Hydrogen effects in III-nitride MOVPE

ARTICLE IN PRESS Journal of Crystal Growth 310 (2008) 4862–4866

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Hydrogen effects in III-nitride MOVPE E.V. Yakovlev a,, R.A. Talalaev a, A.S. Segal a, A.V. Lobanova a, W.V. Lundin b, E.E. Zavarin b, M.A. Sinitsyn b, A.F. Tsatsulnikov b, A.E. Nikolaev b a b

STR Group – Soft-Impact Ltd., P.O. Box 83, 194156 St. Petersburg, Russia Ioffe Physico-Technical Institute, 194021 St. Petersburg, Russia

a r t i c l e in f o

a b s t r a c t

Available online 3 August 2008

Influence of hydrogen on the growth of III-nitride materials by MOVPE is discussed using modeling and experimental study. The main conclusion, coming from the modeling and supported by numerous experimental observations, is that hydrogen affects the growth of III-nitrides in two different ways: via layer etching at elevated temperatures and via surface coverage with metal adatoms. The adatoms are found to accumulate on the surface due to interaction with hydrogen in a wide temperature range, including reduced temperatures. With regard to these effects, one can control such important characteristics as layer composition, growth anisotropies, surface quality, and even material properties (like p-doping level) by adjusting the carrier gas composition and other growth parameters. & 2008 Elsevier B.V. All rights reserved.

PACS: 81.05.Ea 81.15.Gh 82.20.Wt Keywords: A1. Computer simulation A1. Etching A3. Metalorganic vapor phase epitaxy A3. Selective epitaxy B1. Nitrides

1. Introduction Hydrogen is normally used as a carrier gas for MOVPE growth of various semiconductor materials. However, unlike the case of conventional III–V compounds, hydrogen seems to be strongly involved in the gas-phase and surface chemical processes during the growth of III-nitrides, having a strong impact on the growth rates and layer composition, morphological and structural quality of the materials. It is commonly recognized that the hydrogen influence is pronounced at elevated temperatures (above 1000 1C), resulting in GaN etching. The effect of various operating parameters on the GaN etching rate has been studied in a number of papers (e.g., Refs. [1–3]). It has been found that the etching rate is a strong function of the wafer temperature, and the etching can be intensified by changing the gaseous mixture composition in favor of higher hydrogen (or lower ammonia) partial pressures. However, interaction of hydrogen with growing GaN plays an important role also at reduced temperatures (900 1C and below), when the etching rate is negligible and does not affect the overall growth rate. In particular, the adjustment of the H2/NH3 mixture composition during GaN deposition in the temperatures range of 750–900 1C was shown to improve the surface morphology via the overgrowth of pinholes up to their almost complete disappear-

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E-mail address: [email protected] (E.V. Yakovlev). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.07.099

ance [4]. It was concluded that pinhole overgrowth can be enhanced in the presence of metallic adatoms on the surface. With respect to selective epitaxy, the growth temperature and V/III ratio are the parameters largely governing the growth anisotropy. For instance, the authors of Ref. [5] have achieved the maximum anisotropy at high temperatures and ammonia flows. The appearance of slowly growing {11 2¯ 2} sidewall facets was reported in Ref. [6], if a low V/III ratio is used during c-GaN LEO. As the V/III ratio is raised, smooth vertical {11 2¯ 0} facets appear and lateral growth rate increases. The results presented in Ref. [7] indicate that a high level of anisotropy and full coalescence of a-GaN ELOG samples can be achieved only under the conditions corresponding to significant amounts of gallium on the surface. In this paper, we summarize our current understanding of the role of hydrogen in both planar and selective epitaxy of GaN layers of different orientation, based upon the process simulations and experimental study. A series of experiments has been carried out in both horizontal-flow single-wafer reactor Epiquip VP-50RP [1,8], redesigned for III-nitride epitaxial growth, and multi-wafer Planetary Reactors AIX2000HT. A kinetic model of hydrogen interaction with the surface of III-nitride layers was used for the analysis. Gas blending units of both systems allow the use of hydrogen, nitrogen or their mixture at any given proportion as a carrier gas. Ammonia, trimethylgallium (TMGa), trimethylindium (TMIn), trimethylaluminum (TMAl), and triethylgallium (TEGa) were used as precursors, Cp2Mg and SiH4 were used for doping. Growth and

