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Semiconductor Science and Technology
Semicond. Sci. Technol. 31 (2016) 065006 (6pp)

doi:10.1088/0268-1242/31/6/065006

Chlorine-based dry etching of β-Ga2O3Jack E Hogan, Stephen W Kaun, Elaheh Ahmadi, Yuichi Oshima andJames S Speck
Materials Department, University of California, Santa Barbara, CA 93106, USA
E-mail: elaheh@ece.ucsb.edu (Elaheh Ahmadi)
Received 15 December 2015, revised 8 March 2016
Accepted for publication 17 March 2016
Published 14 April 2016
Abstract

Reactive ion etching (RIE) and inductively coupled plasma (ICP) etching techniques were used
to determine the optimal dry etch conditions for β-Ga2O3. RF power and chamber pressure were
examined to study their effects on etch rate and surface roughness for three crystallographic¯ ) by RIE. BCl3 etch rate calibrations were performed on allplanes, i.e., (100); (010); and (201β-Ga2O3 planes studied, in comparison to Cl2. RIE yielded moderate etch rates (<20 nm min−1),
and surface roughness showed no clear trend with RF power. Moreover, the effect of bias power,
plasma power, and the choice of etchant were studied using ICP. The etches performed by ICP
were shown to be superior to RIE in both etch rate and surface roughness, due to the muchhigher plasma densities and uniformities possible with plasma powers beyond those realized in
RIE. The maximum etch rate of 43.0 nm min−1 was achieved using BCl3 in ICP. SF6/BCl3
mixtures, which yield high GaN etch rates, were also studied. However, in contrast to GaN
etching, SF6/BCl3 was found to be far less effective than pure BCl3 in etching β-Ga2O3.
Keywords: β-Ga2O3, chlorine-based etch, dry etch, ICP, RIE(Some ﬁgures may appear in colour only in the online journal)
Introduction

where mass production requires large, single crystal wafers.
This has been accomplished through ﬂoating-zone [7, 8],Czocharalski [9] and edge-deﬁned ﬁlm fed growth [10].
Still, signiﬁcant research is required before β-Ga2O3
devices will be practical for large-scale production. Previous
studies have shown β-Ga2O3 can be n-type doped with Si or
Sn, and Ohmic contacts can be formed with a Ti/Au stackannealed in N2. There are however, few studies of suitableetching conditions for β-Ga2O3 [11, 12]. These conditions
determine the etch rate, roughness, and anisotropy of the
exposed surface—each being essential parameters of any
device fabrication process. In contrast, GaN has extensively
developed etch technologies. Despite the different crystal
structure and anion for GaN and β-Ga2O3, GaCl3 is a likely
volatile product for both materials under chlorine-based etches.
The preferred etch for GaN uses chlorine in the form of BCl3
and Cl2. In this mixture, the Cl2 is the primary etchant, while
the BCl3 removes the native oxide and improves sidewall
passivation [13]. Due to the chemical similarity between GaN
and β-Ga2O3, GaN etch conditions were chosen as the starting
point for the exploration of β-Ga2O3 etch parameters.
Two dry etch techniques are typically employed incompound semiconductor device fabrication—reactive ion

In recent years β-Ga2O3 has garnered attention as an exciting
candidate for the active layer of future semiconducting
devices. This is due in part to its wide bandgap of 4.8 eV,
placing it well into the UV spectrum. As such, β-Ga2O3 iscurrently being developed for optical devices, such as ultraviolet detectors [1], and as a transparent epitaxial layer on
GaN-based light-emitting diodes and lasers [2]. Additionally,
β-Ga2O3 has recently been used for the fabrication of
p-channel nanowire ﬁeld effect transistors [3] and as thesemiconducting material in traditional metal-oxide–semiconductor FETs [4]. An especially promising use of β-Ga2O3
is in vertical electron devices for high-power switching [5].
This is due to the previously mentioned remarkably widebandgap as well as a high estimated breakdown ﬁeld of
8 MV cm−1 [5]. The Baliga ﬁgure of merit—a central metricin power FETs comparing the conduction power losses
between different materials [6]—places β-Ga2O3 above both
SiC and GaN. Another advantage of β-Ga2O3 is the availability of inexpensive single-crystal substrates due to growthby relatively low-cost melt-based growth techniques. This ishighly advantageous in the fabrication of vertical devices
0268-1242/16/065006+06$33.00

