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Control of threshold voltage of AlGaN GaN HEMTs by fluoride-based plasma


IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 53, NO. 9, SEPTEMBER 2006

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Control of Threshold Voltage of AlGaN/GaN HEMTs by Fluoride-Based Plasma Treatment: From Depletion Mode to Enhancement Mode
Yong Cai, Yugang Zhou, Kei May Lau, Fellow, IEEE, and Kevin J. Chen, Member, IEEE
Abstract—This paper presents a method with an accurate control of threshold voltages (Vth ) of AlGaN/GaN high-electron mobility transistors (HEMTs) using a ?uoride-based plasma treatment. Using this method, the Vth of AlGaN/GaN HEMTs can be continuously shifted from ?4 V in a conventional depletion-mode (D-mode) AlGaN/GaN HEMT to 0.9 V in an enhancement-mode AlGaN/GaN HEMT. It was found that the plasma-induced damages result in a mobility degradation of two-dimensional electron gas. The damages can be repaired and the mobility can be recovered by a post-gate annealing step at 400 ? C. At the same time, the shift in Vth shows a good thermal stability and is not affected by the post-gate annealing. The enhancement-mode HEMTs show a performance (transconductance, cutoff frequencies) comparable to the D-mode HEMTs. Experimental results con?rm that the threshold-voltage shift originates from the incorporation of F ions in the AlGaN barrier. In addition, the ?uoride-based plasma treatment was also found to be effective in lowering the gate-leakage current, in both forward and reverse bias regions. A physical model of the threshold voltage is proposed to explain the effects of the ?uoride-based plasma treatment on AlGaN/GaN HEMTs. Index Terms—AlGaN/GaN, depletion mode (D-mode), enhancement mode, ?uoride, gate current, high-electron mobility transistor (HEMT), immobile negative charge, plasma treatment, post-gate rapid thermal annealing (RTA), threshold voltage.

I. INTRODUCTION IDE bandgap AlGaN/GaN high-electron mobility transistors (HEMTs) are emerging as excellent candidates for radio-frequency (RF) and microwave power ampli?ers (PAs) because of their high-power-handling capabilities. Power densities that are one order of magnitude higher than their silicon or GaAs counterparts have been demonstrated [1]–[4]. Their demonstrated low-noise [5]–[10], high-linearity [11], [12], and high breakdown characteristics also imply their usefulness in protection-circuit-free low-noise ampli?ers (LNAs). High-density two-dimensional electron gas (2DEG) induced by spontaneous and piezoelectric polarization effects [13] presents the conventional AlGaN/GaN HEMTs as depletion-mode (D-mode) transistors with a threshold voltage (Vth ) typically around ?4 V. Usually, the Vth of the AlGaN/GaN HEMTs
Manuscript received February 28, 2006; revised June 8, 2006. This work was supported in part by Hong Kong Research Grants Council and National Science Foundation of China under Grant N_HKUST616/04 and an RGC CERG Grant 611706. The review of this paper was arranged by Editor M. Anwar. The authors are with the Department of Electrical and Electronic Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong (e-mail: eecaiyg@ust.hk; eekjchen@ust.hk). Digital Object Identi?er 10.1109/TED.2006.881054

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depends on the design of the epitaxial structure, namely, the Al composition, doping concentration, and thickness of the AlGaN barrier. Methods that can further modify the threshold voltage during the device-fabrication stage will provide additional ?exibilities in device fabrication and circuit applications, especially in single-polarity supply voltage RF and microwave circuits, and digital circuits that require enhancement-mode (E-mode) HEMTs. A common fabrication technique of modifying the HEMTs’ threshold voltage, the so-called “gate-recess” technique, is to reduce the thickness of the barrier layer under the gate metal. In the generally used AlGaN/GaN heterostructure for HEMTs, where Al composition is in the range of 15–35% and the AlGaN barrier thickness is around 20 nm, the reduction in the AlGaN thickness by the gate recess results in a reduced polarization-induced 2DEG density. And with the help of the gate-metal work function, the threshold voltage can be shifted positively. With a deep-enough gate-recess etching, the Vth can reach a positive value and E-mode HEMTs are formed. For a conventional III-V compound semiconductor, such as GaAs- and InP-based HEMTs, there are suf?cient highly selective chemical wet-etching recipes [14] that can be applied to recess etching. The wet etching has the major advantage of low damages. However, a compatible wet-etching method for AlGaN/GaN is still lacking up to now. As an alternative approach, a chloride-based dry inductively coupled plasma reactive ion etching (ICP-RIE) has been employed to ful?ll such a task by several groups [15]–[20]. This approach can effectively modify the Vth of AlGaN/GaN HEMT to positive direction. However, the ICP dry etching has a low etch selectivity between materials and causes subsurface damages [21]. The ICPinduced damages and the associated defects lead to an increase in gate-leakage current. The post-etching rapid thermal annealing (RTA) at 700 ? C was found to be able to repair the damages [16], [19]. However, the RTA at such high temperatures will not be compatible with the gate metal (Ni/Au, for example) and has to be carried out prior to the gate deposition. As a result, photoresist has to be removed after the recess etching to carry out the RTA, followed by the second photolithography step for the gate electrode. Thus, the gate electrode and the recess etching are not self-aligned. To avoid a large access resistance that could be caused by the ungated recess region, the gate electrode is required to be larger than the recess window, an undesirable scheme for highly integrated circuits. Uniformity in the recess-etching depth and, consequently the uniformity

