312 IEEE ELECTRON DEVICE LETTERS, VOL. 35, NO. 3, MARCH 2014
Interface Charge Engineering forEnhancement-Mode GaN MISHEMTs
Ting-Hsiang Hung, Pil Sung Park, Sriram Krishnamoorthy, Digbijoy N. Nath,and Siddharth Rajan, Member, IEEE
Abstract— We demonstrate an efficient approach to engineerthe dielectric/AlGaN positive interface fixed charges by oxygenplasma and post-metallization anneal. Significant suppression ofinterface fixed charges from 2 × 1013 to 8 × 1012 cm−2
was observed. Experimental and theoretical electron mobilitycharacteristics and the impact of remote impurity scatteringwere investigated. The reduction in oxide/semiconductor interfacecharge density leads to an increase of electron mobility, andenables a positive threshold voltage.
Index Terms— AlGaN/GaN HEMT, MISHEMT, ALD,normally-off, E-mode, interface fixed charges, electron mobility,remote impurity scattering, oxygen plasma, post-metallizationanneal (PMA).
I. INTRODUCTION
I II-NITRIDE metal-insulator-semiconductor high electronmobility transistors (MISHEMTs) have gained much atten-
tion due to their potential for energy-efficient power switch-ing. However, achieving normally off or enhancement modetransistors with high threshold (>5 V) and good performanceremains a major challenge. Several approaches have beenreported in the literature [1]–[8]. One of the main factorspreventing normally off GaN transistors is the presence of ahigh density of positive fixed charge of the order of 1 μC/cm2
at the interface of atomic layer deposited (ALD) dielectrics onGaN and AlN [9], [10]. The origin and centroid of the positivefixed charge is not clear. However, the positive fixed chargeprevents normally off operation by causing negative voltageshift of the threshold voltage. It brings substantial impact inE-mode power device. The positive interface fixed charge canalso increase the electron tunneling probability thus increasethe reverse gate leakage current [11]. Furthermore, the inter-face charges act as remote impurity scattering centers that cansignificantly lower the mobility of 2DEG [12]. Our theoreticalcalculation shows the impact of remote impurity scatteringbecomes significant when the semiconductor cap thicknessis reduced, as would be required for E-mode MISHEMTs.An efficient method to suppress the interface fixed charge istherefore needed.
Manuscript received November 5, 2013; revised December 14, 2013;accepted December 22, 2013. Date of publication January 13, 2014; dateof current version February 20, 2014. This work was supported in part by theOffice of Naval Research DEFINE MURI program (ONR N00014-10-1-0937,Dr. Paul Maki). The review of this letter was arranged by Editor M. Passlack.
The authors are with the Department of Electrical and Computer Engi-neering, The Ohio State University, Columbus, OH 43210 USA (e-mail:[email protected]).
Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LED.2013.2296659
Previous work showed that the interface fixed charge couldbe reduced by post-metallization-annealing (PMA) in the caseof ALD Al2O3 on different polarities of GaN, which impliedthe origin of interface fixed charge may be due to the surfacedefects [11]. However, PMA does not efficiently reduce thefixed charge density in the case of dielectric/AlGaN (or AlN)interfaces, making normally-off devices with high thresholdvery challenging to achieve. Oxidizing the AlGaN (AlN)surface to form a native oxide layer is one of the methodsto passivate the surface defects [13], [14]. In this letter, weuse the combination of oxygen plasma and PMA treatmentsto engineer the Al2O3/AlGaN (AlN) interface fixed charges.A normally-off MISHEMT is also demonstrated.
II. EXPERIMENT
A MOCVD-grown 29 nm Al0.3Ga0.7N / 1 nm AlN / GaNHEMT sample on Si substrate with 2DEG sheet carrier densityof 1.1 × 1013 cm−2 (as-received) was used in the study[NTT-AT Co.]. Device fabrication was started fromTi/Al/Ni/Au ohmic contact annealed at 850°C in N2ambient and followed by mesa isolation. For vertically scaleddevices, the Al0.3Ga0.7N layer thickness in the active deviceregion was varied by inductively coupled plasma reactiveion etching (ICP-RIE) using a Cl2/BCl3 gas mixture. Afterphotoresist removal, the samples were exposed to oxygenRIE plasma (20 W, 30 sccm, 5 mTorr) (Plasma ThermSLR770) ex-situ before the Al2O3 deposition. A 20 nmAl2O3 gate dielectric was deposited by ALD with TMAand water as precursors at 300°C. A Ni/Au/Ni metal stackwas then deposited as gate. Five-minute post metallizationanneal (PMA) under 5% H2/95% N2 ambient was conductedto further reduce the positive interface fixed charges. TheAlGaN etching depth was confirmed by both C-V and AFMmeasurements.