1© 2016 IOP Publishing Ltd Printed in the UKSemicond. Sci. Technol. 31 (2016) 065006

J E Hogan et al¯ ) and (010), crystallographic planes shown in red, blue, and green, respectively.
Figure 1. The β-Ga2O3 unit cell with the (100), (201

etching (RIE) and inductively coupled plasma (ICP) etching.
Each technique operates under the principle of ionizing a gas
and propelling the ions towards the substrate surface, but theydiffer in the manner by which the gas is ionized. In RIE, thesample is placed on an electrode that is driven at 13.56 MHz.
As the etchant gas ﬂows over, it is ignited by the potential
difference (RF power), becoming a plasma consisting of
positive ions, radicals, and electrons. The high mobility of theelectrons compared to the ion and radical components causes
the electrode to acquire a negative charge. The positive ionsin turn are accelerated towards the substrate and react with thesurface atoms [14]. In contrast, in ICP etching the plasma is
generated using a magnetic ﬁeld by applying RF power to acoil (plasma power). Once the plasma is ignited, the ions areaccelerated towards the substrate in the same manner as RIE.
The advantage of ICP is that very high ion densities can beachieved leading to higher etch rates than RIE. Additionally,
because the plasma is generated by a magnetic ﬁeld in ICP
etching, a lower bias is needed to propel the ions toward thesample, which reduces surface damage [15]. In either technique, the etch rate is a result of this combined action of
surface chemical reactions and physical ion bombardment.
β-Ga2O3 exists in a monoclinic crystal lattice with lattice
parameters of a=1.22 nm, b=0.304 nm, c=0.580 nm,
α=γ=90°, and β=103.8°. The unit cell shown inﬁgure 1 has two gallium sites: one in tetrahedral geometry;the other octahedral—while the oxygen anions form a distorted cubic close packing arrangement [16]. The (100) planeis the strong cleavage plane and was initially the subject of the
majority of growth studies. However, due to relatively weaksurface bonds growth on this plane is difﬁcult. The lowadhesion energy on the (100) terraces promotes the formationof volatile gallium suboxides which compete with growth[17]. As such, focus has since shifted to the the (010) and¯ ) planes. The (010) plane is a non-cleaveage plane made(201
up of gallium atoms in both tetrahedrally coordinated andoctahedrally coordinated sites, as well as oxygen atoms. The¯ ) plane consists exclusively of octahedrally bonded gal(201lium atoms.
In this paper, we determine the etch rate and etchmorphology for three important crystal planes of β-Ga2O3
using RIE and ICP etching. We demonstrate that ICP etchingwith BCl3 gas results in smooth morphologies and high rates
for the (010) plane.

ExperimentThis study focused on different β-Ga2O3 etch conditions for¯ ). Specithe primary growth planes—(100), (010), and (201
ﬁcally, the effects of power, chamber pressure, and etchantgas were examined using both ICP (Panasonic E6261) andRIE (Plasma-Therm SLR-770) apparatuses in the UCSB
nanofabrication facility.
Free-standing UID and Sn-doped β-Ga2O3 substrates
were diced into ∼5×5 mm2 pieces for etch tests. Eachsample was cleaned using acetone, isopropanol, and deionized water. Timed etches were ﬁrst performed with a Kapton
tape mask and subsequently veriﬁed using a photoresist mask
(Shipley SPR 220 photoresist). In the RIE etch studies,
power, chamber pressure, and etchant gas ﬂow rates were
investigated. The control values for each variable were chosenwith respect to standard GaN etch conditions, for which there
is readily available data. The etch rates were tested at RF
powers of 50, 100, 200, and 250 W. Chamber pressures were
tested at 5, 10, and 20 mTorr. Finally the etchant gas ﬂow
rates for Cl2/BCl3 were tested at 20/0, 15/5, 10/10, 5/15,
and 0/20 sccm. ICP was used to test a variety of etchant gases
and to study the effect of plasma and bias power. In addition
to BCl3, CF4/O2, and SF6, were also tested on gallium oxide.
The etching time for each experiment was 10–30 min.
After removal from the etch chamber, samples wereagain cleaned using the aforementioned solvents. Etchedfeature heights were determined using a Dektak stylus proﬁler. Roughness measurements were performed using atomic
force microscopy (AFM).