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 53, NO. 9, SEPTEMBER 2006

II. PHYSICAL MODEL OF THRESHOLD VOLTAGE For a conventional AlGaN/GaN HEMT with Si modulation doped layer, as shown in Fig. 1(a), the polarization charges need to be taken into account in the calculation of HEMT’s threshold voltage. Modi?ed from a generally used formula [23] by taking into account the effects of charge polarization, surface and buffer traps, the threshold voltage of the AlGaN/GaN HEMT can be expressed as Vth = φB /e ? dσ/ε ? ?EC /e + Ef0 /e
d x

e ? ε
0

dx
0

Nsi (x)dx ? edNst /ε ? eNb /Cb .

(1)

Where the parameters are de?ned as follows: φB σ metal-semiconductor Schottky barrier height. overall net (both spontaneous and piezoelectric) polarization charge at the barrier—AlGaN/GaN interface. AlGaN barrier-layer thickness. Si-doping concentration. conduction-band offset at the AlGaN/GaN heterojunction. Difference between the intrinsic Fermi level and the conduction band edge of the GaN channel. dielectric constant of AlGaN. net-charged surface traps per unit area. effective net-charged buffer traps per unit area. effective buffer-to-channel capacitance per unit area.

d Nsi (x) ?EC
Fig. 1. Cross sections of (a) conventional AlGaN/GaN HEMT and (b) AlGaN/GaN HEMT with immobile negative charges incorporated directly under the gate.

Ef0 ε Nst Nb Cb

in the threshold voltage are another challenging issues for this method. Recently, we demonstrated a technique of fabricating highperformance self-aligned E-mode AlGaN/GaN HEMTs using the ?uoride-based plasma treatment [22]. No change in AlGaN thickness is required in this method. The control of the threshold voltage was realized through a modulation of energy band by F? ions implanted in the AlGaN/GaN heterostructure during the plasma treatment. The ?uorine ions have a strong electronegativity and are negatively charged, effectively raising the potential in the AlGaN barrier and the 2DEG channel. As a result, the Vth can be shifted to positive values, and E-mode HEMTs can be fabricated. A post-gate annealing at a gateelectrode-compatible temperature of 400 ? C proves to be effective in recovering the plasma-induced damages. The plasma treatment and the gate-electrode formation are self-aligned, maintaining a low access resistance, a feature that is critical in achieving desirable device characteristics including high transconductance, low ON-resistance and low knee-voltage. In this paper, we propose a physical model to explain how the negatively charged ?uorine ions affect the threshold voltage in an AlGaN/GaN HEMT. Detailed experimental results of using this technique in HEMTs’ fabrication will also be presented. This paper is organized as follows. A physical model of the threshold voltage is discussed in Section II. In Section III, the devicefabrication process is described. The effects of CF4 plasma treatment and post-gate annealing on DC and RF characteristics of AlGaN/GaN HEMTs are shown and discussed in Section IV. A conclusion of our work is given in Section V.