Mobility measurements were done using gated transmis-sion line measurement (gated TLM) combined with C-Vcharacterization by Agilent B1500 semiconductor device ana-lyzer equipped with medium power source/monitor units(MPSMUs) and multi frequency capacitance measurement unit(MFCMU). Frequency of 1 MHz was used for C-V measure-ment. The gated TLM-measured mobility eliminates the effectof contact resistance and can be repeated after different PMAtreatments on the same device. The gate recessed length (Lg)varied from 2 um to 20 um and gate metal overhang lengthwas 0.4 um in each side [Fig. 2(a)]. The AlGaN/AlN thick-ness after recess was determined by the C-V measurement.
HUNG et al.: INTERFACE CHARGE ENGINEERING FOR E-MODE GaN MISHEMTs 313
Fig. 1. (a) Experimental (symbols) and theoretical (lines) C-V characteristicsof non-recessed MISHEMT. (b) Energy band diagram with different positiveoxide/AlGaN interface fixed charge density.
Fig. 2. (a) The schematic of recessed MISHEMT structure. The AFM images(b) before and (c) after 20 nm AlGaN etching by chlorine-based ICP-RIE.
The theoretical mobility calculation was based on our previousstudy [12] and further includes other scattering mechanismssuch as dislocation scattering, dislocation strain field scatteringand interface roughness scattering [15]. A surface roughnessof 0.49 nm and a dislocation density of 2 × 109cm−2 was usedfor calculations. The interface fixed charge density was variedfrom 2 × 1013 cm−2 to 8 × 1012 cm−2 to match the measureddensity after different oxygen plasma and PMA conditions.
III. RESULTS AND DISCUSSION
Fig. 1(a) shows the experimental C-V characteristics of anon-recessed MISHEMT with no treatment (circle), O2 plasma(triangle) and O2 plasma/PMA (diamond), together with the-oretical C-V curves for positive interface fixed charge densityof 2 × 1013 cm−2, 1.14 × 1013 cm−2 and 1 × 1013 cm−2,respectively (simulated by Poisson-Schrodinger solver [16]).The threshold voltage (Vth) shifted from −16 V to −12 V afterO2 plasma/PMA treatments and the C-V hysteresis character-istics were also improved. We found a significant reduction ofthe interface fixed charges after O2 plasma treatment. Energyband diagram with different positive interface fixed charges[Fig. 1(b)] shows a remarkable change after O2 plasma. TheAl2O3/AlGaN positive fixed charges become less than thenegative polarization charge, which leads to net negative inter-face charges. This enables an efficient approach to engineerAl2O3/AlGaN interface fixed charges, and could be used todesign normally-off MISHEMTs with high threshold voltage.
After finding the significant impact of O2 plasma andPMA, we investigated the effect on electron mobility forvertically scaled MISHEMTs. The schematic of AlGaN-recessed MISHEMT structure is shown in Fig. 2(a). Sur-face morphology and roughness of AlGaN before and afterICP-RIE etching were confirmed by atomic force microscope
Fig. 3. (a) The experimental (symbols) and theoretical (lines) C-V charac-teristics with and without PMA after oxygen plasma of 9 nm AlGaN/AlNMISHEMT. (b) Mobility profiles of 9 nm AlGaN/AlN MISHEMT with andwithout PMA after oxygen plasma treatment.
(AFM). Fig. 2(b) and (c) shows the AFM images before andafter 20 nm AlGaN etching. There is no significant differ-ence in morphology and roughness (0.40 nm and 0.49 nm)of the surface which indicates that the chlorine-based dryetching did not cause significant physical damage on theAlGaN surface. Fig. 3(a) shows the experimental (symbols)and theoretical (solid line) C-V characteristics of recessed(8 nm AlGaN/1 nm AlN) MISHEMTs before and after 400°CPMA with O2 plasma treatment. The Vth shifted from −7Vto −4V and the interface fixed charges were substantiallyreduced to 8 × 1012 cm−2 after both O2 plasma and PMAtreatments. In the absence of plasma treatment and PMA,MISHEMTS with 20 nm Al2O3/AlGaN/GaN cap were foundto have high interface fixed charges (>4 × 1013 cm−2). Inthe case of thin AlGaN cap, this led to significant mobilitydegradation (<400 cm2V−1s−1 for 6 nm AlGaN cap), directlydemonstrating the impact of remote interface charge scatteringon the electron mobility.
