [25]. The number of gas-phase two-body collisions scales with Pod, so this quantity can be thought of as a measure of how complete the expansion is, with higher values giving better approximations to the ideal kinetic energy given by Eq. (2). For example, Abuaf et al. have employed a jet of 1% Xe in He expanding from a 328 pm nozzle aperture with a stagnation pressure of 25 Torr, and showed that the Xe attained only 69% of the maximum achievable velocity, while a nozzle pressure of 700 Torr increases the Xe velocity to 99% of the maximum [29]. One interesting and problematic side effect of mixing gases in a jet is the focusing of the heavier component to an angular distribution significantly tighter than that for a pure jet. Fernandez de la Mora et al. have observed and described the aerodynamic conditions leading to this effect for seeded jets of heavy molecules and fine particles [30, 31]. Other groups have also observed this effect under film growth conditions, as will be discussed below. The total flux deliverable by a jet is limited in most systems by pumping speed. In order to keep background interference at a minimum, it is necessary to maintain a pressure in the growth ambient low enough so that the mean free path is of the order of the nozzle to substrate distance. For separations of the order of inches, this requires pressures of 10 3 Torr or lower. Practically speaking, this puts limits on the nozzle pressure and orifice diameter which can be used in a given deposition system. In general, high pumping speeds are essential for supersonic jet deposition.

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2.2. Review of the dynamics of dissociative chemisorption The dynamics of dissociative chemisorption is an important issue in low-pressure CVD film growth since the growth rates are typically limited by surface processes. Molecular beams formed by skimming supersonic jets have been used extensively to investigate the dynamics of surface adsorption and decomposition of various molecules [32]. Semiconductor surfaces have not been studied in this way as thoroughly as various metal surfaces [33]. However, the knowledge gained from metal surface studies can be used to guide studies on semiconductor surfaces. For example, studies of the interaction of oxygen on Pt(1 1 1) [34] and Ru(00 1) [35], nitrogen on W(1 00) [36,37], and ethane on It(1 1 0)-(1 x 2) [38, 39] have elucidated reaction mechanisms which are involved in many surface chemical processes. It has been found that molecules which strike a surface with low kinetic energy have an energy-dependent probability of being "trapped" into a physically adsorbed potential well. Once in this so-called precursor state, there is a kinetic competition between desorption to the gas phase and chemisorption, where surface temperature provides the thermal activation for these processes. The trapping probability of an incident molecule decreases with increasing kinetic energy since an increasing amount of kinetic energy must be dissipated to the surface for physisorption to occur. This chemisorption mechanism is referred to as the "trapping-mediated" reaction pathway. In the limit of high kinetic energy, however, it has been found that the adsorption probability increases with increasing kinetic energy. This socalled "direct" reaction pathway utilizes kinetic energy to overcome activation barriers, and the molecule can thus directly access the chemisorbed state. Since kinetic energy, as opposed to thermal energy from the surface, provides a majority of the activation energy for adsorption, the dissociation probability is determined primarily by the incident kinetic energy, and is somewhat independent of surface temperature. One of the most studied systems is the interaction of methane with various metal surfaces such as W(1 1 0) E40], Ni(1 1 1) 1-41], Pt(1 1 1) [42], and Ir(1 1 0)-(1 x 2) E43~6]. In these

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studies of methane and the other studies mentioned above, the reaction probability was shown to increase with kinetic energy, with only a weak dependence on surface temperature. For example, the reaction probability of 1.0 eV CH 4 on Ir(1 1 0) was 0.2, completely independent of surface temperature over the range of 550 1150 K [-45]. Some semiconductor systems have been studied which display dissociative chemisorption dynamics consistent with the direct reaction pathway. Engstrom et al. have employed supersonic molecular beam scattering techniques to investigate the dissociative adsorption of silane (Sill4) and disilane (Si2H6) on Si(1 0 0) and (1 1 1) surfaces and also to study methylsilane (CH3SiH3) dissociation on 13SIC(1 00). Translational energies in excess of 0.5-1.0 eV were found to dramatically increase the dissociative adsorption probability of these molecules [47 51]. This enhancement is ascribed to a transition from the trapping-mediated reaction pathway at low energies to a direct dissociative adsorption pathway at high energies. The results of these surface dynamics investigations have strong implications for growth of semiconductor thin films using free jets. In particular, a high reaction probability can be achieved, relatively independent of substrate temperature, with sufficient precursor kinetic energy. However, the film growth rate may be limited by the rate of desorption of deposition by-products (e.g. H 2 desorption from Si using SizH6) , requiring substantial wafer thermal energy.