ARTICLE IN PRESS E.V. Yakovlev et al. / Journal of Crystal Growth 310 (2008) 4862–4866

2. Results and discussion 2.1. Basic concept formulation and model verification A kinetic model of hydrogen interaction with the surface of IIInitride layers was originally suggested in Ref. [1], and model revisions have been made in the past to provide a better agreement with the available experimental data on GaN-etching rate and AlGaN growth rate/composition versus process parameters. Comparison of the model predictions and experimental data on the etching rate versus the temperature, hydrogen partial pressure, and carrier gas composition can be found in Ref. [1]. According to the developed model, interaction of GaN with hydrogen–ammonia ambient is described by the combination of two coupled processes, including reversible decomposition of GaN by H2 with the formation of free ammonia and adsorbed gallium (R1) and reversible evaporation of the adsorbed gallium (R2): GaNðsÞ þ ðadsÞ þ 1:5H2 2GaðadsÞ þ NH3

(R1)

GaðadsÞ2Ga þ ðadsÞ

(R2)

where (ads) denotes a vacant adsorption site and Ga(ads) is for the adsorbed gallium atom. The two reaction rates, W1 and W2, are described by the conventional relationships relating the forward and backward rate constants, species partial pressures, and surface coverage with gallium adatoms. Assuming that the adsorbed gallium is saturated at the surface, i.e. W1 ¼ W2, one can derive the relationships for the stationary surface coverage with gallium and for the overall GaN-etching rate. After that, the model allows the calculation of the etching rate and surface coverage with metal adatoms as a function of the hydrogen and ammonia concentrations, reactor pressure, and substrate temperature. The reaction rate constants have been adjusted to provide the best agreement of the calculated GaN-etching rate as a function of species partial pressures and temperature with the experimental data [1]. It has been found that the data of Ref. [1] are consistent with the assumption that all the reaction rate constants, except for that of gallium desorption, are weakly temperature dependent. The latter constant is well approximated by the Arrhenius dependence on temperature with the activation energy of about 2.2 eV and eventually determines the overall activation energy of the GaN etching. Note that the experimentally observed GaN-etching activation energy depends on the temperature range, surface polarity, and other factors, generally varying in the range of 1.5–2.7 eV [1–3], so that the activation energy for the reaction rate, used in the model, lies somewhere in the middle of this range. The upper limit of the scatter range is close to the activation energy of the liquid gallium evaporation, for which reason some authors suggest that the GaN surface is covered with a gallium adsorbate (see, for instance, Ref. [3]). According to the model, the reaction (R2) is slow in comparison with (R1) under the typical operating conditions and gives an insignificant contribution into the adsorbed gallium balance. As a result, the steady-state surface coverage with gallium can be approximately found as 3=2 Kþ 1 ðP H2 Þ yGa  þ 3=2 K 1 ðPH2 Þ þ K 1 P NH3

K7 1

,

(1)

are the forward and backward rate constants of the where reaction (R1), PH2 and PNH3 are the surface partial pressures of hydrogen and ammonia, respectively. As stated above, the (R1)

