Improved temperature-dependent characteristics of a sulfur-passivated AlGaAs/InGaAs/GaAs pseudomorphic high-electron-mobility transistor

Improved Temperature-Dependent Characteristics of a Sulfur-Passivated AlGaAs/InGaAs/GaAs Pseudomorphic High-Electron-Mobility Transistor

Po-Hsien Lai, Ssu-I Fu, Yan-Ying Tsai, Ching-Wen Hung, Chih-Hung Yen, Hung-Ming Chuang, and Wen-Chau Liu ^z

Institute of Microelectronics, Department of Electrical Engineering, National Cheng-Kung University, Tainan, 70101, Taiwan

The temperature-dependent characteristics of 共NH

兲

S

-passivated AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistors were studied and demonstrated. Due to the use of sulfur passivation, remarkable improvements in device performance, including higher forward turn-on voltage, higher reverse breakdown voltage, lower reverse leakage current, higher transconduc- tance, lower on-resistance, more linear operating regime, and superior microwave performance, were obtained. In addition, the sulfur-passivated devices also show good properties in the higher operating temperature regime and relatively thermally stable performance over the operation temperature range 300–510 K. Therefore, the studied device with 共NH

兲

S

treatment provides promise for high-performance digital and microwave device applications.

© 2006 The Electrochemical Society. 关DOI: 10.1149/1.2199433兴 All rights reserved.

Manuscript submitted February 23, 2006; revised manuscript received March 16, 2006. Available electronically May 10, 2006.

Due to progressive improvements in growth techniques, hetero- structure field-effect transistors 共HFETs兲 have been studied exten- sively for high-speed microwave applications. ^1-3 Nevertheless, the III-V HFETs may suffer from surface-related mechanisms. The sur- face of III-V semiconductor materials such as AlGaAs is known to be plagued by a high density of interface states. ⁴ Furthermore, the extrinsic surface, formed by the typical gate-recess etching pro- cesses, traps a significant portion of the electrons and causes the increase of the depletion layer between the source and the gate edge near the source. ^5,6 This adversely influences the effect of applied gate bias on the active channel and results in reduced transconduc- tance and poor long-term device stability. Therefore, the lack of an adequate surface passivation for the III-V HFETs severely degrades device performance and restricts application of compound semicon- ductor circuits. In order to overcome this undesirable problem, the surface passivation becomes a crucial requirement for fabricating high-performance electronic and optoelectronic devices. ^7-9 In par- ticular, sulfur passivation in a wet chemical solution of 共NH 4 兲 2 S

was shown to be an effective way to substantially lower AlGaAs surface recombination centers. ^10,11

In this work, the temperature-dependent characteristics of a sulfur-passivated AlGaAs/InGaAs/GaAs pseudomorphic high- electron-mobility transistor 共PHEMT兲 were studied and demon- strated. Sulfur passivation was employed to a gate junction to avoid the difficulty in fabricating high-performance metal/AlGaAs Schottky contacts. The passivation method controls defective states originating from 共i兲 air oxidation of the AlGaAs surface before Schottky metallization and 共ii兲 interfacial reaction during metalliza- tion. In other words, the sulfur passivation effectively reduces the series resistances and interface states. This also indicates that the Fermi-level pinning effect produced by native surface oxides and nonradiative recombination centers can be eliminated. ^11,12 There- fore, good device performance, high-temperature operation capabil- ity, and relatively thermally stable characteristics are simultaneously obtained.

Experimental

The epitaxial structure of the studied device was grown on a 共100兲-oriented semi-insulating 共S.I.兲 GaAs substrate by a metallor- ganic chemical vapor deposition 共MOCVD兲 system. The layer structure consisted of a 4000 Å GaAs buffer layer, a 200 Å AlAs buffer layer, a 50 Å Al _0.25 Ga _0.75 As space layer, a ␦-doped sheet ␦ 2 共n ⁺ 兲 = 1 ⫻ 10 ¹² cm ⁻² , a 50 Å Al _0.25 Ga _0.75 As space layer, a

150 Å In _0.15 Ga _0.85 As channel layer, a 50 Å Al _0.25 Ga _0.75 As space layer, a ␦-doped sheet ␦ 1 共n ⁺ 兲 = 4 ⫻ 10 ¹² cm ⁻² , a 400 Å n-Al _0.25 Ga _0.75 As 共n = 3 ⫻ 10 ¹⁷ cm ⁻³ 兲 Schottky barrier layer, a 300 Å n-GaAs 共n = 3 ⫻ 10 ¹⁷ cm ⁻³ 兲 space layer, and a 500 Å n ⁺ -GaAs 共n ⁺ 艌 3 ⫻ 10 ¹⁸ cm ⁻³ 兲 cap layer. After epitaxial growth, wet chemical etching, conventional vacuum evaporation, and lift-off techniques were used to fabricate mesa-type devices. Drain-source ohmic contacts were formed on the n ⁺ -GaAs cap layer by alloying evaporated AuGeNi/Au metals at 430°C for 3 min. Then n ⁺ -GaAs cap and n-GaAs space layers were removed and the sample 共device A 兲 was immediately passivated by soaking in the ammonia–sulfide 关共NH 4 兲 2 S