The last two terms in (1) describe the effects of the surface traps and buffer traps, respectively. The AlGaN surface is at x = 0, and the direction pointing to the channel is the positive direction for the integration. Now, let us consider a case, in which a certain amount of immobile negative charges is introduced into the AlGaN barrier layer under the gate, as shown in Fig. 1(b). Because of electrostatic induction, these immobile negative charges can deplete 2DEG in the channel, raise the energy band, and hence modulate Vth . Including the effect of the negative charges con?ned in the AlGaN barrier, the modi?ed threshold voltage [from (1)] is given by Vth = φB /e?dσ/ε??EC /e+Ef0 /e
d x

?

e ε
0

dx
0

(Nsi (x)?NF (x)) dx?edNst /ε?eNb /Cb .

(2)

The positive-charge distribution pro?le Nsi (x) is replaced by the net charge distribution Nsi (x) ? NF (x), where NF (x) is the concentration of the negatively charged ?uorine ion. The surface-trap density (Nst ) could be modi?ed by the plasma treatment. The investigation on the surface traps’ variation as a result of the plasma treatment is ongoing.

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Fig. 2. Fluorine atoms’ distributions in AlGaN/GaN heterostructures treated by CF4 plasma and various post-gate RTA, measured by SIMS. The untreated device is used as a reference.

It was found that the ?uorine ions, which were incorporated into the AlGaN barrier layer by CF4 plasma treatment, could effectively shift the threshold voltage positively [24]. The F? ions’ incorporation in the AlGaN layer was con?rmed by secondary-ion-mass-spectrum (SIMS) measurements, as shown in Fig. 2. During CF4 plasma treatment, ?uorine ions are implanted into AlGaN/GaN heterostructure in a self-built electrical ?eld stimulated by the RF power. This process is similar to the plasma-immersion ion implantation (PIII) [25]–[27], a technique that has been developed to realize ultrashallow p-n junctions in advanced silicon technology. It is also concluded from the results shown in Fig. 2 that the implanted ?uorine ions have a good thermal stability in the AlGaN layer up to 700 ? C. It should be noted that, although the presence of the F? ions are con?rmed as the cause of the threshold-voltage shift, it is not clear what sites, either interstitial or substitutional, the F? ions occupy. Further investigation on the trap states associated with the F? ions is needed for a clear understanding. Recently, we have carried out a deep-level transient spectroscopy (DLTS) on the HEMT samples treated by CF4 plasma. Our observation is that the F? ions incorporated in the AlGaN barrier introduce a deep-level state that is at least 1.8 eV below the conduction-band minimum. As a result, the ?uorine ions are believed to introduce a negatively charged acceptorlike deep level in the AlGaN. By applying Poisson’s equation and Fermi–Dirac statistics, we simulated the conduction-band pro?les and the electron distributions of AlGaN/GaN HEMT structures with and without ?uorine ions incorporated in AlGaN layer. Both structures have the same epitaxial structure, shown in Fig. 1(a). For the F? ions incorporated HEMT structure, the negatively charged F? ions’ pro?le was extracted from SIMS measurement results of the ?uorine atoms’ distribution of an AlGaN/GaN HEMT structure that was treated by CF4 plasma at 150 W for 150 s and converted to an E-mode HEMT [22]. The simulated conductionband diagrams at zero gate bias were plotted in Fig. 3(a) and (b). For the simulated conduction band of E-mode HEMT, as shown in Fig. 3(b), the ?uorine concentration is approximated by using a linear distribution that the peak F concentration is 3 × 1019 cm?3 at the AlGaN surface, and the F concentration is assumed to be negligible at the AlGaN/GaN interface. A

Fig. 3. Simulated conduction-band diagrams of (a) conventional D-mode AlGaN/GaN HEMT without CF4 plasma treatment and (b) E-mode AlGaN/ GaN HEMT with CF4 plasma treatment; (c) shows the electron concentrations of these two devices.

total F? sheet concentration of ? 3 × 1013 cm?2 is suf?cient to not only compensate the Si doping (? 3.7 × 1013 cm?2 ) in the AlGaN barrier but also compensate the piezoelectric and spontaneous polarization-induced charges (? 1 × 1013 cm?2 ). Two signi?cant features can be observed. First, compared to the untreated AlGaN/GaN HEMT structure, the plasma-treated structure has its 2DEG channel’s conduction-band minimum above Fermi level, indicating a completely depleted channel and E-mode HEMT. As shown in the electron pro?les [shown in Fig. 3(c)], there are no electrons in the channel under the zero gate bias in the plasma-treated structure, indicating an E-mode HEMT operation. Second, the immobile negatively charged F? ions cause an upward bending of the conduction band, especially in AlGaN barrier, yielding an additional barrier height ΦF , as shown in Fig. 3(b). Such an enhanced barrier