Fig. 3(b) shows the experimental and theoretical electronmobility profile before and after 400°C PMA with O2 plasmatreatment. We find that the mobility increases, especially inthe lower 2DEG density regime, after 400 °C PMA. Thissuggests that while the effect of remote impurity scattering canbe observed experimentally, there is still some disagreementbetween theoretical and experimental curves. It is possiblethat in addition to the net interface fixed charges, trappedstates may also need to be considered since they can alsoact as scattering centers. However, our results confirm thatthe combination of oxygen plasma and PMA leads to animprovement of electron mobility in recessed MISHEMTs,and that there is a correlation between interface fixed chargesand electron mobility, as predicted earlier [12].
Since the existence of Al2O3/AlGaN or Al2O3/AlN [10]positive interface fixed charges can bring the threshold tonegative direction, fabrication of normally-off MISHEMTsbecomes a challenge. The ability to reduce the interface fixedcharge is important. Fig. 4(a) demonstrates the device structureof normally-off MISHEMT. The active region recess length is2 um and source-to-drain distance is 6 um. A thin AlN layerwas present after dry recess etching, based on comparisonof the drain current level to a MISFET where the channelwas etched by an additional 3 nm [dotted line in Fig. 4(b)].Using the recipe developed above, native oxide was formed
314 IEEE ELECTRON DEVICE LETTERS, VOL. 35, NO. 3, MARCH 2014
Fig. 4. (a) The schematic of E-mode MISHEMT. (b) Linear and (c) LogTransfer characteristics and (inset) C-V profile of E-mode MISHEMT.(d) Id-Vd characteristics.
by O2 plasma and 400 °C PMA treatment for 5 minutes informing gas ambient was done. Fig. 4(b) shows the trans-fer characteristics of the normally-off device at Vds = 7V.The drain current density was more than seven times largerthan 3 nm over etched MISFET and the transconductance(gm) was 43 mS/mm. Fig. 4(c) shows the transfer charac-teristics in log scale. Here we define the threshold voltageat Ids = 10 μA/mm, which gives us Vth = 1.5 V. Thegate current density is at the order of 10−8 A/mm fromVg = 0 V to 7 V. The C-V characteristics for the transistor andMIS diode (18 nm Al2O3/n+ GaN, no oxidation/PMA) [11]are shown in the inset of Fig. 4(c). The hysteresis �Vfor the transistor is 0.8V, and the threshold voltage fromC-V matches well with the transconductance curve for thetransistor. The positive threshold voltage shift in the MIS diode(from −2.8 V to +1.5 V) due to the oxidation/PMA stepcould be useful for achieving high positive threshold voltagein enhancement mode devices. Oxidation/PMA still is notable to completely eliminate the hysteresis which is causedby the interface deep traps [17]. The Ids-Vds characteristicis shown in Fig. 4(d). Gate bias (Vgs) was applied from0 V to 10 V and more than 140 mA/mm drain currentdensity was achieved at Vds = 7V. The on-resistance is20 �·mm. Further improvement of off-state current needs tobe achieved by optimizing device isolation. We analyzed thedevice characteristics to extract the field effect mobility in thechannel. Based on these estimates, the field effect mobility(100 cm−2V−1s−1) is relatively low. We believe this maybe due to etch damage. Therefore, reducing the gate recessetch damage may provide a pathway to higher mobility andcurrent. Nevertheless, the device demonstration shown hereconfirms that the combination of O2 plasma treatment andpost-metallization annealing can be used to eliminate fixedpositive charges, and achieve normally off transistors.
IV. CONCLUSION
A comprehensive study of interface fixed charge engi-neering approaches and E-mode MISHEMT is reported. Theimpact of remote impurity scattering on scaled MISHEMTwas demonstrated experimentally. The combination of oxygenplasma and PMA treatments can efficiently reduce positiveinterface fixed charges and improve the electron mobility, andenable the demonstration of normally-off devices. An E-modeMISHEMT with Vth = 1.5 V was shown in this letter, butwith optimized device design, the charge engineering approachdemonstrated here could lead to devices with significantlyhigher threshold.
ACKNOWLEDGMENT
The authors greatfully acknowledge discussions withDr. Wu Lu from ECE Department, The Ohio State University.
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