2.3. Review of supersonic jet epitaxy Supersonic jet film growth is a relatively new technique which is still in its infancy. However, there have been a few studies of the application of supersonic jet epitaxy to the growth of crystalline semiconductor films. While the examples in the following section do not describe growth of IIInitrides, they are directly relevant to supersonic jet epitaxy in general. In one of the first attempts to grow semiconductor films from jets, Eres et al. used a pulsed jet of 5% digermane (Ge2H6) seeded in He to grow heteroepitaxial films of Ge on GaAs(1 0 0) surfaces at 480-680°C. The pulsed jet gave remarkably high

growth rates along the jet centerline, approaching 100 nm/s [52]. This large peak Ge growth rate is attributed to a combination of the high digermane flux delivered to the surface by the pulsed supersonic jet (up to 102o cm-2 s 1 with a nozzle-substrate separation of 1 cm) and the high thermal decomposition rate of digermane on the surface [53, 54]. However, we estimate that the digermane kinetic energy is likely approaching 1.2 eV, which may also enhance the film growth rates. The measured film thickness was found to vary dramatically as a function of angle from the nozzle axis due to the focusing effect. In fact, the film thickness distribution was found to agree quite well with the measured flux distribution of digermane exiting the nozzle (which followed a c o s 22 0 relationship) [55]. A pulsed jet has also been employed in the homoepitaxial growth of GaAs [56, 57]. Unseeded trimethylgallium (TMG) was delivered with a pulsed supersonic jet to a heated GaAs(1 0 0) surface, along with a continuous arsine (ASH3) flux from a standard cracking effusion cell. The GaAs growth rate in this system was dramatically lower than in the pulsed jet study of Ge epitaxy mentioned above. However, submonolayer controlled growth could be achieved by adjusting the duty cycle of the jet pulses. This level of control over the growth of GaAs could be useful in the growth of some types of heterostructures where alternating film layers must be of a precise thickness. Pacheco et al. have achieved Si homoepitaxy by directing a supersonic jet of disilane seeded in H2 at a Si(1 0 0) surface heated to 500-650°C. Enhancement of the reaction probability with increasing kinetic energies was shown by comparing growth from a seeded jet to both an unseeded (low energy) jet and ultra-high vacuum chemical vapor deposition (UHV-CVD) [58]. These results are consistent with the surface science measurements made by Engstrom et al. [47-51], and demonstrate the usefulness of applying surface dynamics studies to film growth. A cos2°0 growth distribution was measured from Si grown using a supersonic jet of 1% Si2H6 in Hi [59], again illustrating the dramatic focusing effect of seeded jets. With disilane seeded in He, the growth profile improved somewhat to a cosS0 distribution. This improvement

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in uniformity with He seeding results from more efficient momentum transfer (in the radial direction) between the disilane and the carrier gas due to more similar molecular weights, although the precursor kinetic energy is lower in the helium-seeded case due to a smaller mass ratio (mp/(m)). The resulting homoepitaxial silicon films were single crystalline, and have a smooth surface morphology as determined by atomic force microscopy (AFM) [60]. It should be noted that the growth rates in this study were limited by hydrogen desorption, and the application of supersonic jets to epitaxial growth may be more beneficial in a system or growth regime without such a limitation. Other groups have attempted growth on Si(1 0 0) at lower temperatures (400~50°C) with a pulsed jet of SizH 6 seeded in H 2 [61, 62]. However, the growth rates were very low (< 2 A/min), as deposition was again limited by the rate of hydrogen desorption at these low temperatures. Instead of using kinetic energy to activate growth, jets may be coupled with other excitation techniques which create highly reactive precursors for film growth. One very simple method is to heat the nozzle to a temperature at which significant pyrolysis occurs, producing very reactive radicals. By using a jet which is supersonic, the source is able to deliver a very high intensity of precursors to the growth surface. One example of this technique was demonstrated by Lee et al., who used a supersonic pyrolysis jet of methyl radicals along with a concurrent flux of atomic hydrogen from an effusive plasma source to grow diamond crystallites from small seed crystals at temperatures of 65(~850°C [63]. A plasma can also be created within a supersonic jet nozzle, producing a highly reactive effluent. This type of arrangement was used to grow the Si3N4 insulating layer of a metal-nitride-Si (MNS) capacitor [64]. A jet of silane (Sill4) seeded in He, concentrically surrounded by an effusive source of N2 in He, were both enclosed in a microwave cavity. Due to the highly reactive nature of the excited precursors, the Si3N4 films could be deposited on a silicon substrate at room temperature. The MNS capacitors displayed interface trap densities comparable to a good SiO2/Si interface, and high resistance to radiation and hot electron damage, which demon-

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strate the high quality of the Si3N4 films deposited from supersonic jets.