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can signi?cantly suppress the gate Schottky diode current of AlGaN/GaN HEMT in both the reverse and forward bias regions. Detailed experimental results on the threshold-voltage control and the gate-leakage-current suppression will be given in Section IV. III. DEVICE FABRICATION The AlGaN/GaN HEMT structure used in this paper, as shown in Fig. 1(a), was grown on (0001) sapphire substrate in an Aixtron AIX 2000 HT metalorganic-chemical-vapordeposition (MOCVD) system. All the devices shown here were fabricated from the same 2-inch wafer. Detailed devicefabrication steps have been given in [22]. Compared to the fabrication of conventional AlGaN/GaN HEMTs, two distinguished steps were added. First, before the gate-electrode deposition, the gate region was treated with CF4 plasma in an STS RIE system. The treatment was carried out at a room temperature, and the gas ?ow was controlled to be 150 sccm. The plasma-treated gate region and the gate electrode were selfaligned. Devices with different Vth were fabricated by applying different CF4 plasma power and treatment time. Second, after the gate-metal deposition, the sample was annealed at 400 ? C for 10 min. We chose this RTA temperature with cautions, because the RTA at temperatures higher than 500 ? C can degrade both the gate Schottky contact and the source/drain ohmic contacts. It will be shown later that the plasma treatment induced damages can be effectively recovered with the RTA temperature at 400 ? C. For comparison, the HEMT without CF4 plasma treatment was also fabricated on the same sample and in the same processing run. All the devices were unpassivated in order to avoid any confusion caused by the passivation layer, which may change the stress in the AlGaN layer and alter the piezoelectric polarization [28], [29]. All the HEMT devices have a gate length of 1 ?m, a source-gate spacing of Lsg = 1 ?m and a gate-drain spacing of Lgd = 2 ?m. IV. EFFECTS OF CF4 PLASMA TREATMENT AND POST-GATE ANNEALING A. Threshold-Voltage Control DC current–voltage (I–V ) characteristics of the fabricated devices were measured using an HP4156A parameter analyzer. Transfer characteristics and transconductance (gm ) characteristics are shown in Fig. 4(a) and (b), respectively. Taking the conventional HEMT (i.e., without CF4 plasma treatment) as the baseline devices, the threshold voltage of all the other CF4 plasma-treated HEMTs are shifted to the positive direction. De?ning Vth as the gate-bias intercept of the linear extrapolation of the drain–current at the point of peak transconductance (gm ), the Vth of all the devices were extracted and listed in Table I. For the conventional HEMT, Vth is ?4 V. For the HEMT treated by CF4 plasma at 150 W for 150 s, Vth is 0.9 V, which corresponds to the E-mode HEMT. A maximum Vth shift of 4.9 V was achieved. In order to further reveal the effects of CF4 plasma treatment, the dependencies of Vth on both CF4 plasma treatment time and RF power are plotted in Fig. 5.

Fig. 4. DC I–V transfer characteristics of AlGaN/GaN HEMT at different CF4 plasma-treatment conditions: (a) Id versus Vgs and (b) gm versus Vgs . TABLE I EXTRACTED BARRIER HEIGHTS AND IDEALITY FACTORS OF GATE SCHOTTKY DIODES WITH DIFFERENT PLASMA TREATMENTS FROM FORWARD GATE CURRENTS; THRESHOLD VOLTAGES ARE ALSO LISTED

Fig. 5. Threshold voltage’s dependencies on (a) plasma power and (b) treatment time.

The higher is the plasma power and the longer is the treatment time, the larger is the shift in Vth . With the increase in the plasma treatment time, more ?uorine ions were implanted into

CAI et al.: CONTROL OF THRESHOLD VOLTAGE OF AlGaN/GaN HEMT BY FLUORIDE-BASED PLASMA TREATMENT

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Fig. 6. AFM image showing the insigni?cant etching effect of the CF4 plasma treatment on the AlGaN layer.