Results
Reactive ion etching

To determine the viability of different etch conditions, thesurface roughness and etch rate achieved were assessed for2

Semicond. Sci. Technol. 31 (2016) 065006

J E Hogan et al

etchants is their selectivity to the photoresist. Cl2 quickly
removed the photo resist at 200 W while the BCl3 showed no
ability to etch it. This supports the idea that each etchingprocesses contain different active species present in theplasma, though GaCl3 and O2 are nonetheless expected as an
etch products in either case.
Increasing the chamber pressure from 5 to 20 mTorr
increased the etch rate on the (100) and (010) planes from 4.2
to 8.1 nm min−1 and 7.1 to 10.5 nm min−1, respectively. As
expected, this effect is linear, since higher etchant ion densities result in a greater frequency of ion-substrate collisions.
¯ ) planes were consistently etched at a
The (010) and (201higher rate than the (100) surface. The (100) plane has strong
lateral bonds and a relatively weak bonds normal to the plane
direction resulting in a strong cleavage plane that is difﬁcult
to etch. We speculate that the surface is made entirely ofoxygen atoms (see ﬁgure 1), and the electronegative oxygen
provides a surface resistance to etching involving Cl− or
BCl− anions. The electrostatic repulsion of the reactive
anions with the surface oxygen anions decreases the ability toetch the surface and the recessed gallium cations are, in effect,
inaccessible and protected, leading to a decreased etch rate.
Unlike the (100) plane, the non-cleavage (010) plane is¯ ) plane is a weakcomposed of gallium atoms, and the (201cleavage plane composed of both gallium and oxygen atoms.
While the exact etching mechanism is not entirely understood, the availability of surface gallium atoms on the (010)¯ ) planes provides insight that suggests a favorableand (201
reaction between gallium and the impinging activated chlorine. This is also justiﬁed through the noted etch product of
GaCl3. The accessibility of these reactant gallium cations,
therefore, can explain the observed face-dependent trend inetch rates. However, this does not consider possible surface
reconstructions or other factors inﬂuencing the lattice, such as
differing bond angles and energies. The other possibility is
the difference in the density of the dangling bonds on different planes. Based on the assumption that dangling bonds
are vulnerable to the etching gas. Therefore, higher density of¯ ) planes could result indangling bonds on (010) and (201higher etch rates.

Figure 2. (a) RIE etch rate for differing RF powers with a constantCl2:BCl3 ratio of 10:10 sccm. (b) RIE etch rate for differing Cl2:BCl3
ratios with an RF power of 200 W. In each case the chamberpressure is 10 mTorr.

each case. In changing the etch conditions, RF power,
chamber pressure, and etchant gas ﬂow rates (Cl2/BCl3 sccm)¯ ) surfaces
were all varied on β-Ga2O3 (100), (010), and (201
using RIE.
Our initial work focused on the RF power supplied to theCl2/BCl3 gas mixture, where increases in power signiﬁcantly
impacted the plasma activation, yielding an approximately
linear relationship between RF power and etch rate. As shownin ﬁgure 2(a) an increase of RF power from 50 to 250 W
corresponded to an etch rate increase from 2.0 to
11.5 nm min−1 for the (010) plane. At RF powers greater than
250 W, the reﬂected power of the system markedly increased;
thus, 250 W was the highest RF power tested. With 200 W RF
power adding no risk of plasma instability and providing etchrates greater than 5 nm min−1 for each studied crystal plane,
this RF power was used in the remainder of the Cl2/BCl3
ﬂow rate study.
The etchant gas ratio had a signiﬁcant effect on the etchrate. It was found that a pure BCl3 etch was much more
effective than a pure Cl2 etch, and a mixture of the two
resulted in a etch rate roughly related to the weighted averageof each etchant as shown in (ﬁgure 2(b)). This linear relationship when using mixtures of BCl3 and Cl2 may be indicative of minimal interaction between the etch species.
Interestingly BCl3 etches at similar rates for all three planes¯ ) planes at a higher rate
while Cl2 etched the (010) and (201than the (100) plane. Another difference between the two

