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GaAs pseudomorphic high electron mobility transistor grown by solid-source molecular beam epitaxy
Microelectronics Journal Microelectronics Journal 30 (1999) 23–28
Fabrication and characteristics of In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs pseudomorphic high electron mobility transistor grown by solid-source molecular beam epitaxy S.F. Yoon*, B.P. Gay, H.Q. Zheng, K.S. Ang, H. Wang, G.I. Ng School of Electrical and Electronic Engineering (Block S2), Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Accepted 5 June 1998
Abstract In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs pseudomorphic high electron mobility transistor (p-HEMT) structures were grown by solid-source molecular beam epitaxy (SSMBE) using a valved phosphorus cracker cell. The electron mobility and sheet carrier density at room temperature were 1700 cm 2/Vs and 3.3 ⫻ 10 12 cm ¹2, respectively. Despite the low two-dimensional electron gas (2DEG) mobility, a peak transconductance (G m) of 267 mS/mm and peak drain current density (I ds) of 360 mA/mm were measured for a p-HEMT device with 1.25 mm gate length. A high gate–drain breakdown voltage (BV gd) of 33 V was measured, a value which is more than doubled compared to that of a conventional Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs device. The drain–source breakdown voltage (BV ds) was 12.5 V. The results showed that the In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs material system grown by SSMBE using the valved phosphorus cracker cell for the In 0.48Ga 0.52P layers is clearly viable for p-HEMT device applications. 䉷 1998 Elsevier Science Ltd. All rights reserved. Keywords: High electron mobility transistor; Indium gallium phosphide
1. Introduction Pseudomorphic AlGaAs/InGaAs/GaAs high electron mobility transistors (p-HEMTs) have been one of the most established of III–V compound semiconductor transistors for varied applications in wireless communication. These devices have demonstrated excellent performance for high-speed and low-noise microwave applications. However, InGaP/InGaAs/GaAs pseudomorphic HEMTs have also recently attracted considerable attention due to the several advantages in using the InGaP/InGaAs/GaAs material system compared to the AlGaAs/InGaAs/GaAs system. First of all, the absence of Al in the InGaP/InGaAs/GaAs material system results in the elimination of problems related to the presence of DX centres, such as degradation of the drain current (I ds) and transconductance (G m) at low temperature [1,2]. The relatively less reactive InGaP, compared to AlGaAs, to oxidation enables more stable Schottky gate formation and reduction in the 1/f noise due to smaller surface recombination effects [1,2]. The InGaP material also has a very high wet etch selectivity with respect to * Corresponding author
GaAs. Hence, the gate recess step is much simplified since the etching will stop precisely at the InGaP layer, and problems related to gate recess over-etch can be avoided. This will also help to ensure that the gate contact–semiconductor interface will be uniform, a factor which is very critical to the DC and RF performance of the device [3]. From a material standpoint, the high valence band offset of the InGaP/InGaAs heterointerface acts as a barrier for holes, thus having the advantage of reducing the gate leakage current due to holes [4]. In the past, InGaP has been grown by various techniques such as liquid phase epitaxy (LPE) [5], metal–organic chemical vapour deposition (MOCVD) [6], gas source molecular beam epitaxy (GSMBE) [7] and chemical beam epitaxy (CBE) [8]. While these techniques have been successful for the growth of high quality InGaP layers, so far environmental concerns have limited their usage due to the toxicity of the hydrides and metalorganics used in the deposition process. Apart from these, there have been recent reports on the growth of InGaP by solid source molecular beam epitaxy (SSMBE) through sublimation of the GaP compound [9]. Despite the fact that high quality InGaP layers have been achieved, such a technique is relatively unconventional as it risks contamination by the group III
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elements, hence creating additional complications in the growth process. In the experiments we have performed, the growth of InGaP was carried out by SSMBE using a valved phosphorus cracker cell. This new evaporation technique for solid phosphorus overcomes the conventional problems associated with the handling of hydrides and metalorganics employed in growth processes involving the use of gas sources. The use of a valve in the cracker cell enables precise delivery and control of the phosphorus flux required for good control of the growth process without rapid depletion of the source materials. Due to the relatively short time since the introduction of this new evaporation technique for group V elements in SSMBE, there have been few reports on transistor structures fabricated from phosphorus-containing compounds grown using this method. Hence, in this paper we report the feasibility of fabricating an In 0.48Ga 0.52P/In 0.20Ga 0.52As/GaAs p-HEMT with In 0.48Ga 0.52P being used as the Schottky layer and spacer. Unlike previous studies which used GSMBE [1,2,10], metal organic vapour phase epitaxy (MOVPE) [4] or MOCVD [11,12], in our case the In 0.48Ga 0.52P layers were grown using the valved phosphorus cracker cell technique in a SSMBE system. The DC performance of the p-HEMT device with 1.25 mm gate length will be presented and compared against a typical Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs p-HEMT of the same gate length, as well as with those from other reports [1,2,12,13]. Details of the device fabrication process will be discussed.