AlGaN layer. The increased ?uorine ion concentration leads to a reduced electron density in the channel, and causes the positive shift of Vth . When the plasma power increases, on one hand, ?uorine ions obtain a higher energy, and on the other hand, ?uorine ion ?ux increases due to the enhanced ionization rate of CF4 . With higher energy, ?uorine ions can reach at a deeper depth closer to the channel. The closer they are to the channel, the more effective are the ?uorine atoms to deplete 2DEG, and a larger shift in Vth is achieved. The increased ?uorine ions ?ux has the same effect on Vth as the increase of the plasma treatment time by raising the ?uorine atoms concentration in AlGaN layer. It should be noted that the nearly linear Vth versus time and Vth versus power relationships imply the possibilities of a precise control of Vth of AlGaN/GaN HEMTs. Although the Vth is shifted by CF4 plasma treatment, the gm is not degraded. As shown in Fig. 4(b), all the devices’ maximum gm are in the range of 149–166 mS/mm, except for that treated at 150 W for 60 s, which has a higher peak gm of 186 mS/mm. It is suspected that this singularity point was caused by the nonuniformity in epitaxial growth. Con?rmed by an atomic-forcemicroscope (AFM) measurement conducted on a CF4 -treated patterned sample (with part of the sample treated and other parts protected from the plasma treatment), the CF4 plasma treatment only results in an AlGaN-thickness reduction of less than 1 nm, as shown in Fig. 6. Thus, the almost constant transconductance indicates that the 2DEG mobility in the channel is maintained in our device fabrication. The key step in maintaining the transconductance is the post-gate annealing process. B. Recovery of Plasma-Induced Damages by Post-Gate Annealing As well known, the plasma normally induces damages and creates defects in semiconductor materials, and consequently degrades carriers’ mobility. RTA is an effective method to repair

Fig. 7. DC I–V transfer characteristics of the AlGaN/GaN HEMTs with and without the plasma treatment: (a) Id versus Vgs . (b) gm versus Vgs . The effect of the RTA is shown by the curves before and after the RTA.

these damages and recover the mobility. In the CF4 plasmatreated AlGaN/GaN HEMTs, we observed drain–current and transconductance degradation just after the plasma treatment. In Fig. 7, the drain–current and transconductance measured on an untreated device and a treated device (200 W, 60 s) before and after RTA (400 ? C for 10 min.) are plotted. Fig. 8 compares the output characteristics of the treated device before and after the RTA. The drain–current was 76% and the transconductance was 51% higher after the RTA in the treated device. The RTA process can recover majority of the mobility degradation in the plasma-treated device, while showing an insigni?cant effect on the conventional untreated device. Therefore, we can conclude that the recovery of Id and gm in the CF4 plasma-treated device is the result of the effective recovery of the 2DEG mobility at this RTA condition. Compared to a higher annealing temperature of 700 ? C, which is needed to recover damages induced by chlorine-based ICP-RIE in the case of recessedgate [16], [19], this lower RTA temperature implies that the CF4 plasma treatment creates lower damages than the chlorinebased ICP-RIE. It also enables the RTA process to be carried out after the gate deposition, ful?lling the goal of a self-aligned process. If the previous de?nition of Vth is used, the Vth of the CF4 plasma-treated device seems to be shifted from 0.03 to ?0.29 V after the RTA. When the start point of gm , as shown in Fig. 7(b), or the start point of Id at the logarithm scale, as shown in the inset of Fig. 7(a), is used as the criteria to evaluate Vth , the Vth of the CF4 plasma-treated device is not changed after the RTA. The good thermal stability of Vth is consistent

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Fig. 8. DC I–V output characteristics of the AlGaN/GaN HEMTs with plasma treatment before and after RTA, at 400 ? C for 10 min.

