AlN grown by CBE for power device applications

Guillaume Gomme, Adrien Cutivet, Boussairi Bouzazi, Abderrahim Rahim Boucherif, Tom Macelwee, Christophe Rodriguez, Meriem Bouchilaoun, Hubert Pelletier, Philippe-Olivier Provost, Hassan Maher, et al.

► To cite this version:

Guillaume Gomme, Adrien Cutivet, Boussairi Bouzazi, Abderrahim Rahim Boucherif, Tom Macelwee, et al. AlN grown by CBE for power device applications. AIP Advances, 2020, 10 (6), pp.065123. 10.1063/1.5142615. hal-02895418

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# AIN grown by CBE for power device applications

Guillaume Gommé ⬒ ; Adrien Cutivet ⬒ ; Boussairi Bouzazi ; Abderrahim Rahim Boucherif ⬒ ;

Tom MacElwee ; Christophe Rodriguez ; Meriem Bouchilaoun ; Hubert Pelletier ; Philippe-Olivier Provost ⬒ ;

Hassan Maher ⬒ ; Richard Ares ⬒

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# AlN grown by CBE for power device applications

Cite as: AIP Advances 10, 065123 (2020); doi: 10.1063/1.5142615

Submitted: 16 December 2019 • Accepted: 26 May 2020 •

Published Online: 15 June 2020

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Guillaume Gommé,<fnref n="1" />,ᵃ) Adrien Cutivet,<fnref n="1" /> Boussairi Bouzazi,<fnref n="1" /> Abderrahim Rahim Boucherif,<fnref n="1" />

Tom MacElwee,<fnref n="2" /> Christophe Rodriguez,<fnref n="1" /> Meriem Bouchilaoun,<fnref n="1" /> Hubert Pelletier,<fnref n="1" /> Philippe-Olivier Provost,<fnref n="1" />

Hassan Maher,<fnref n="1" /> and Richard Ares<fnref n="1" />

## AFFILIATIONS

## ABSTRACT

Thermal and electrical properties of aluminum nitride (AlN) epilayers grown by chemical beam epitaxy (CBE) were investigated. A high growth rate of 5.9 ± 0.4 µm/h was achieved using trimethyl aluminum and ammonia as group III and V precursors, respectively, at a growth temperature below 600 °C. The thermal conductivity and breakdown field of 10 µm thick AlN epilayers were measured to be 57 W/(m·K) and 1.04 10⁶ V/cm, respectively. These results demonstrate the potential of CBE as an alternative growth method for the development of thick AlN layers in high power device applications.

© 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5142615

## I. INTRODUCTION

Aluminum nitride (AlN) layers have been widely used in optoelectronic applications, such as high electron mobility transistors (HEMT),<fnref n="1" /> light-emitting diodes (LED),<fnref n="2" /> and micro-electromechanical systems (MEMS)³ because of their outstanding properties, namely wide bandgap, dielectric properties, and high thermal conductivity. In addition, AlN is commonly used as a buffer layer for the growth of high-quality III-nitride structures leading to exceptional performances.⁴ Nevertheless, because of the miniaturization of the microelectronic devices and the simultaneous increase of power, high local thermal heating occurs in high power devices. Furthermore, power device performances are limited to the breakdown voltage, which occurs when the high electric field reaches the Si substrates underneath the gate-drain region of the devices in high power GaN transistors grown on Si, for example.⁵ Hence, it is crucial to overcome these issues with the use of materials that ensure heat dissipation and at the same time prevent breakdown through the substrate. Several solutions have already been proposed, such as the use of silicon dioxide (SiO₂),⁶ silicon nitride (Si₃N₄),⁷ or polymer.⁸ However, the properties of these materials do not ensure good thermal evacuation and high breakdown simultaneously. On the other hand, monocrystalline AlN is a very promising candidate to