, 5% 兴 solution for 10 min at room temperature. After sur- face passivation, the sample was rinsed in deionized 共DI兲 water, followed by blown-dry N ₂ gas. This procedure was used to remove thin, whitish residual amorphous sulfur on the surfaces of AlGaAs and ohmic contact metal. For comparison, another device 共denoted device B 兲 prepared with the same process only without 共NH 4 兲 2 S

treatment was also included in this work. Finally, the gate Schottky contacts were achieved by evaporating Pt/Au metals on the n-Al _0.25 Ga _0.75 As Schottky barrier layer with the gate dimension of 1 ⫻ 100 ␮m ² . The experimental dc current–voltage 共I–V兲 charac- teristics were measured by an HP4156A semiconductor parameter analyzer. The microwave performances of the studied devices were measured by an HP8510C network analyzer in conjunction with Cascade probes at different temperature. During the process, device characteristics are monitored systematically to assure that the pro- cess is reproducible.

Results and Discussion

Figure 1 shows forward turn-on voltage 共V on 兲, gate-drain break- down voltage 共BV GD 兲, and reverse gate leakage current 共I

兲 as a function of temperature. The V _on , BV _GD , and I

were measured un- der the gate current of 1 mA/mm, −0.5 mA/mm, and the gate-drain voltage of V _GD = −22 V, respectively. For device A 共with sulfur passivation 兲 and B 共without sulfur passivation兲, the V on values were decreased from 0.994 to 0.69 V and 0.936 to 0.598 V, respectively, as the temperature was increased from 300 to 510 K. The corre- sponding BV _GD were decreased from 36.4 to 21.5 V and 32.4 to 16.9 V, while the corresponding I

values were increased from 0.6 to 571 ␮A/mm and 7 to 1830 ␮A/mm. The I

of device B, was several times higher than that of device A. This indicates that the high density defects were produced near the AlGaAs interface of device B, which caused the increases of nonradiative recombination centers and surface leakage current. Thus, the Schottky and break- down characteristics deteriorated. The properties of V _on , BV _GD , and

z

E-mail: wcliu@mail.ncku.edu.tw

Journal of The Electrochemical Society, 153 共7兲 G632-G635 共2006兲

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I

of device A were remarkably improved over a wide temperature range 共300–510 K兲. This may be due to DI water rinsing after the 共NH 4 兲 2 S

treatment, which helped to remove the deposited sulfur in the forms of sulfate and polysulfur. ^9,13 This is a crucial process to reduce the surface leakage current. In addition, the temperature deg- radation rates in V _on 共⳵V on / ⳵T兲, BV GD 共⳵BV GD / ⳵T兲, and I

共 ⳵I

⳵T兲 of device A were only −1.46 mV/K, −66.1 mV/K, and 2.02 ␮A/mm K, respectively, as the temperature was increased from 300 to 510 K. The corresponding temperature degradation rates of device B were −1.60 mV/K, −68.9 mV/K, and 7.14 ␮A/mm K, respectively. Device A still maintained high V on , high BV _GD , and low I

values at higher temperature regimes, imply- ing that the sulfur passivation can reduce the generation of leakage current resulting from the increase of temperature. Therefore, the sulfur-passivated device exhibited improved thermal stability of metal/AlGaAs Schottky contact among a wide range of operating temperatures.

Figure 2a-c shows typical common-source I–V characteristics of the studied devices at 300, 390, and 510 K. The applied gate-source voltage was kept at V _GS = −0.5 V/step. Device A shows good pinch- off and saturation characteristics with high transconductance and without significant gate leakage current, even at a higher tempera- ture of 510 K. This implies that the high breakdown voltage associ- ated with good Schottky behavior and carrier confinement effect is obtained in device A. Besides, the drain saturation current 共I DS 兲 of device A 共B兲, measured at V DS = 3.5 V and V _GS = + 1.0 V, were 523 共497兲, 485 共460兲, and 433 共392兲 mA/mm at 300, 390, and 510 K, respectively. The deviations of I _DS were 17.2 and 21.1% for device A and B as the temperature was increased from 300 to 510 K. Obviously, the degradation of I _DS values caused by the increase of temperature is relatively insignificant in device A.