with the previously mentioned good thermal stability of ?uorine atoms in AlGaN layer. C. Suppression of Schottky Gate-Leakage Current AlGaN/GaN HEMTs always show much higher reverse gateleakage currents than the values theoretically predicted by the thermionic-emission (TE) model. The higher gate currents degrade the device’s noise performance [30] and raise the standby power consumption. In particular, forward gate currents limit the gate input voltage swing, hence the maximum drain–current [31]. Many efforts were made to suppress gate currents of AlGaN/GaN HEMTs. These efforts include using the gate metal with higher work function (i.e., P t, M o [32], Cu [33] etc.), modifying the HEMTs structure (such as adding GaN cap [34]) or diversion to metal-insulator-semiconductor heterostructure ?eld-effect transistors (MISHFETs) [35]–[37]. In the CF4 plasma-treated AlGaN/GaN HEMTs, we observed the suppressions of gate currents in both reverse and forward bias regions. And gate-current suppressions show dependencies on CF4 plasma-treatment conditions. Fig. 9(a) shows gate currents of AlGaN/GaN HEMTs with different CF4 plasma treatments. Fig. 9(b) is the enlarged plot of the forward gate bias region. In reverse bias region, compared to the conventional HEMT without CF4 plasma treatment, the gate-leakage currents of all the CF4 plasma-treated AlGaN/GaN HEMTs decreased. The higher is the plasma RF power and the longer is the plasma-treatment time, the lower is the gate-leakage current. At Vg = ?20 V, the gate-leakage current drops by more than four orders of magnitude from 1.2 × 10?2 A/mm for conventional HEMT to 7 × 10?7 A/mm for the AlGaN/GaN HEMT plasma treated at 200 W, 60 s. In forward region, the gate currents of all the CF4 plasma-treated AlGaN/GaN HEMTs also decrease. As a result, the turn-on voltages of the gate Schottky diode are extended, and the gate input voltage swings are increased. Using 1 mA/mm as the criterion [34], the turn-on voltage of the gate Schottky diode increases from 1 V for conventional HEMT to 1.75 V for the CF4 plasma-treated AlGaN/GaN HEMT at 200 W, 60 s. The suppression of the gate-leakage current in the CF4 plasma-treated AlGaN/GaN HEMT can be explained as follows. During CF4 plasma treatment, ?uorine ions are in-

Fig. 9. Gate currents of AlGaN/GaN HEMT with different CF4 plasma treatments: (a) both reverse and forward gate currents and (b) enlarged forward gate currents.

corporated into the AlGaN layer. These ions with a strong electronegativity act as immobile negative charges that cause the upward conduction-band bending in the AlGaN barrier layer due to the electrostatic induction effect. Thus, an additional barrier height ΦF , as shown in Fig. 3(b), is formed, and the effective metal-semiconductor barrier height is increased from ΦB to ΦB + ΦF . This enhanced barrier height can effectively suppress the gate Schottky diode current in both reverse and forward bias regions. With higher plasma power and longer treatment time, the ?uorine ion concentration in the AlGaN layer increases, and the effective barrier height is raised further, leading to a more signi?cant gate-current suppression. In Table I, we list the effective barrier heights and ideality factors that were extracted from the forward region of the measured gate currents by using the TE model. The effective barrier height of conventional HEMT is 0.4 eV, while the effective barrier height increases to 0.9 eV for the CF4 plasmatreated HEMT at 200 W, 60 s. The effective barrier heights of the CF4 plasma-treated HEMT also show a trend of increase with the plasma power and treatment time, except for the HEMT treated at 150 W, 20 s, which has a relatively higher effective barrier height. This exception is thought to be due to the process variations. The fact that the extracted effective barrier height is much lower than the theoretically predicted values and very large ideality factors (> 2.4) indicates that the gate currents of fabricated AlGaN/GaN HEMTs are not dominated by the TE mechanism but other mechanisms, such as vertical tunneling [38], surface barrier thinning [39], and trap-assisted tunneling [40], etc. Thus, the barrier heights and

CAI et al.: CONTROL OF THRESHOLD VOLTAGE OF AlGaN/GaN HEMT BY FLUORIDE-BASED PLASMA TREATMENT

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TABLE II ON-WAFER MEASURED ft AND fmax OF AlGaN/GaN HEMTS WITH DIFFERENT PLASMA TREATMENTS

Fig. 10. Dependencies of ft and fmax on gate bias for fabricated E-mode HEMT, where Vds is ?xed at 12 V.