3. Activated nitrogen for IIl-nitride growth 3.1. MOCVD and modified MBE growth issues Metalorganic chemical vapor deposition is a very successful technique for the growth of GaN films. The high growth temperatures required (> 1000°C) are helpful in producing high-quality epilayers. In fact, one of the highest crystalline quality GaN films, as determined by a high-resolution X-ray diffraction (HR-XRD) rocking curve FWHM of 37 arcsec, was grown by MOCVD [65]. For comparison, modified MBE growth of GaN has only produced FWHM values as low as ,-~5 arcmin despite the lower growth temperatures and thus a lower level of thermally-induced strain compared to MOCVD [66-68]. The growth rates achievable with the high temperatures in MOCVD growth are also correspondingly high: in excess of 1 lam/h. However, these high growth temperatures can have deleterious effects as well. Interface sharpness degradation and phase separation of ternary alloys can both occur via thermally activated diffusional processes. Despite these latter drawbacks, MOCVD has attracted more attention from the semiconductor industry than MBE, in part because it is better suited for scaleup to high production levels. As mentioned above, it is difficult to produce p-type doping with the MOCVD technique. MOCVD-grown GaN films which are doped with Mg or Zn in situ typically have a high resistivity, though slightly p-type. Amano et al. discovered that low energy electron beam irradiation (LEEBI) of Mg- and Zn-doped MOCVD-grown films would activate the previously inactive dopants [69, 70]. Soon after, Nakamura et al. demonstrated that annealing Mg-doped GaN films in nitrogen would activate the Mg dopant just as well as the LEEBI treatment [12]. Since the Mg dopants could be inactivated again by annealing in an NH3 ambient, it was concluded that hydrogen was passivating the Mg dopants [71]. The precursors which are employed in MOCVD processes are typically hydride

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gases of the required compound semiconductor elements. As such, hydrogen is always present in the MOCVD growth environment, necessitating an extra annealing step to remove the hydrogen from p-doped films. On the other hand, the majority of MBE precursors for III-N growth do not contain hydrogen, obviating the need for a post-growth anneal step [72, 73]. Another advantage of MBE techniques is the ability to grow high-quality epitaxial GaN at significantly lower temperatures (600-750°C) compared to MOCVD (> 800°C), thereby limiting the diffusional processes which lead to degradation of interfaces and phase separation. Lower growth temperatures have been achieved through the use of "activated" nitrogen, which refers to a nitrogen bearing species that has been energetically promoted to a state which is more reactive. This can be ionic, atomic, translationally accelerated, or combinations of the above.

3.2. Effusive sources of activated nitrogen Most growth techniques which use activated nitrogen for the growth of III-nitrides have employed nitrogen sources which are effusive in nature. Within the effusive category, there are three primary methods of generating the activated nitrogen: ionization, electron-cyclotron-resonance (ECR) discharges, and radio frequency (RF) plasmas. There has been comparatively little work on the use of ionic nitrogen species for III-nitride growth. Fu and co-workers grew GaN on sapphire (0 0 0 1) and ~-SiC(0 0 0 1) at 60(~750°C using reactive ionbeam molecular beam epitaxy (RIMBE), wherein a discharge plasma ion source was used to deliver primarily N~- ionic species, along with Ga from an effusion cell [74]. The substrate was biased with a variable retarding voltage to investigate the effect of energetic ion bombardment ( ~ 1(~30 eV) on film quality. Optical film properties improved with bias voltage, but the crystal quality as determined by HR-XRD did not. Biasing of sapphire substrates resulted in nonuniformity of film properties across the wafer due to sample charging effects. However, the use of conductive ~-SiC substrates significantly improved the uniformity. In another study, the