address thermal heating and breakdown issues with a theoretical thermal conductivity of 320 W/(m·K) at room temperature⁷ and a breakdown field of 1.2 MV/cm for the wurtzite structure.⁹ Hence, AlN has a great potential for high power applications where thermal management is necessary as reported in Ref. 10. Nevertheless, the thermal conductivity and the breakdown field of AlN layers are affected by several parameters, most of them related to various crystal imperfections, such as grain size, impurities, and dislocations. Each of these defects tends to reduce the thermal conductivity and the breakdown of the AlN layers. Several deposition techniques exist for the growth of AlN, such as magnetron reactive sputtering,¹¹⁻¹⁶ laser ablation,¹⁷ hybrid vapor phase epitaxy (HVPE),¹⁸ molecular beam epitaxy (MBE),¹⁹⁻²¹ metalorganic chemical vapor deposition (MOCVD),²²⁻²⁵ or chemical beam epitaxy (CBE).²⁶,²⁷ To be advantageous, the growth technique used must produce AlN layers with a good thermal conductivity and breakdown field, at a high enough growth rate to reduce the impact on the process time. The MOCVD technique has demonstrated AlN layers with high growth rates.²⁸ However, high growth temperatures above 1000 °C are commonly used by MOCVD, which might be unsuitable for power device applications or regrowth schemes that require limited thermal budgets. On the other hand, MBE growth temperatures used for AlN layers can be decreased down to 200 °C, but growth rates then fall to a few

<footnote n="1">¹Institut Interdisciplinaire d'Innovation Technologique (3IT), Université de Sherbrooke, 3000 Boulevard Université, Sherbrooke, Quebec J1K 0A5, Canada</footnote>

<footnote n="2">²GaN Systems, 1145 Innovation Drive, Ottawa, Ontario K2K 3G8, Canada</footnote>

<footnote>ᵃ) Author to whom correspondence should be addressed: guillaume.gomme@usherbrooke.ca</footnote>
hundred nanometers per hour,²⁹ limiting its interest for mass pro-
duction. Among all the other growth techniques mentioned, mag-
netron reactive sputtering is of interest to grow AlN because of
its high growth rate and relatively low growth temperatures and
remains to this day as the incumbent approach. CBE has demon-
strated its ability to grow good quality III-V semiconductors with
high growth rates³⁰,³¹ and lower growth temperatures compared to
MOCVD. Hence, CBE could be a potential candidate for the growth
of AlN layers as an alternative cluster tool. In this study, we investi-
gate the growth of AlN layers on Si by CBE. The thermal conductiv-
ity and breakdown field of the AlN layers grown by CBE have been
assessed.

II. EXPERIMENTAL PROCEDURE

Two-inch n-type Si (111) substrates with a resistivity of
2–5 mΩ cm were used for the growth of AlN films. Prior to any
growth, the substrates were cleaned ex-situ using a Piranha solution
(H₂SO₄:H₂O₂) and then dipped in hydrofluoric acid (HF) in order
to remove the native oxide layer. AlN epitaxial layers were grown
in a CBE reactor using a custom injector design, as described by
Boucherif et al.⁷² Trimethylaluminum (TMAI) and ammonia (NH₃)
were used as Group-III and Group-V elements, respectively, for the
growth of AlN layers. The growth temperature was varied between
500 °C and 600 °C. The *in situ* reflectometry system was used to
measure the growth rate of the different epilayers. NH₃ and TMAI
fluxes and the growth temperature were investigated to determine
the highest achievable growth rate. The morphological quality of
AlN epilayers was assessed by atomic force microscopy (AFM). The
thermal conductivity of the different AlN epilayers was measured
by the 3ω method¹³,³³ using a Keithley 6221 wave current genera-
tor and a SR850 digital lock-in amplifier. Ni/Au metal stripes with
a width of 20 µm were deposited by electron beam evaporation on
the AlN layers for 3ω measurements. Considering the stripes width
used in this study, the cross-plane thermal conductivity of the AlN
epilayers was assessed as described in Ref. 34. Finally, the breakdown
field analysis of the AlN epilayers grown on Si was carried out by

current-voltage (I-V) measurements using a B1505A power device
analyzer.