ideality factors, which are extracted by using the TE model, are not accurate. Nevertheless, they provide suf?cient qualitative information for explaining the mechanism of the gate-current suppression in CF4 plasma-treated AlGaN/GaN HEMTs. Dynamic I–V characterizations were conducted by using an Accent DIVA D265 system to investigate the effects of CF4 plasma treatment on drain–current dispersion. The pulsewidth is 0.2 ?s and the pulse separation is 1 ms. Quiescent point is at VGS slightly (? 0.5 V) below the pinch-off and VDS = 15 V. Compared to static I–V characteristics, the maximum drain–current of conventional D-mode HEMT dropped by 63%, while that of E-mode HEMT with CF4 plasma treatment at 150 W for 150 s dropped by 6%. These results indicate that the CF4 plasma treatment, at least, does not turn the current dispersion to worse. The alleviation of drain–current drops for E-mode HEMT is likely due to a raised gate bias of the quiescent point (VGS = 0 V for E-mode HEMT, VGS = ?4.5 V for D-mode HEMT). D. RF Small-Signal Characteristics On-wafer small-signal RF characterization of all the fabricated AlGaN/GaN HEMTs were carried out at the frequency range of 0.1–39.1 GHz using Cascade microwave probes and Agilent 8722ES network analyzer. Open-pad de-embeddings with the S-parameters of dummy pads were carried out to eliminate a parasitic capacitance of the probing pads. The current gain and maximum stable gain/maximum available gain (MSG/MAG) of all devices with 1-?m long gate were derived from the deembedded S-parameters as a function of frequency. The current cutoff frequency (ft ) and maximum oscillation frequency (fmax ) were extracted from current gains and MSG/MAGs at unit gain. It has been observed that the intrinsic ft and fmax are generally 10–15% higher than the extrinsic ones without the de-embeddings process. In [22], the current gain and MSG/MAG as functions of frequency have been shown for the E-mode HEMT. Here, as a supplement, the dependencies of ft and fmax on the gate bias are shown in Fig. 10 for the E-mode HEMT. Both ft and fmax are relatively constant at both low and high gate bias, indicating a good linearity. Table II lists ft and fmax of all samples. For the conventional HEMT, ft and fmax are 13.1 and 37.1 GHz, while for the CF4 plasma-treated HEMTs, ft and fmax are around

10 and 34 GHz, a little lower than that of the conventional HEMT, except for the HEMT treated at 150 W, 60 s. This higher ft and fmax in this exceptional device are consistent with the higher gm presented before, and are attributed to a material nonuniformity and process variation. The slightly lower ft and fmax in the CF4 plasma-treated HEMTs indicate that the postgate RTA at 400 ? C can effectively recover the 2DEG mobility degraded by the plasma treatment, but the recovery is less than 100%. It suggests that the optimization of the RTA temperature and time is needed to further improve the 2DEG mobility, while not degrading the gate Schottky contact.

V. CONCLUSION We presented in details a method of precisely controlling the threshold-voltage of AlGaN/GaN HEMTs by the ?uoridebased plasma-treatment technique. A charge control model was presented to explain the underlying mechanisms of the technique. The ?uorine ions incorporated in the AlGaN barrier layer during the plasma treatment can positively shift the threshold voltage, When this shift is large enough, so that the threshold voltage reaches a positive value, E-mode HEMTs are realized. Because of the self-aligned nature of the treatment process, low access resistance can be maintained and desirable device performance including high transconductance, low ON-resistance, and low knee voltage are realized. Compared to the chloridebased plasma, the CF4 plasma treatment was also found to create damages that are easier to repair, which in turn, facilitates a post-gate annealing process at 400 ? C, a temperature that is within the tolerant range of the gate metals. The post-gate annealing was found to be effective in repairing the damages and mitigating the degradation of 2DEG mobility induced by the plasma damages. The device characteristics, as a function of plasma power and plasma-treatment time, are presented in details. The CF4 plasma treatment also shows a prominent effect on suppression of both reverse and forward gate current. In particular, the suppressed forward gate current extends the range of the gate input voltage, a critical factor in maximizing the drain–current in E-mode HEMTs. The techniques described in this paper enable the fabrication of E-mode HEMTs as well as the monolithic integration of E/D-mode HEMTs, both of which are expected to provide useful circuit applications. The monolithic integration of E/D-mode HEMTs has been recently implemented in direct-coupled FET logic circuits [41].

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 53, NO. 9, SEPTEMBER 2006

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CAI et al.: CONTROL OF THRESHOLD VOLTAGE OF AlGaN/GaN HEMT BY FLUORIDE-BASED PLASMA TREATMENT

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Yong Cai was born in Nanjing, China, in 1971. He received the B.S. degree from the Department of Electronics Engineering, Southeast University, Nanjing, in 1993, and the Ph.D. degree from the Institute of Microelectronics, Peking University, Beijing, in 2003. He is currently a postdoctoral Research Associate with the Department of Electrical and Electronic Engineering, Hong Kong University of Science and Technology, working on wide-bandgap GaN-based devices and circuits.