reaction rate slowly changes with temperature, so that the surface coverage also appears to be a weak function of the temperature. A typical temperature dependence of the gallium surface coverage, corresponding to GaN etching in hydrogen/ammonia atmosphere, is shown in Fig. 1. Following the model, the surface is likely to be considerably covered with the adsorbed gallium under the typical growth/etching conditions and for a wide temperature range. To summarize, the reaction (R2) describes a strongly temperature-activated process of gallium desorption that determines the etching rate and is considerable only at high temperatures, whereas the reaction (R1) describes a fast quasi-equilibrium process of ‘‘crystal bulk-adsorption layer’’ interaction which occurs in a wide temperature range and is mostly responsible for the surface coverage with gallium. The results of model application to the description of GaN etching in the Epiquip reactor are presented in Figs. 2 and 3. Fig. 2(a) demonstrates that the etching rate reduces when the ammonia partial pressure is raised, which is related to a gradual weakening of the reaction (R1). The etching rate reduction is accompanied by lowering of the surface coverage, as shown in Fig. 2(b). A similar trend was observed in Ref. [9]: a decrease in the NH3 flow rate resulted in ‘‘Ga-rich’’ surface. Fig. 3(a) shows that the measured etching rate is a weak function of the operating pressure in a wide pressure range, and this trend is reproduced well by the modeling. At the same time, the model predicts an increase of the gallium surface coverage with pressure. This effect can be easily understood from the formula for the surface coverage (Eq. (1)): while the numerator increases as p3/ 2 , the denominator increases slower with pressure. This results in an overall increase of the surface coverage with pressure. 2.2. Effect of the surface coverage on the growth characteristics The above model has been verified quantitatively using the experimental data relating to high-temperature regimes, when the etching rate is high and makes a noticeable contribution to the overall growth rate and layer composition. However, following the model assumptions, hydrogen should affect III-nitride deposition also at reduced temperatures, via the surface coverage by metal adatoms. Under such conditions, when the etching rate is low, direct measurements of any characteristics (e.g., surface coverage) that can be directly compared to the modeling results are difficult,

0.55

0.50 Ga coverage, ML

etching rates were measured in-situ by optical reflectance monitoring.

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0.45

Pressure: 200 mbar 0.40

H2 Flow: 4.5 slm NH3 Flow: 2.5 slm

0.35

900

950

1000 1050 Temperature, °C

1100

1150

Fig. 1. Surface coverage with gallium as a function of the temperature during GaN etching.

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0.16

H2 partial pressure = 129mbar T = 1070°C

0.12

Experiment Etching rate, nm/s

Etching rate, nm/sec

0.3

Computation

0.2

0.1

Experiment Computation

0.08

T = 1085°C

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H2 Flow: 4.5 slm NH3 Flow: 2.5 slm

0.0 0

20

40 60 80 100 120 NH3 partial pressure, mbar

140

0.00

160

0

200

400 600 Pressure, mbar

800

1000

1.0 0.7

T = 1070°C

0.6 Ga coverage, ML

Ga coverage, ML

0.8

H2 partial pressure = 129mbar

0.6

0.4

0.2

0

20

40 60 80 100 120 NH3 partial pressure, mbar

140

160

Fig. 2. GaN-etching rate (a) and gallium surface coverage (b) versus the ammonia partial pressure. The total pressure in the reactor can be found as the sum of the H2 and NH3 partial pressures.

so the results described below serve as qualitative confirmation of the fact that hydrogen influences the growth peculiarities even at reduced temperatures. Strong effect of hydrogen on the GaN growth process at temperatures well below 1000 1C, predicted by the model, was observed in a number of experiments. Effect of carrier gas composition on p-GaN growth rate anisotropy in sidewall epitaxy was studied in Ref. [10]. p-GaN layers with nominal thickness of about 400 nm were grown on sidewalls of GaN mesa-stripes, formed by selective area epitaxy on Si3N4. It has been found [10] that both at high temperature (above 1000 1C), when the etching rate is comparable to the deposition rate, and at reduced temperatures (below 900 1C), when the etching rate is negligible, the effect of hydrogen on the growth anisotropy is qualitatively the same. SEM images of the p-GaN (lighter layer above the stripe) are shown in Fig. 4 for the hydrogen and nitrogen carrier gases. The use of hydrogen (or a nitrogen–hydrogen mixture) promotes lateral growth, and the mesa-stripe top appears to be the underlying n-GaN. In the absence of hydrogen (in nitrogen carrier gas), the growth anisotropy changes, and vertical growth along (0 0 0 1) direction dominates in nitrogen, as shown in Fig. 4(b). We attribute these