2. Device fabrication The In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs p-HEMT structure shown in Fig. 1 was grown in a Riber MBE32 system equipped with a Riber KPC250 valved phosphorus cracker cell and a Riber VAC500 valved arsenic cracker cell. Details of InGaP growth using this system have been previously reported by Yoon et al. [14]. First of all, a 1 mmthick undoped GaAs buffer layer is grown on a GaAs (100) semi-insulating substrate at a substrate temperature (T s) ˚ -thick In 0.20Ga 0.80As of 600⬚C, followed by a 140 A
Fig. 1. Layer structure of the pseudomorphic In 0.48Ga 0.52P/In 0.20Ga 0.80As/ GaAs HEMT device.
pseudomorphic undoped channel layer grown at T s ¼ ˚ -thick undoped 530⬚C. This is then followed by a 30 A In 0.48Ga 0.52P spacer layer which is grown above the channel. Next, a silicon delta-doped layer with a sheet carrier concentration of 5 ⫻ 10 12 cm ¹2 is introduced to supply the twodimensional electron gas (2DEG) into the channel layer. A ˚ -thick undoped In 0.48Ga 0.52P Schottky layer was then 300 A ˚ -thick n þ GaAs cap layer grown, followed by a final 450 A with a silicon doping concentration of 5 ⫻ 10 18 cm ¹3. The In 0.48Ga 0.52P Schottky layer and spacer were grown at T s ¼ 500⬚C, while the GaAs cap layer was grown at T s ¼ 580⬚C. T s was maintained at 500⬚C during silicon delta doping. Device fabrication essentially involves only three masking steps, namely mesa, ohmic and gate. Conventional optical lithography and wet etching techniques were used to fabricate the In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs p-HEMT. Fabrication starts with the mesa formation, during which the cap layer was etched using a mixture of H 2SO 4:H 2O 2:H 2O (4:1:35 by volume) solution while a mixture of HCl:H 2O (6:5 by volume) was used to etch the InGaP Schottky layer and spacer. The mesa isolation was completed by removing the remaining layers using the same H 2SO 4:H 2O 2:H 2O solution until the buffer layer is reached. Ni/GeAu/Ni/Au metallisation was used for the formation of the ohmic source and drain contacts, which were fabricated by the lift-off technique. Contact annealing was performed at 405⬚C for 60 s on a hotplate under ambient N 2. The contact resistance which was measured using the transmission line method was 0.08 Q mm. The gate was then patterned using image reversal optical lithography, and recess etching carried out using a mixture of CH 3COOH:H 2O 2:H 2O (25:1:75 by volume) solution. The process of gate recess etching was made simpler due to the high etch selectivity of InGaP compared to GaAs. Finally, the gate contact was formed by evaporating Ti/Au, followed by a lift-off process. The fabricated In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs p-HEMT device has a gate length of 1.25 mm, defined using optical lithography.
3. Results and discussion The 2DEG mobility in the sample was 1700 cm 2/Vs at 300 K and 2000 cm 2/Vs at 77 K, and the sheet carrier density was 3.3 ⫻ 10 12 cm ¹2 and 3.7 ⫻ 10 12 cm ¹2, respectively. The carrier mobility was lower compared to the conventional Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs p-HEMT structure of almost identical doping level and layer thickness. This low mobility could have been contributed partly by the presence of a rough InGaP/InGaAs interface between the In 0.20Ga 0.80As channel layer and In 0.48Ga 0.52P spacer, an effect which influences the 2DEG carrier mobility [10]. It is possible that the interfacial roughness arises from intermixing between the two group V elements (As and P) at the region of the interface resulting from switching of the two elements during MBE growth. This has the effect of creating
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Fig. 2. Gate current (I g) as a function of gate voltage (V gs) for a conventional Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs p-HEMT and In 0.48Ga 0.52P/ In 0.20Ga 0.80As/ GaAs p-HEMT.