ICP etching

ICP was used to investigate the performance of a wider
variety of etchant gases; the effect of plasma (ICP) and forward (capacitively coupled plasma) powers were also analyzed. Speciﬁcally, the etchant gases used were BCl3, a
mixture of BCl3/SF6, and CF4/O2.
The combination of BCl3/SF6 has previously beenshown to increase the etch rate and selectivity of GaN [18].
Addition of other gases is expected to reduce the probabilityof boron–chlorine recombination, leading to higher active
chlorine, which is line with the proposed mechanism [19].
Etchant gas ﬂow ratios of 15:5 sccm and 5:15 sccm BCl3:SF6
were compared to BCl3 alone. Chamber conditions used
plasma and bias powers of 200 and 30 W, respectively. As
shown in table 1, the etch rates for β-Ga2O3 involved a signiﬁcant reduction when using either gas mixture as opposed3

Semicond. Sci. Technol. 31 (2016) 065006

J E Hogan et al

Figure 3. (a) Etch rate as a function of ICP plasma power and RIE RF power, (b) etch rate as a function of ICP bias power. In each casechamber conditions were 15 mTorr and 20 sccm BCl3.
Table 1. ICP Etch rates for various etchant gases on (010). Chamber

to BCl3 alone; this is the opposite effect to GaN. The BCl3
rich gas mixture had a higher etch rate of 3.1 nm min−1,
compared to 1.5 nm min−1 for the CF4/O2 mixture, but bothcan be considered ineffective compared to the etch rate of
12 nm min−1 for BCl3 at the same conditions. This marked
decrease in etch rate can likely be attributed to the formationof GaF3 which acts as an etch inhibitor because of its low
volatility [18].
Using BCl3, etch studies were conducted to alter plasma
and bias power. These are shown in ﬁgure 3. These results,
although limited in number, suggest etch rate and plasma
power are linearly related. Considering plasma density isdependent on plasma power it is likely this relationship islinear as well. Additionally the etch mechanism seems to berelated primarily to the incident ion ﬂux to the surface, rather
than any surface diffusion/reactions or desorption rates.

conditions are 15 mTorr with source/bias power 200/30 W.
Etchant gas

BCl3
20 sccm

CF4/O2
20/2 sccm

BCl3/SF6
15/5 sccm

BCl3/O2
5/15 sccm

12.0

1.5

3.1

2.3

Etch rate(nm min−1)

required to accomplish similar etch rates. In ﬁgure 4, comparing the 100 W bias power cases, there appear to be fewer
inhomogeneous surface features on the ICP sample, though as
indicated the etch rate is higher. This can be rationalized by
the higher plasma density promoting isotropic chemical
etching that partial negate the formation of unwanted featurescaused by physical bombardment; this combined physicochemical action is the reason for both higher etch rates and amore uniform surface.

Surface roughness
Mesa structure

Surface morphologies of samples etched under various conditions are shown in ﬁgure 4. ICP yielded lower surface
roughness than RIE for equivalent etch depths. This is likely
due to the different mechanisms for ionization of the etchantgas between the two systems. In RIE a large forward bias isrequired to ionize the etchant gases and accelerate them
towards the surface. At higher forward biases ions impact thesurface with greater kinetic energy which may lead to physical sputtering in addition to chemical etching. Furthermore,
uneven etch proﬁles were observed on RIE-etched samples
for all RF powers, possibly the result of incomplete removalof etch products acting as micro-masks. This may be
explained by non-uniformity in the plasma as the RF power in
RIE appears to be too low to homogeneously ignite theplasma. This deleterious effect appears to be absent in ICP,
since the plasma is generated through a magnetic ﬁeld prior toapplying forward plasma. The resulting effect is two-fold,much higher plasma densities are possible in ICP in comparison to RIE and, therefore, much lower forward bias is