same group successfully produced p-type GaN with Mg doping, confirming the ability of MBE to produce p-type GaN without a post-growth anneal [73]. However, crystal quality remains an issue. Greene et al. has studied the use of N~- and N H + ionic species as nitrogen precursors in RIMBE [-75 77] using a similar plasma discharge ion source. Single crystalline epitaxial GaN films were grown with this method at 450-850°C on sapphire and MgO substrates, and the epitaxial orientational relationships and microstructures were studied extensively with transmission electron microscopy (TEM), XRD, and reflection high-energy electron diffraction (RHEED). It was found that GaN film growth using N~ suffered from ioninduced damage and Ga desorption with increasing ion energies (3(~90eV) at temperatures of 45~700~'C. However, the film quality improved significantly with growth temperature, with a minimum X-ray rocking curve F W H M of 5 arcmin. GaN growth using NH + was found to deliver enough nitrogen into the growing film to allow higher growth temperatures of 800-850°C, resulting in improved crystal quality and carrier mobility. However, it is unknown whether ionic NH + or co-delivered neutral ammonia is the dominant nitrogen precursor for film growth at these higher temperatures. In addition to using ionic nitrogen species, the column lII metal may be ionized and accelerated to the growth surface, as in the plasma-assisted ionized source beam epitaxy (PAISBE) growth technique [78]. GaN was grown by PAISBE on sapphire (0 0 0 1) at 60~800°C with an effusive nitrogen RF plasma source and a concurrent flux of gallium from a modified effusion cell. Ga + was produced with an electron beam ionizer, and accelerated with an array of biased plates. The results of this preliminary study indicate that the use of unaccelerated Ga + leads to a significant improvement in film crystallinity over a neutral Ga source. Interestingly, lattice damage in GaN films grown with the 400 eV accelerated Ga + ions is not apparent, as the film crystallinity was similar to that grown with unaccelerated Ga +. The bulk of GaN growth with effusive sources of activated nitrogen has been done with ECRtype microwave sources and a simultaneous flux

B.A. Ferguson, C.B. Mullins /'Journal of Crystal Growth 178 (1997) 134-146

of atomic Ga from an effusion cell. In a typical configuration, a cylindrical cavity filled with N2 at a low pressure is excited by a coaxial solenoid magnet which couples 2.45 G H z microwave energy to the N2 gas [79]. This type of activated nitrogen source delivers an effusive stream of atomic, ionic, and molecular nitrogen to the substrate. Typical N2 dissociation efficiencies of up to 10% are reported with such an apparatus [67, 80], yielding G a N growth rates of 50 nm/h to 0.2 lam/h [67, 68, 80, 81]. In this regime, the growth rate is limited by the flux of activated nitrogen delivered to the surface [1]. Higher microwave power levels can be applied to increase the growth rates up to 0.65 lam/h [81], but this leads to degraded film quality, presumably due to energetic ion bombardment. To date, the crystal quality of G a N films produced with this growth technique [66 68] is not as good as has been achieved using M O C V D (5.2 arcmin and 37 arcsec X-ray F W H M , respectively) [65]. Radio frequency (RF) plasma sources of activated nitrogen can also be used to produce both G a N and InN. A 13.56 MHz RF effusive plasma source was used to deliver activated nitrogen, along with coincident In or Ga fluxes from effusion cells, for growing InN on GAP(1 1 1), or G a N on GaAs(1 0 0), respectively. Due to the thermal instability of InN, films were grown at temperatures of 300~00°C while G a N films were grown at 600°C [82]. X-ray rocking curves were not measured in this preliminary study, preventing comparison of film qualities.

3.3. Supersonic jet sources of activated nitrogen Supersonic plasma jet sources have been produced which provide large fluxes of activated nitrogen. In particular, Pollard described a radio frequency discharge nozzle jet which produced beams of atomic nitrogen [83]. This design utilized a RF coil operating at 39 MHz and 100 W which was placed coaxially around a ceramic nozzle tube with a restricted exit orifice. The restricted orifice allows for a higher nozzle pressure than effusive sources, which provides the hydrodynamic flow conditions