additional scattering centres at the InGaP/InGaAs interface and caused degradation in the 2DEG mobility. A similar effect was previously reported by Missous et al. [13] on In 0.48Ga 0.52P/In 0.15Ga 0.85As/GaAs p-HEMT grown using SSMBE from a GaP source. Future investigations will include transmission electron microscopic (TEM) analysis of the InGaP/InGaAs interface prepared under various MBE growth conditions, and these findings will be reported separately. The DC characteristics of the In 0.48Ga 0.52P/In 0.20Ga 0.80As/ GaAs 1.25 mm gate length p-HEMT were measured using the HP4155A semiconductor parameter analyser. Fig. 2 shows a plot of the gate current (I g) as a function of the gate voltage (V gs). The gate–drain breakdown voltage (BV gd), which is defined as the voltage for the gate current to reach 50 mA, was 33 V. The high value of BV gd measured in the In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs p-HEMT is comparable to those reported for devices grown by GSMBE [1,2] or MOCVD [11] where the gate–drain breakdown voltages are in the range of 25–34 V. Fig. 2 also shows the I g versus V gs characteristic of one of our conventional Al 0.30Ga 0.70As/ In 0.20Ga 0.80As/GaAs 1.25 mm-gate length p-HEMT devices. As can be seen, the gate–drain breakdown voltage is more than doubled in the In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs device compared to the Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs device. The higher breakdown voltage in the In 0.48Ga 0.52P/ In 0.20Ga 0.80As/GaAs device indicates a clear contribution of the undoped In 0.48Ga 0.52P Schottky layer and channel [1,11]. Using the method reported in [15], the measured drain–source breakdown voltage, BV ds was 12.5 V, indicating a high breakdown strength and thus making the In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs p-HEMT device a
promising candidate for high-power applications. The In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs p-HEMT was also found to have a higher turn-on voltage (2.5 V) compared to the Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs device (0.6 V). A larger turn-on voltage is beneficial for good device linearity and also indicates the possibility of having a larger input voltage swing. Fig. 3 shows that a peak transconductance (G m) of 267 mS/mm was achieved in our device. This value is higher than that reported by Yang et al. [12], and shows that the performance of our p-HEMT device grown by SSMBE is comparable to that of a device with similar structure and dimension but grown by MOCVD. Our device is also generally better in characteristics compared to that reported by Missous et al. [13] which used GaP as the source of phosphoprus in their SSMBE system. The transconductance is also slightly higher compared to that of the Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs p-HEMT (240 mS/ mm). This could very likely be due to the presence of the InGaP Schottky layer, which has a very high etch selectivity with respect to GaAs and is less susceptible to oxidation compared to AlGaAs. These advantages enabled better control over the gate formation process, resulting in a uniform gate metal–semiconductor interface and hence good DC characteristics. Experiments are in progress to investigate the characteristics of the InGaP/InGaAs interface between the channel layer and spacer, and the results are expected to further enhance the transconductance of the device. The maximum drain current (I ds) for our device was 360 mA/mm. Fig. 3 also shows that the G m versus V gs characteristic is relatively broad, demonstrating that the device has good
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Fig. 3. Plot of transconductance (G m) and drain current (I ds) as a function of gate voltage (V gs) for the In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs p-HEMT.
linearity and a wide operation region. Defining the ‘width’ of the broad region as the difference between two I ds values corresponding to 90% of maximum G m [1], we obtained an operational drain current latitude of 250 mA/mm, as shown in Fig. 4. The I ds versus V ds characteristic, as shown in Fig. 5, exhibits a sharp pinch-off behaviour. Due to the high etch selectivity of the InGaP/GaAs layer, good control during gate recessing was achieved, and the deviation of the threshold voltage was only ⬃0.1 V. This clearly demonstrates the significant advantage of having the InGaP etchstop Schottky layer. The peak current density (I ds) obtained
was 360 mA/mm. This value is similar in magnitude to that reported by Yang et al. [12], as well as to that of the Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs p-HEMT. Geiger et al. [4] have reported InGaP/InGaAs heterostructure field effect transistor (HFET) devices with higher current density, but this was achieved with quarter micron gate length dimension and the use of a doped channel in the device structure. Hence our results indicate that molecular beam epitaxial growth of the In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs material system using the valved phosphorus cracker cell is clearly viable for applications in high electron mobility transistors.