Yugang Zhou was born in Hubei Province, China, in 1975. He received the B.S. and Ph.D. degrees from the Department of Physics, Nanjing University, Nanjing, China, in 1996 and 2001, respectively. From September 2001 to September 2004, he worked as a Postdoctoral Research Associate with the Department of Electrical and Electronic Engineering, Hong Kong University of Science and Technology. In September 2004, he joined the Advanced Packaging Technology Ltd., Hongkong. Before September 2004, he mainly worked on MOCVD growth, device fabrication, and device physics of GaN-based HFETs. After then, he focused on the fabrication of GaN-based on high-power LED.

Kei May Lau (S’78–M’80–SM’92–F’01) received the B.S. and M.S. degrees in physics from the University of Minnesota, Minneapolis, in 1976 and 1977, respectively, and the Ph.D. degree in electrical engineering from Rice University, Houston, TX, in 1981. From 1980 to 1982, she was a Senior Engineer with M/A-COM Gallium Arsenide Products, Inc., where she worked on epitaxial growth of GaAs for microwave devices, development of high-ef?ciency and millimeter-wave impact ionization avalanche transit-time diodes, and multiwafer epitaxy by the chloride transport process. In the fall of 1982, she joined the faculty of the Department of Electrical and Computer Engineering, University of Massachusetts Amherst (Umass Amherst), where she became a Full Professor in 1993. She initiated metal–organic chemical vapor deposition, compound semiconductor materials and devices programs at Umass Amherst. Her research group performed studies on heterostructures, quantum wells, strained layers, III–V selective epitaxy, as well as high-frequency and photonic devices. She spent her ?rst sabbatical leave in 1989 at the Massachusetts Institute of Technology Lincoln Laboratory. She developed acoustic sensors at the DuPont Central Research and Development Laboratory, Wilmington, DE, during her second sabbatical leave (1995–1996). In the fall of 1998, she was a Visiting Professor with the Hong Kong University of Science and Technology (HKUST), where she joined the regular faculty in the summer of 2000. She established the Photonics Technology Center for R&D efforts in wide-bandgap semiconductor materials and devices. She became a Chair Professor of Electrical and Electronic Engineering at HKUST in July 2005. Prof. Lau was a recipient of the National Science Foundation Faculty Awards for Women Scientists and Engineers in the U.S. She served on the IEEE Electron Devices Society Administrative Committee and was an Editor of the IEEE TRANSACTIONS ON ELECTRON DEVICES (1996–2002). She also served on the Electronic Materials Committee of the Minerals, Metals and Materials Society of the American Institute of Materials Engineers.

Kevin J. Chen (M’95) received the B.S. degree from the Department of Electronics, Peking University, Beijing, China, in 1988, and obtained the Ph.D. degree from the University of Maryland, College Park, in 1993. From January 1994 to December 1995, he was a Research Fellow with the NTT LSI Laboratories, Atsugi, Japan, engaging in the research and development of functional quantum effect devices and heterojunction ?eld-effect transistors (HFETs). In particular, he developed the device technology for monolithic integration of resonant tunneling diodes and HFETs (MISFET and HEMT) on both GaAs and InP substrates for applications in ultra-high-speed signal processing and communication systems. He also developed the Pt-based buried gate technology that is widely used in the enhancement-mode HEMT devices. From 1996 to 1998, he was an Assistant Professor with the Department of Electronic Engineering, City University of Hong Kong, carrying out research on high-speed device and circuit simulations. He then joined the Wireless Semiconductor Division, Agilent Technologies, Inc. (formerly HewlettPackard Company), Santa Clara, CA, in 1999, working on enhancementmode pseudomorphic HEMT RF power ampli?ers used in dual-band global standard for mobile communications/digital cellular system wireless handsets. His work at Agilent covered RF characterization and modeling of microwave transistors, RF IC, and package design. In November 2000, he joined the Department of Electrical and Electronics Engineering, Hong Kong University of Science and Technology, where he is currently an Associate Professor. At HKUST, his group has carried out research on novel III-nitride device and fabrication techniques, silicon-based RF/microwave passive components, III-nitride and silicon-based microelectromechanical systems, RF packing technology, and microwave ?lter design. He has authored or coauthored over 130 publications in international journals and conference proceedings.


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