141

necessary for a supersonic expansion. Beams of nitrogen seeded 1.75% in He yielded dissociation fractions of up to 60%, which is most likely aided by the reduction in recombination of dissociated nitrogen by the carrier gas. Activated nitrogen flux intensities of up to 1.5 x 1018 sr- 1 s- 1 were produced by this jet arrangement, whereas effusive sources are typically one to two orders of magnitude lower. Lieber et al. used such a plasma supersonic jet source to produce a new synthetic material, [3C3N4 [84]. Due to its strong, short covalent bonds, this material was predicted to be as hard or harder than diamond. [3-C3N 4 w a s synthesized by directing the plasma jet at a substrate held at 165-600°C with concurrent pulsed laser ablation of a graphite target. Interestingly, the growth temperature had little effect on the C ' N ratio in the films. The synthesis of this unique material was assisted in this case by the high flux of reactive atomic nitrogen from the supersonic jet source. A similar supersonic plasma jet nitrogen source was employed for G a N growth by Sellidj et al. [85, 86]. A plasma jet based on the design of Pollard's jet source was modified for operation at 13.56 MHz. A nozzle with a 100 gm aperture was pressurized to 120 150Torr with a mixture of 1% N2 in He, yielding a deposition chamber pressure of 5 x 10 .4 Torr. The RF coil, which is supplemented with a tuning circuit for impedance matching, was operated at 150 W RF power, and the resulting plasma jet was directed at sapphire (0 0 0 1) substrates with a concurrent Ga flux from a standard effusion cell for G a N growth. After substrate cleaning and before deposition, the sapphire surface was exposed to the atomic nitrogen jet without a Ga flux, which nitrided the surface to provide a thin A1N layer for epitaxial growth. A two-step G a N deposition procedure was used, which included deposition of a 500 ~, G a N buffer layer at 500°C and deposition of G a N films at 600-750°C. This procedure has been shown to promote 2-D layer-by-layer growth [67, 81]. All G a N films were epitaxial single crystalline, with the basal planes of the film and substrate aligned. The X-ray diffraction peak F W H M decreased with increasing growth temperature, down to 38 arcmin at 750°C. Although a F W H M of 38 arcmin is comparable to

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other MBE-based GaN growth techniques, the crystal quality in this case is probably degraded by energetic ion bombardment from the plasma source. Thus, appropriate biasing techniques could be employed in an attempt to increase the crystal quality. GaN growth rates of approximately 0.65 gm/h were achieved at 600-750°C using constant reactant fluxes, independent of substrate temperature. This lack of growth rate dependence on temperature is likely due to the high sticking probability of the atomic nitrogen and gallium delivered to the growth surface. Decreasing X-ray FWHM values with higher growth temperatures therefore indicate that the substrate thermal energy served primarily to activate diffusional processes which increase the crystal quality of the growing film. Relatively high growth rates were achieved in this system due to a combination of several factors: a short nozzle-tosubstrate separation (2 in), high dissociation efficiency (~ 65 %), and the inherently high intensity of a directed supersonic jet. Use of a 1% N2 in He seeded mixture provided for efficient delivery of dissociated nitrogen due to a reduced radical recombination rate. Other inert gases such as argon could have been used as the carrier gas as well, and would likely have provided for increased deposition uniformity across the wafer due to more efficient momentum transfer between the carrier gas and nitrogen. Argon would have also made a better plasma gas medium, with its 35% lower first ionization potential. In addition, seeding is typically used in supersonic jet deposition to increase the sticking probability by accelerating the precursor molecule to kinetic energies above thermal levels. However, in this case the precursor kinetic energy will most likely not increase the sticking probability because the atomic nitrogen is already quite reactive. Indeed, the precursor kinetic energy might hinder growth because it must be dissipated by the surface for the nitrogen atom to stick. On the other hand, it is possible that the precursor kinetic energy may help in ordering surface adatoms through local heating caused by the collision of energetic molecules. This effect has been predicted to improve crystal quality [87-89], although kinetic energies that are too high (> 1(~20 eV) may cause surface damage.