Fig. 4. Plot of transconductance (G m) as a function of drain current (I ds) for the In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs p-HEMT.
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Fig. 5. Plot of drain current (I ds) as a function of source–drain voltage (V ds) for various gate voltages (V gs) for the In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs p-HEMT.
4. Conclusions In conclusion, we report in this paper the successful fabrication of a In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs pseudomorphic HEMT grown by solid source molecular beam epitaxy using a valved phosphorus cracker cell for the InGaP Schottky layer and spacer. A relatively high transconductance of 267 mS/mm and drain current density of 360 mA/mm, respectively, were obtained. The gate–drain breakdown voltage of 33 V is more than doubled the value in a conventional Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs pHEMT of almost identical 2DEG concentration and layer thickness. These characteristics are possible because of the use of the InGaP Schottky layer and spacer which eliminate problems associated with the use of Al in a conventional Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs device. The high etch selectivity between InGaP and GaAs simplifies the HEMT fabrication process, hence enabling good control of the DC characteristics of the device. Our results have shown that the In 0.48Ga 0.52P/ In 0.20Ga 0.80As/GaAs material system grown by solid source MBE using the valved phosphorus cracker cell is clearly a viable technology for p-HEMT device applications.
References [1] Y.S. Lin, T.P. Sun, S.S. Lu, Ga 0.51In 0.49P/In 0.15Ga 0.85As/GaAs pseudomorphic doped-channel FET with high-current density and high-breakdown voltage, IEEE Electron Device Letters 18 (4) (1997) 150–152. [2] Y.S. Lin, S.S. Lu, Y. J Wang, High-performance Ga 0.51In 0.49P/GaAs airbridge gate MISFET’s grown by gas-source MBE, IEEE Transactions on Electron Devices 44 (6) (1997) 921–928.
[3] M. Tam, M. King, C.S. Wu, M. Sanna, H. Edwards, R. McGlothin, Impact of gate recess profile on GaAs microwave device performance using AFM, GaAs MANTECH (1997) 18–21. [4] D. Geiger, E. Mittermeier, J. Dickmann, C. Geng, R. Winterhof, F. Scholz, E. Kohn, InGaP/InGaAs HFET with high current density and high cut-off frequencies, IEEE Electron Device Letters 16 (6) (1995) 259–261. [5] G.B. Stringfellow, The importance of lattice mismatch in the growth of Ga xIn 1¹xP epitaxial crystals, J. Appl. Phys. 43 (8) (1972) 3455–3460. [6] Y. Ohba, M. Ishikawa, H. Sugawara, M. Yamamoto, T. Nakanisi, Growth of high-quality InGaAlP epilayers by MOCVD using methyl metalorganics and their application to visible semiconductor lasers, Journal of Crystal Growth 77 (1986) 374–379. [7] C.H. Yan, D.Z. Sun, H.X. Guo, X.B. Li, S.R. Zu, Y.H. Huang, Y.P. Zheng, M.Y. Kong, High-quality InGaP and InGaP/InAlP multiple quantum well grown by gas-source molecular beam epitaxy, Journal of Crystal Growth 136 (1994) 306–309. [8] Ph. Maurel, J.C. Garcia, Ph. Regreny, J.P. Hirtz, E. Vassilakis, A. Parent, M. Baldy, C. Carriere, Room temperature 600 mW CW output power per facet from single GaInAs/GaAs/GaInP large area laser diode grown by CBE, Electronics Letters 29 (1993) 91–93. [9] T. Shitara, K. Eberl, Electronic properties of InGaP grown by solidsource molecular-beam epitaxy with a GaP decomposition source, Appl. Phys. Lett. 65 (1994) 356–358. [10] S. Loualiche, A. Ginudi, A. Le Corre, D. Lecrosnier, C. Vaudry, L. Henry, C. Guillemot, Low-temperature DC characteristics of pseudomorphic Ga 0.18In 0.82P/InP/Ga 0.47In 0.53As HEMT, IEEE Electronic Device Letters 11 (4) (1990) 153–155. [11] L.W. Laih, S.Y. Cheng, W.C. Wang, P.H. Lin, J.Y. Chen, W.C. Liu, W. Lin, High-performance InGaP/InGaAs/GaAs step-compositioned doped-channel field-effect transistor (SCDCFET), Electronics Letters 33 (1) (1997) 98–99. [12] Y.F. Yang, C.C. Hsu, E.S. Yang, Integeration of GaInP/GaAs heterojunction bipolar transistor and high electron mobility transistors, IEEE Electron Device Letters 17 (7) (1996) 363–365. [13] M. Missous, A.A. Azta, A. Sandhu, InGaP/InGaAs/GaAs high electron mobility transistor structure grown by solid source molecular beam epitaxy using GaP as phosphorus source, Jpn. J. Appl. Phys. 36 (6a) (1997) 647–649.