0.5 0.4 0.3

T = 1085°C

0.2

H2 Flow: 4.5 slm

0.1

NH3 Flow: 2.5 slm 0

200

400 600 Pressure, mbar

800

1000

Fig. 3. GaN-etching rate (a) and gallium surface coverage (b) versus the pressure in the reactor.

differences in the deposition behavior to the accumulation of gallium on the surface due to interactions with hydrogen and its subsequent diffusion from the top to the sidewall of the stripes, enhancing lateral growth in case of the H2 carrier gas. Thus, the use of nitrogen is preferable with respect to covering all the stripe facets with p-GaN layer. It should also be noted that the growth of Mg-doped layers using the nitrogen carrier gas results in p-type conductivity in all investigated growth conditions and stripe orientations, which is not the case for a hydrogen-containing carrier gas. A similar mechanism underlines another important observation: the decrease of the pit-type defect density and improvement of the surface quality of GaN grown at temperatures of about 850 1C can be achieved, if conditions enhancing GaN etching (e.g., reduction of the ammonia partial pressure) are established in the reactor [4]. The same effect was observed by the addition of a certain amount of TMIn to the gas phase. In both cases the improvement in morphology was associated with an accumulation of metallic (Ga and In, respectively) atoms in the adsorption layer at the growing surface. This conclusion was supported by surface kinetics model computations, showing the increase in the surface coverage under the conditions promoting the morphology improvement.

ARTICLE IN PRESS E.V. Yakovlev et al. / Journal of Crystal Growth 310 (2008) 4862–4866

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Fig. 4. SEM images of the p-GaN (lighter layer above the stripe) grown by sidewall epitaxy in hydrogen (a) and nitrogen (b).

Fig. 5. Non-coalesced a-GaN ELOG structures grown under identical growth conditions, using hydrogen (a) and nitrogen (b) as a carrier gas.

Optimization of a-ELOG process [7] has also revealed a principal role of surface coverage by gallium adatoms on the growth anisotropy. It has been found that a high growth anisotropy and good faceting of the stripes can be achieved only using hydrogen as a carrier gas and a low partial pressure of ammonia, providing a high surface coverage with gallium. Examples of non-coalesced a-GaN ELOG structures grown in hydrogen and nitrogen are shown in Fig. 5. Accumulation of gallium on the surface due to etching by hydrogen and its subsequent diffusion from the top to the sidewall plane promote lateral growth. To provide coalescence of the laterally growing stripes, one should further reduce the ammonia flow entering the reactor at the coalescence stage, thus increasing further the gallium surface coverage and its diffusion to the sidewalls. This observation is consistent with both the model (see Fig. 2(b)), predicting the elevation of the surface coverage with lowering ammonia flow rate, and data from [9], reporting on Ga-rich surface at low NH3 flows.

etching, and numerous experimental observations suggest that this is a two-stage process, including ‘‘crystal bulk-adsorption layer’’ interaction that proceeds in a quasi-equilibrium mode and gallium desorption, representing a strongly temperatureactivated process determining the overall etching rate. The former stage may provide a significant coverage of the surface with gallium adatoms even at reduced temperatures, when the etching rate is negligible. The ability to adjust the surface coverage via the variations of the carrier gas composition and other growth parameters is assumed to underlie various experimental findings related to both planar and selective growth of GaN layers.

3. Summary

References

The interactions of hydrogen with the surfaces of III-nitride layers grown by MOVPE have been studied using both modeling and experimental analysis. The kinetic model, describing GaN

Acknowledgement This work was supported by the Russian Foundation for Basic Research, Grant 07-02-01246-a.

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