4. Supersonic jets of molecular precursors Atomic or activated nitrogen as a reactive nitrogen precursor can be replaced with supersonic jets of nitrogen-bearing molecules for III-N growth. As mentioned above, the reaction probability of many precursors can be increased by the translational acceleration provided by a seeded jet. This enhancement can play a significant role in the growth of III-nitrides because most growth techniques are rate-limited by the amount of reactive nitrogen delivered to and incorporated into the film. Many of the growth methods described above use N2 as a nitrogen source, which is one of the most difficult molecules to break apart due to its strong triple bond. Ammonia, by comparison, has a bond dissociation energy roughly half that of N2 (4.7 eV for NH3 vs. 9.8 eV for N2), which should facilitate the incorporation of nitrogen into a growing film. Despite this advantage, dissociative chemisorption of ammonia is likely the rate-limiting step in MOCVD GaN deposition, with growth temperatures above 800°C necessary for epitaxial growth [6]. In fact, ammonia adsorbs on GaAs(1 0 0) only at low temperatures (less than 150 K), and desorbs intact at 300 K with only trace amounts of decomposition [90, 91]. Thus, the energy barrier to dissociative chemisorption is likely above the vacuum zero potential level, necessitating some sort of activation for surmounting the barrier. Supersonic jet epitaxy of III-nitrides using seeded jets of ammonia may be able to overcome this difficulty using increased precursor kinetic energy to enhance the reaction probability. Additionally, instead of using conventional effusion cells for delivery of column III atoms, supersonic jets of organometallic molecules can be employed. Since supersonic jet epitaxy relies primarily on surface decomposition of precursors, it is instructive to look at the surface chemistry of these organometallic molecules. The initial dissociative chemisorption probability of triethylgallium and trimethylgallium on GaAs(1 0 0) is of the order of unity at room temperature. However, this reaction probability was observed to decrease as the normal component of precursor kinetic energy increases up to ,--0.8 eV, which is consistent with a trappingmediated reaction pathway [92-95]. It should be

B.A. Ferguson, C.B. Mullins / Journal of C~stal Growth 178 (1997) 134 146

noted, however, that these measurements were made on clean surfaces, and that the surface will have a temperature- and flux-dependent steadystate coverage of alkyl ligands under growth conditions, thereby covering some adsorption sites. Since organometallics such as those used in MOCVD have a significantly higher molecular weight than typical seeding gases (H2 or He), the theoretical precursor kinetic energy in a seeded jet (as given by Eq. (2)) can be quite high (much greater than 1 eV), although velocity slippage will limit the actual kinetic energy attained. This kinetic energy may not benefit organometallic precursor adsorption, because the surface chemistry studies on clean GaAs give no indication of a direct reaction mechanism, although the situation may be somewhat different on a GaN growth surface. In addition, organometallic precursors with ethyl or longer-chain ligands may remove ligands from the surface via the [3-hydride desorption pathway, and will most likely lead to lower carbon levels in the film than methylated organometallic precursors [96, 97]. Finally, it should be noted that the introduction of hydrogen into the growth environment will likely require a post-growth annealing step to activate p-type dopants. Ho et al. have used separate supersonic jets of triethylaluminum (TEA) and NH3, each seeded in N2, He, o r H 2 to grow A1N at 550-775°C on Si(1 1 1) and (1 0 0) substrates [98, 99]. Single crystalline epitaxial A1N films were obtained only on Si(1 1 1), while growth on Si(1 0 0) produced highly oriented polycrystalline films. In addition, it was found that the surface preparation method had a significant effect on the growth of aluminum nitride. Growth on as-received Si wafers was minimal, while wafers etched in H F to remove the native oxide yielded a 200 nm thick A1N film under the same growth conditions, and an additional in situ sputter-anneal cleaning cycle increased the A1N growth to 350 nm. The effect of the choice of seeding gases, and therefore the precursor kinetic energy, was also studied. Considering the surface dynamics measurements described above, one would expect TEA to adsorb relatively easily on the growth surface, while NHa adsorption should limit the growth rate. Based on this, the ammonia kinetic energy should influence the A1N growth rate more