Because the sulfur-passivated device has more stable group III ele- ments without dangling bonds on the AlGaAs surface, this certainly suppressed the interface traps and reduced the leakage current over operating temperatures ranging from 300 to 510 K. ^11,14-16 There- fore, based on the 共NH 4 兲 2 S

treatment, the studied device is cer- tainly suitable for high-temperature applications.

The maximum extrinsic transconductance 共g m,max 兲 and on- resistance 共R on 兲 as a function of temperature are illustrated in Fig. 3.

R _on is determined from the slope of the linear region by the extrapo- lation of I _DS vs V _DS curves. The g _m,max value of device A 共B兲 was decreased from 240 共179兲 to 211 共154兲 mS/mm as the temperature was increased from 300 to 510 K. On the contrary, the correspond-

ing R _on was increased from 2.6 共4.0兲 to 3.7 共5.5兲 ⍀ mm. The on- resistance 共R on 兲 and intrinsic transconductance 共g mi 兲 can be ex- pressed as ^17,18

Figure 1. Forward turn-on voltage 共V

兲, gate-drain breakdown voltage 共BV

兲, and reverse gate leakage current 共I

兲 as a function of temperature.

The V

, BV

, and I

values are measured under the I

= 1 mA/mm, I

= −0.5 mA/mm, and V

= −22 V, respectively.

Figure 2. Typical common-source I–V characteristics at 共a兲 300, 共b兲 390, and 共c兲 510 K.

G633 Journal of The Electrochemical Society, 153 共7兲 G632-G635 共2006兲 G633

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R _on =

^⳵V ⳵I ds ^ds

= R _ch + R

关1兴

and

g _mi = g

1 − g

R

关2兴 where R _ch is the channel resistance and R

the parasitic resistance.

Using measured values of R _on and g

and calculated values of R _ch , the g _mi values are 503 and 474 mS/mm at 300 and 510 K, respec- tively. The corresponding R

values of device A 共B兲 are estimated to be about 2.2 共3.6兲 and 2.6 共4.4兲 ⍀ mm, respectively. It is known that the parasitic resistance and interface state density significantly affect the device characteristics. However, due to the improved Schottky characteristics and reduced parasitic resistance of device A, the depletion layer between the drain-source and the gate edge was eliminated. Hence, upon sulfur passivation, the gate control ability and transconductance performance were improved. More- over, the decreased rate of g _m,max and related temperature coefficient in R _on 共 ⳵R on / ⳵T兲 of device A were only 12% and 5.24 ⫻ 10 ⁻³ ⍀ mm/K, which were lower than those of device B 共14% and 6.31 ⫻ 10 ⁻³ ⍀ mm/K兲 over the temperature range be- tween 300 and 510 K. This proves that the studied device with 共NH 4 兲 2 S

treatment also exhibits relatively temperature-independent characteristics in terms of g _m,max and R _on .

The transconductance 共g

兲 vs drain saturation current 共I DS 兲 of device A at different temperatures are depicted in Fig. 4. The inset shows the I _DS operating regime as a function of temperature. The biased voltage was fixed at V _DS = 3.5 V. The I _DS operating regime was defined as the I _DS range where g _m,max is larger than 90% of its peak value. The width of the I _DS operating regime was decreased from 348 共329兲 to 242 共187兲 mA/mm for device A 共B兲 as the tem- perature was increased from 300 to 510 K. These flat and broad I _DS operating regimes of device A 共B兲 were higher than 62 共62兲% and 55 共45兲% of the maximum output current at 300 and 510 K, respec- tively. Apparently, the I _DS operating regimes of device A were larger than those of device B. In addition, the I _DS degradation rate with temperature 共 ⳵I/I兲共1/⳵T兲 of device A 共−1.45 ⫻ 10 ⁻³ /K 兲, from 300 to 510 K, was relatively insignificant. This good device linear- ity over a wide temperature range 共300–510 K兲 indeed shows the high-quality Schottky contact of the gate electrode after sulfur pas- sivation. This good behavior also demonstrates that the 共NH 4 兲 2 S

treatment is effective in eliminating the interface state formation at the metal/AlGaAs Schottky contact and suppressing the generation of perimeter and intrinsic leakage currents. Thus, for device A, good

properties of amplification performance are achieved, even if the temperature was elevated to 510 K. In addition, the dependencies of normalized I _DS and normalized g _m,max on the stress time, under stress conditions of V _DS = 7.0 V and V _GS = 0 V at 360 K, are shown in Fig. 5. The I _DS and g _m,max values decrease and then nearly maintain at the same magnitude as the stress time increases. After 188 h of stress testing, the variations of I _DS 共g m,max 兲 for devices A and B were lower than 2.2 共2.8兲% and 4.4 共5.5兲%, respectively. This implies that the 共NH 4 兲 2 S

treatment is effective in eliminating the interface state formation at the metal/AlGaAs Schottky contact and protecting the AlGaAs surface from oxidation. Therefore, the sulfur- passivated device exhibits relatively temperature-independent and thermal stability characteristics in terms of I _DS and g _m,max .