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[14] S.F. Yoon, K.W. Mah, H.Q. Zheng, P.H. Zhang, Effects of substrate temperature on the properties of In 0.48Ga 0.52P grown by molecular beam epitaxy using a valved phosphorus cracker cell, Journal of Crystal Growth (1998) in press.
[15] S.R. Bahl, J.A. del Alamo, A new drain-current injection technique for the measurement of off-state breakdown voltage in FETs, IEEE Transactions on Electron Devices 40 (8) (1993) 1558–1560.
Fabrication and characteristics of In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs pseudomorphic high electron mobility transistor grown by solid-source molecular beam epitaxy S.F. Yoon*, B.P. Gay, H.Q. Zheng, K.S. Ang, H. Wang, G.I. Ng School of Electrical and Electronic Engineering (Block S2), Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Accepted 5 June 1998
Abstract In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs pseudomorphic high electron mobility transistor (p-HEMT) structures were grown by solid-source molecular beam epitaxy (SSMBE) using a valved phosphorus cracker cell. The electron mobility and sheet carrier density at room temperature were 1700 cm 2/Vs and 3.3 ⫻ 10 12 cm ¹2, respectively. Despite the low two-dimensional electron gas (2DEG) mobility, a peak transconductance (G m) of 267 mS/mm and peak drain current density (I ds) of 360 mA/mm were measured for a p-HEMT device with 1.25 mm gate length. A high gate–drain breakdown voltage (BV gd) of 33 V was measured, a value which is more than doubled compared to that of a conventional Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs device. The drain–source breakdown voltage (BV ds) was 12.5 V. The results showed that the In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs material system grown by SSMBE using the valved phosphorus cracker cell for the In 0.48Ga 0.52P layers is clearly viable for p-HEMT device applications. 䉷 1998 Elsevier Science Ltd. All rights reserved. Keywords: High electron mobility transistor; Indium gallium phosphide
1. Introduction Pseudomorphic AlGaAs/InGaAs/GaAs high electron mobility transistors (p-HEMTs) have been one of the most established of III–V compound semiconductor transistors for varied applications in wireless communication. These devices have demonstrated excellent performance for high-speed and low-noise microwave applications. However, InGaP/InGaAs/GaAs pseudomorphic HEMTs have also recently attracted considerable attention due to the several advantages in using the InGaP/InGaAs/GaAs material system compared to the AlGaAs/InGaAs/GaAs system. First of all, the absence of Al in the InGaP/InGaAs/GaAs material system results in the elimination of problems related to the presence of DX centres, such as degradation of the drain current (I ds) and transconductance (G m) at low temperature [1,2]. The relatively less reactive InGaP, compared to AlGaAs, to oxidation enables more stable Schottky gate formation and reduction in the 1/f noise due to smaller surface recombination effects [1,2]. The InGaP material also has a very high wet etch selectivity with respect to * Corresponding author
GaAs. Hence, the gate recess step is much simplified since the etching will stop precisely at the InGaP layer, and problems related to gate recess over-etch can be avoided. This will also help to ensure that the gate contact–semiconductor interface will be uniform, a factor which is very critical to the DC and RF performance of the device [3]. From a material standpoint, the high valence band offset of the InGaP/InGaAs heterointerface acts as a barrier for holes, thus having the advantage of reducing the gate leakage current due to holes [4]. In the past, InGaP has been grown by various techniques such as liquid phase epitaxy (LPE) [5], metal–organic chemical vapour deposition (MOCVD) [6], gas source molecular beam epitaxy (GSMBE) [7] and chemical beam epitaxy (CBE) [8]. While these techniques have been successful for the growth of high quality InGaP layers, so far environmental concerns have limited their usage due to the toxicity of the hydrides and metalorganics used in the deposition process. Apart from these, there have been recent reports on the growth of InGaP by solid source molecular beam epitaxy (SSMBE) through sublimation of the GaP compound [9]. Despite the fact that high quality InGaP layers have been achieved, such a technique is relatively unconventional as it risks contamination by the group III