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than that of TEA. On the contrary, it was found that the growth rate increased dramatically with TEA kinetic energy, but only slightly with NH3 kinetic energy. In particular, seeding both precursors in N2 yields a growth rate of < 2.5 nm/h, while seeding TEA or NH3 in He yielded 22.5 and <2.5 nm/h, respectively, and seeding both in He gives 35.0 nm/h. However, it should be noted that TEA kinetic energies can be roughly an order of magnitude higher than those for NH3 with the same carrier gas, due to the large difference in precursor molecular weights. For example, the calculated maximum kinetic energies are 1.8 and 0.21 eV for TEA and NH3 seeded in He, respectively. More work is needed to investigate this interesting effect on the growth rate. Lamb et al. have employed separate supersonic jets of NH3 and triethylgallium (TEGa) seeded in He to grow GaN on sapphire at 58(~600°C [100, 101]. The preliminary results indicate that the stoichiometry of the deposited films, as determined by Auger spectroscopy, could be easily adjusted with the NH3 : TEGa flow ratio. Films grown directly on sapphire (0 0 0 1) substrates were composed of discontinuous crystalline whiskers. Increasing ammonia kinetic energy was shown to change the grain crystalline orientation from random to highly oriented, with the basal plane of each grain parallel to that of the substrate [1003. The use of a GaN buffer layer deposited at 500°C provided a template for continuous, but highly oriented polycrystalline films [101]. Another possible method for growth of III-nitrides using supersonic jets would be to use a single source precursor which contains both the column III atom and nitrogen. These precursors are heavier than the organometallics mentioned above, thus possibly resulting in even higher incident kinetic energies. These molecules are typically unstable, decomposing thermally at relatively low temperatures (< 300°C). This will most likely lead to a high reaction probability upon collision with the growth surface at high kinetic energies. One possible candidate for jet growth of GaN from a single source precursor is bis(dimethylamido)gallium azide, which has been shown to produce epitaxial GaN on sapphire at 580°C under CVD conditions, although with a high carbon concentration in the film [6, 102].

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The only study which demonstrates compound semiconductor film growth from a supersonic jet of a single source precursor has been reported by Ferguson and co-workers [86]. A jet of (t-Bu)2(GaAs)(t-Bu)2 seeded in He was directed at a heated GaAs(1 0 0) substrate to deposit epitaxial GaAs at 400-500°C. The maximum growth rate was only 0.05 gin/h, which was limited primarily by the low precursor vapor pressure of 10-2 Torr, resulting in a low jet seeding fraction and thus a low flux. In addition, the film thickness was nonuniform, as expected for supersonic jet deposition. It should be pointed out that since single source precursors typically have very high molecular weights, uniformity will suffer not only from the point source nature of a jet, but also from the focusing effect in seeded jets which becomes more pronounced as the mass ratio with the carrier gas increases.

5. Summary The growth of III-nitride semiconductors is a very active research field due to their applicability for short wavelength optoelectronic devices. GaN is the most widely studied material of the III-nitrides, and extension of the results to InN and A1N growth should be possible. Two general methods of application of supersonic jets for III-N growth have been demonstrated. In the first method, a supersonic plasma jet source can be used to provide atomic nitrogen for growth with a concurrent flux of a column III metal from an effusion cell. In this system, very high GaN growth rates (0.65 gm/h) were achieved due to the high flux of atomic nitrogen delivered from the jet. Argon may have served as a better carrier gas than helium for this application, due to its better plasma performance, and a possible improvement of the growth uniformity. Another III-nitride supersonic jet epitaxial growth method uses the acceleration of precursor molecules in a seeded jet to enhance reactivity. A seeded supersonic jet can accelerate precursors to kinetic energies one to two orders of magnitude higher than average thermal energies. This acceleration has been shown in many cases, including some semiconductor systems, to increase the reaction probability of the incident precursors. Indeed, the

growth rate of AlN deposited using supersonic jets of organometallic precursors and ammonia increased with precursor kinetic energy. Since supersonic jets are inherently directional, and heavy precursors seeded in jets of light carrier gases are focused even more within the jet, the resulting deposition is highly nonuniform. This arises from the fact that the cessation of intermolecular collisions occurs just outside the nozzle aperture. In order to apply supersonic jets to semiconductor manufacturing, a method for obtaining uniform deposition over large areas must be developed. The use of multiple jets would certainly help increase the area over which deposition will occur. However, a moving jet/moving substrate arrangement [-103] will most likely have to be employed to provide uniform deposition. In conclusion, supersonic jets have been shown to possess unique properties which can be utilized effectively to deposit epitaxial films of III-nitrides. The translational acceleration of precursors in a seeded jet provides for enhanced reactivity. However, the nonuniformity of the deposition produced by a jet needs to be eliminated for acceptance of this growth method by the industrial community. Epitaxial growth of semiconductors from supersonic jets is a relatively young field which is still developing. Much more work is needed to investigate the potential of this deposition method.

Acknowledgements The authors thank the National Science Foundation (NSF) for support through the UT-Austin Science and Technology Center for the Synthesis, Growth, and Analysis of Electronic Materials under Grant No. CHE8920120. Additionally, C.B.M. thanks the Office of Naval Research Young Investigator Program, the NSF Presidential Young Investigator Program, and the State of Texas Advanced Research Program for additional support.

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