III. RESULTS AND DISCUSSIONS

A. Growth of thick AlN

As far as we know, very few reports exist on the growth of AlN
by CBE.<sup>35,36</sup> Moreover, none of them report the growth of thick AlN
films and their thermal and electrical properties. The AlN epilayers
are well known as buffer layers in high quality GaN heterostruc-
tures.<sup>37,38</sup> However, low growth rates are commonly seen in high
quality III-N, which is impractical for the thick AlN required for
isolation layers, supporting the use of reactive magnetron sputter-
ing in industrial settings. In the first part of our approach, different
growth conditions were investigated to increase the growth rate of
AlN grown by CBE. Figure 1 shows the growth rate of the AlN epi-
layers grown at 600 °C as a function of the NH3 flow, for different
TMAI flows from 0.24 sccm to 1.68 sccm. An increase in the growth
rate was clearly observed for higher TMAI fluxes, independently of
the NH3 flux by Lobanova *et al.*<sup>29</sup> when growing AlN by MOCVD.
We observe that the growth rate increases with an increase in NH3
for low fluxes, until reaching the stoichiometry, followed by a slow
decrease, as NH3 becomes larger. We interpret these data as two
competing processes on the growth surface. For low NH3 flux val-
ues, essentially all available N atoms produced by the cracking of the
NH3 molecules are consumed in the reaction with Al from TMAI to
grown AlN. This mode is represented by a semi-linear increase in the
growth rate, as the NH3 flux increases. Meanwhile, another process
can be considered. Because of the relatively low surface temperature
used for the growth, a significant portion of the NH3 molecules that
reach the surface remains uncracked and do not contribute to the
growth rate. These unreacted molecules have a finite residency time
(temperature dependent) on the surface before re-evaporating back
to the vacuum reactor. During this residency, the NH3 molecules
likely induce steric hindrance, covering some reaction sites and pre-
venting some of the TMAI from reacting and contributing to the

<figure><img src="image_13.png" /><figcaption>FIG. 1. Growth rates (nm/h) of AlN epilayers as a function of the NH₃ flux for different TMAI fluxes, namely 0.24 sccm (square), 0.49 sccm (triangle), 0.72 sccm (lozenge), and 1.68 sccm (circle).</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>FIG. 2. Growth rates in nm/hr of AlN layers grown on Si by CBE as a function of temperature for TMAI and NH3 fluxes of 1.68 sccm and 20 sccm, respectively.</figcaption></figure>
<table><caption>TABLE I. Growth conditions, rms roughness, thermal conductivity, and breakdown field of the AlN epilayers grown on Si.</caption><thead><tr><th>Sample number</th><th>Growth temperature (°C)</th><th>TMAI flux (sccm)</th><th>NH<sub>3</sub> flux (sccm)</th><th>Growth rate (µm/h)</th><th>rms roughness measured by AFM (nm)</th><th>Thermal conductivity [W/(m K)]</th><th>Breakdown field (10<sup>6</sup> V/cm)</th></tr></thead><tbody><tr><td>No. 1</td><td>550</td><td>1.68</td><td>20</td><td>2.5 ± 0.2</td><td>18.26</td><td>55 ± 5</td><td>0.54 ± 0.01</td></tr><tr><td>No. 2</td><td>600</td><td>1.68</td><td>20</td><td>5.9 ± 0.4</td><td>24.28</td><td>57 ± 5</td><td>1.04 ± 0.01</td></tr></tbody></table>

growth, resulting in an effective reduction in the growth rate, as the NH₃ fluxes reach higher values. Steric hindrance becomes more important as the NH₃ flux increases. This effect can explain the growth rate evolution, from a linear increase at low NH₃ fluxes, to a maximum value, followed by a slow decrease for high NH₃ fluxes. The actual process has several concurrent phenomena. For instance, TMAI is known to improve the decomposition of NH₃,⁴⁰ creating some catalytic effect in the growth. However, this catalytic mechanism is a probable explanation for the growth of AlN at relatively low temperatures by CBE, compared to other techniques. A growth rate of 5.9 ± 0.4 µm/h was obtained for the TMAI and NH₃ fluxes of 1.68 sccm and 20 sccm, respectively.

Figure 2 shows the growth rate of AlN as a function of temperature using the conditions that showed the highest growth rate found previously, namely TMAI and NH₃ fluxes of 1.68 sccm and 20 sccm, respectively. We observed a drastic decrease in the growth rate of AlN for lower growth temperature, probably due to a diminution in the thermally stimulated decomposition of TMAI as observed in other groups.⁴¹,⁴² Nonetheless, a growth rate of 850 ± 68 nm/h was found at a growth temperature of 500 °C, which is similar to the growth rate that can be observed for AlN grown by magnetron reactive sputtering.⁴³,⁴⁴ In addition, we calculated an activation energy of 26 kcal/mol, which is coherent with the activation energy of the reaction between TMAI and NH₃ found by other groups.⁴⁵,⁴⁶ Hence, using the conditions summarized in Table I, 10 µm thick AlN epitaxial layers were grown. The root mean square (rms) roughness of 18.26 nm and 24.28 nm were found for samples No. 1 and No. 2, respectively. Figure 3 shows 10 × 10 µm² AFM images of the AlN epilayer surface. We observed larger AlN crystallites at a growth temperature of 550 °C [Fig. 3(a)] compared to that at 600 °C [Fig. 3(b)]. This roughness increase for lower growth temperatures can be explained by a lower diffusion length

of the chemical species leading to bigger crystallites as observed by Caicedo et al.⁴⁷