0026-2692/99/$ - see front matter 䉷 1998 Elsevier Science Ltd. All rights reserved. PII: S0 02 6 -2 6 92 ( 98 ) 00 0 77 - 9
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elements, hence creating additional complications in the growth process. In the experiments we have performed, the growth of InGaP was carried out by SSMBE using a valved phosphorus cracker cell. This new evaporation technique for solid phosphorus overcomes the conventional problems associated with the handling of hydrides and metalorganics employed in growth processes involving the use of gas sources. The use of a valve in the cracker cell enables precise delivery and control of the phosphorus flux required for good control of the growth process without rapid depletion of the source materials. Due to the relatively short time since the introduction of this new evaporation technique for group V elements in SSMBE, there have been few reports on transistor structures fabricated from phosphorus-containing compounds grown using this method. Hence, in this paper we report the feasibility of fabricating an In 0.48Ga 0.52P/In 0.20Ga 0.52As/GaAs p-HEMT with In 0.48Ga 0.52P being used as the Schottky layer and spacer. Unlike previous studies which used GSMBE [1,2,10], metal organic vapour phase epitaxy (MOVPE) [4] or MOCVD [11,12], in our case the In 0.48Ga 0.52P layers were grown using the valved phosphorus cracker cell technique in a SSMBE system. The DC performance of the p-HEMT device with 1.25 mm gate length will be presented and compared against a typical Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs p-HEMT of the same gate length, as well as with those from other reports [1,2,12,13]. Details of the device fabrication process will be discussed.
2. Device fabrication The In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs p-HEMT structure shown in Fig. 1 was grown in a Riber MBE32 system equipped with a Riber KPC250 valved phosphorus cracker cell and a Riber VAC500 valved arsenic cracker cell. Details of InGaP growth using this system have been previously reported by Yoon et al. [14]. First of all, a 1 mmthick undoped GaAs buffer layer is grown on a GaAs (100) semi-insulating substrate at a substrate temperature (T s) ˚ -thick In 0.20Ga 0.80As of 600⬚C, followed by a 140 A
Fig. 1. Layer structure of the pseudomorphic In 0.48Ga 0.52P/In 0.20Ga 0.80As/ GaAs HEMT device.
pseudomorphic undoped channel layer grown at T s ¼ ˚ -thick undoped 530⬚C. This is then followed by a 30 A In 0.48Ga 0.52P spacer layer which is grown above the channel. Next, a silicon delta-doped layer with a sheet carrier concentration of 5 ⫻ 10 12 cm ¹2 is introduced to supply the twodimensional electron gas (2DEG) into the channel layer. A ˚ -thick undoped In 0.48Ga 0.52P Schottky layer was then 300 A ˚ -thick n þ GaAs cap layer grown, followed by a final 450 A with a silicon doping concentration of 5 ⫻ 10 18 cm ¹3. The In 0.48Ga 0.52P Schottky layer and spacer were grown at T s ¼ 500⬚C, while the GaAs cap layer was grown at T s ¼ 580⬚C. T s was maintained at 500⬚C during silicon delta doping. Device fabrication essentially involves only three masking steps, namely mesa, ohmic and gate. Conventional optical lithography and wet etching techniques were used to fabricate the In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs p-HEMT. Fabrication starts with the mesa formation, during which the cap layer was etched using a mixture of H 2SO 4:H 2O 2:H 2O (4:1:35 by volume) solution while a mixture of HCl:H 2O (6:5 by volume) was used to etch the InGaP Schottky layer and spacer. The mesa isolation was completed by removing the remaining layers using the same H 2SO 4:H 2O 2:H 2O solution until the buffer layer is reached. Ni/GeAu/Ni/Au metallisation was used for the formation of the ohmic source and drain contacts, which were fabricated by the lift-off technique. Contact annealing was performed at 405⬚C for 60 s on a hotplate under ambient N 2. The contact resistance which was measured using the transmission line method was 0.08 Q mm. The gate was then patterned using image reversal optical lithography, and recess etching carried out using a mixture of CH 3COOH:H 2O 2:H 2O (25:1:75 by volume) solution. The process of gate recess etching was made simpler due to the high etch selectivity of InGaP compared to GaAs. Finally, the gate contact was formed by evaporating Ti/Au, followed by a lift-off process. The fabricated In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs p-HEMT device has a gate length of 1.25 mm, defined using optical lithography.