We investigated the steepness of mesa structure of a sample
formed by ICP etching using a condition under which both
good surface morphology and high etch rate was achieved.
BCl3 with a ﬂow rate of 20 sccm was used as the etchant gas
for this sample. The etch was performed for half an hourusing source and bias powers of 200 and 30 W, respectively.
ﬁgure 4 shows a scanning electron microscope (SEM) imageof the sample. It was found that a relatively, steep wall wasachieved using the abovementioned etch conditions.

Summary
The effects of etchant gas, chamber pressure, and power forboth RIE an ICP etching processes has been investigated on¯ ) planes of β-Ga2O3 to determine
the (100), (010), and (201the optimal conditions for both etch rate and surface
4

Semicond. Sci. Technol. 31 (2016) 065006

J E Hogan et al

Figure 4. Roughness data using AFM taken for both RIE and ICP etches at differing powers on the (010) plane. Roughness is computed as
the average of three 5×5 μm2 scans.

roughness. The RIE and ICP etching conditions are summarized in tables 2 and 3, respectively. The two planes thatare most promising for power electron devices—(010) and(201)—performed similarly across all tests, while the (100)plane showed lower etch rates. Of the etchant gases tested(BCl3, Cl2, CF4, and SF6), BCl3 produced the highest etchrate. In contrast to previous studies on GaN, the addition of
SF6 to BCl3 to the etch gas did not increase the etch rate, butrather decreased it drastically. Increasing RF power in RIE
increased the etch rate without showing a clear pattern insurface roughness. The maximum RF power of 250 W produced an etch rate of 19 nm min−1 on (010). RIE was outperformed by ICP likely due to higher plasma density. ICP
conditions of 400 W/100 W plasma/bias power (highest
tested) using BCl3 had an etch rate of 33 nm min−1 with lower
surface roughness than any RIE samples. The results of thisstudy indicate that ICP is preferred over RIE to etch β-Ga2O3,

Figure 5. SEM image (bird’s-eye view) of an sample etched by ICP

with 20 sccm BCl3 for half an hour using plasma and bias powers of
200 W and 30 W, respectively.

Table 2. A summary of RIE etch conditions studied in this work.

RIE
RF power (W)Cl2/BCl3 (sccm)
Pressure (mTorr)
Plane
Results shown inPower study
50, 100, 200, 250
10/10
10
¯ )
a, b, (201
Figure 1(a)100, 200, 250
0/20
15
b
Figure 3(a)5

Gas ratio study

Pressure study

200
20/0, 15/5, 10/10, 5/15, 0/20
10
¯ )a, b, (201
Figure 1(b)10/10
5,10,20a,btext

Semicond. Sci. Technol. 31 (2016) 065006

J E Hogan et al

Table 3. A summary of ICP etch conditions studied in this work.

ICP

Plasma power study

Plasma power (W)Bias power (W)BCl3/CF6 (sccm)
CF4/O2 (sccm)
Pressure (mTorr)
Plane
Results shown in

100, 200, 400
30
60

Figures 3(a), 4

Bias power study
100
15, 30, 60
20/0—

Figure 3(b)400
15, 30, 60, 100

15bFigures 3(b), 5

Gas ratio study
200
30
15/5, 5/15, 20/0
—

—
20/2

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and using BCl3 as the etchant is advised. Acceptable surface
roughness was shown below 400 W/100 W plasma/bias
power.

Acknowledgment
This work was supported by the Air Force Ofﬁce of ScientiﬁcResearch (AFOSR, Program Manager Dr Ali Sayir) throughgrant number FA9550-14-1-0112. Additional support for JEH,
SWK and JSS was provided by the MRSEC Program of the US
National Science Foundation under Award No. DMR-1121053.

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