Figure 6 reveals the unity current gain cutoff frequency 共 f

兲, maximum oscillation frequency 共 f max 兲, and I DS operating regime 共⬎0.8 maximum values of f

, f _max 兲 as a function of temperature.

The inset shows microwave characteristics of device A at 300, 350, and 400 K. The biased voltages were fixed at V _DS = 3.5 共3.5兲 V and Figure 3. Maximum extrinsic transconductance 共g

兲 and on-resistance

共R

兲 as a function of temperature.

Figure 4. Transconductance 共g

兲 vs drain saturation current 共I

兲 of device A at different temperature. The inset shows I

operating regime 共⬎0.9g

兲 as a function of temperature. The biased voltage is fixed at V

= 3.5 V.

Figure 5. Normalized I

and normalized g

vs stress time under stress conditions of V

= 7.0 V and V

= 0 V at 360 K.

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V _GS = −0.5 共−1.0兲 V of device A 共B兲. For device A 共B兲, the f

and f _max were 21.2 共17.7兲 GHz and 73.4 共65.4兲 GHz at 300 K, respec- tively. Even at 400 K, the corresponding f

and f _max still maintain 19.5 共14.7兲 and 59.3 共53.0兲 GHz, respectively. Obviously, it is found that device A shows improved microwave characteristics. This is caused by reduction of the surface-bound charges and the corre- sponding capacitance upon 共NH 4 兲 2 S

treatment. ^6,11 Hence, due to the lower C

, the f

and f _max values of device A are higher than those of device B at 300–400 K. In addition, the studied device A 共B兲 maintains 80% of its f

and f _max peak values over large I _DS operating regimes of 475 共320兲, 470 共305兲, and 455 共300兲 mA/mm at T = 300, 350, and 400 K, respectively. The degradation rate 共⳵I DS /I _DS 兲共1/⳵T兲 in I DS operating regimes of 共−4.21 ⫻ 10 ⁻⁴ /K 兲 of device A is relatively insignificant as the temperature is increased from 300 to 400 K. Because the 共NH 4 兲 2 S

treatment offers an effec- tive passivation on the AlGaAs surface, this certainly suppresses the generation of interface state density and surface leakage current. ^11,19-21 Thus, upon the 共NH 4 兲 2 S

treatment, the studied de- vice with good linearity and high-temperature operation capability in frequency behaviors can be obtained.

Conclusion

The influences of 共NH 4 兲 2 S

treatment on an AlGaAs/InGaAs/GaAs PHEMT were studied and fabricated.

Experimentally, for a 1 ⫻ 100 ␮m ² gate-dimension sulfur- passivated PHEMT, the forward turn-on voltage of 0.994 共0.69兲 V,

gate-drain breakdown voltage of 36.4 共21.5兲 V, gate leakage current of 0.6 共571兲 ␮A/mm at V GD = −22 V, maximum transconductance of 209 共159兲 mS/mm, on-resistance of 2.6 共3.7兲 ⍀ mm, and linear operating regime of 348 共242兲 mA/mm were obtained, respectively, at 300 共510兲 K. The corresponding microwave properties of f

and f _max are 21.2 共19.5兲 and 73.4 共59.3兲 GHz at 300 共400兲 K, respec- tively. Moreover, the degradations of device performance with the increase of temperature are insignificant. Therefore, the studied de- vice provides the promise for high-temperature and high- performance microwave applications.

Acknowledgments

Part of this work was supported by the National Science Council of the Republic of China under contract no. NSC-94-2215-E-006- 060 and no. 94-2215-E-197-002. The authors are also grateful to National Nano Device Laboratories 共NDL兲 for radio frequency mea- surements.

National Cheng Kung University assisted in meeting the publication costs of this article.

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Figure 6. Unity current gain cutoff frequency 共 f

兲, maximum oscillation frequency 共 f

兲, and I

operating regime 共⬎0.8 maximum values of f

, f

兲 as a function of temperature. 共Inset兲 Microwave characteristics of de- vice A at 300, 350, and 400 K. The biased voltages are fixed at V

= 3.5 共3.5兲 V and V

= −0.5 共−1.0兲 V for device A 共B兲.

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