## B. Thermal and electrical characteristics

The 3ω method and I-V measurements were carried out to assess the thermal conductivity and the breakdown voltage of the AlN epilayers grown on samples No. 1 and No. 2. The results are summarized in Table I. Thermal conductivities of 55 W/(m K) and 57 W/(m K) were measured for sample No. 1 and sample No. 2, respectively. These results are comparable to the thermal conductivity values that were reported for AlN grown by magnetron reactive sputtering, where the thermal conductivity is measured between ~2 W/(m K) and 250 W/(m K).¹¹⁻¹⁵ It is worth mentioning that all thermal conductivities reported for AlN grown by magnetron reactive sputtering are for thinner layers (~1 µm). Belkerk et al. show that the thermal conductivity increases for thicker AlN layers.¹⁶ One hypothesis to understand why the AlN epilayers grown by CBE do not show higher thermal conductivity is a probable carbon or oxygen contamination. Indeed, Slack et al. indicate that the higher oxygen contamination decreases the AlN thermal conductivity.⁷ Furthermore, since the growth temperature is relatively low in our experiment compared to the usual growth temperature used by MBE or MOCVD, the AlN epilayers grown by CBE at 550 °C and 600 °C are expected to show a lower structural quality. Other groups reported an oxygen contamination up to 1 × 10¹⁸ atm/cm³ in AlN grown by MBE¹⁸ or metalorganic MBE²⁷ that might be assumed to be in the same range for AlN epilayers in this study. Nevertheless, it is worth mentioning that this experiment shows that the AlN films with reasonably good thermal properties can be grown by CBE at low temperatures. Further optimization would be required to truly explore the full potential in terms of layer quality, but it is beyond the scope of this study.

<figure><img src="image_15.png" /><figcaption>FIG. 3. 10 × 10 µm² AFM images of 10 µm thick AlN layers grown by CBE at 550 °C (a) and 600 °C (b).</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>FIG. 4. I-V measurements performed on samples No. 1 and No. 2 following the schematic set up in the inset. Breakdown voltages were found at 1080 V and 2000 V for samples No. 1 and No. 2, respectively.</figcaption></figure>

I-V measurements (Fig. 4) were performed on samples No. 1 and No. 2, where the voltage was increased from 0 V up to the breakdown with a voltage step of 5 V and a current compliance of 100 µA. The breakdown fields of 0.54 $10^6$ V/cm and 1.04 $10^6$ V/cm were found for samples No. 1 and No. 2, respectively. These results are in good agreement with breakdown fields reported in the literature.⁴⁹ Indeed, An et al. showed the AlN breakdown fields between 0.3 $× 10^6$ V/cm and 2.1 $× 10^6$ V/cm, which are in good agreement with our results. Other groups show breakdown fields for AlN layers grown by magnetron reactive sputtering reaching up to $5 × 10^6$ V/cm.⁵⁰,⁵¹ However, these properties were obtained either by following annealing up to 1000 °C or from the studies reported on a reduced growth rate for their deposition (lower that 600 nm/h).

Hence, the AlN layers grown by CBE below 600 °C showed promising electrical and thermal properties that can be used in microelectronic applications where high breakdown fields are necessary while keeping advantageous thermal dissipation capabilities.

## IV. CONCLUSIONS

We presented the electrical and thermal characteristics of the thick AlN epilayers grown by CBE at 550 °C and 600 °C. A high growth rate of $5.9 ± 0.4$ µm/h was achieved for TMAI and NH3 fluxes of 1.68 sccm and 20 sccm, respectively. Under these conditions, 10 µm thick AlN layers show a thermal conductivity and a breakdown field of 57 W/(m K) and $1.04 × 10^6$ V/cm, respectively, which are comparable to the AlN layers grown by the reactive magnetron sputtering results. These preliminary results demonstrate the potential of CBE reactors as an alternative approach for the growth of thick AlN layers in mass-produced microelectronic applications.

## DATA AVAILABILITY

The data that support the findings of this study are available within the article.

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