3. Results and discussion The 2DEG mobility in the sample was 1700 cm 2/Vs at 300 K and 2000 cm 2/Vs at 77 K, and the sheet carrier density was 3.3 ⫻ 10 12 cm ¹2 and 3.7 ⫻ 10 12 cm ¹2, respectively. The carrier mobility was lower compared to the conventional Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs p-HEMT structure of almost identical doping level and layer thickness. This low mobility could have been contributed partly by the presence of a rough InGaP/InGaAs interface between the In 0.20Ga 0.80As channel layer and In 0.48Ga 0.52P spacer, an effect which influences the 2DEG carrier mobility [10]. It is possible that the interfacial roughness arises from intermixing between the two group V elements (As and P) at the region of the interface resulting from switching of the two elements during MBE growth. This has the effect of creating
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Fig. 2. Gate current (I g) as a function of gate voltage (V gs) for a conventional Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs p-HEMT and In 0.48Ga 0.52P/ In 0.20Ga 0.80As/ GaAs p-HEMT.
additional scattering centres at the InGaP/InGaAs interface and caused degradation in the 2DEG mobility. A similar effect was previously reported by Missous et al. [13] on In 0.48Ga 0.52P/In 0.15Ga 0.85As/GaAs p-HEMT grown using SSMBE from a GaP source. Future investigations will include transmission electron microscopic (TEM) analysis of the InGaP/InGaAs interface prepared under various MBE growth conditions, and these findings will be reported separately. The DC characteristics of the In 0.48Ga 0.52P/In 0.20Ga 0.80As/ GaAs 1.25 mm gate length p-HEMT were measured using the HP4155A semiconductor parameter analyser. Fig. 2 shows a plot of the gate current (I g) as a function of the gate voltage (V gs). The gate–drain breakdown voltage (BV gd), which is defined as the voltage for the gate current to reach 50 mA, was 33 V. The high value of BV gd measured in the In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs p-HEMT is comparable to those reported for devices grown by GSMBE [1,2] or MOCVD [11] where the gate–drain breakdown voltages are in the range of 25–34 V. Fig. 2 also shows the I g versus V gs characteristic of one of our conventional Al 0.30Ga 0.70As/ In 0.20Ga 0.80As/GaAs 1.25 mm-gate length p-HEMT devices. As can be seen, the gate–drain breakdown voltage is more than doubled in the In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs device compared to the Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs device. The higher breakdown voltage in the In 0.48Ga 0.52P/ In 0.20Ga 0.80As/GaAs device indicates a clear contribution of the undoped In 0.48Ga 0.52P Schottky layer and channel [1,11]. Using the method reported in [15], the measured drain–source breakdown voltage, BV ds was 12.5 V, indicating a high breakdown strength and thus making the In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs p-HEMT device a
promising candidate for high-power applications. The In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs p-HEMT was also found to have a higher turn-on voltage (2.5 V) compared to the Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs device (0.6 V). A larger turn-on voltage is beneficial for good device linearity and also indicates the possibility of having a larger input voltage swing. Fig. 3 shows that a peak transconductance (G m) of 267 mS/mm was achieved in our device. This value is higher than that reported by Yang et al. [12], and shows that the performance of our p-HEMT device grown by SSMBE is comparable to that of a device with similar structure and dimension but grown by MOCVD. Our device is also generally better in characteristics compared to that reported by Missous et al. [13] which used GaP as the source of phosphoprus in their SSMBE system. The transconductance is also slightly higher compared to that of the Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs p-HEMT (240 mS/ mm). This could very likely be due to the presence of the InGaP Schottky layer, which has a very high etch selectivity with respect to GaAs and is less susceptible to oxidation compared to AlGaAs. These advantages enabled better control over the gate formation process, resulting in a uniform gate metal–semiconductor interface and hence good DC characteristics. Experiments are in progress to investigate the characteristics of the InGaP/InGaAs interface between the channel layer and spacer, and the results are expected to further enhance the transconductance of the device. The maximum drain current (I ds) for our device was 360 mA/mm. Fig. 3 also shows that the G m versus V gs characteristic is relatively broad, demonstrating that the device has good
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Fig. 3. Plot of transconductance (G m) and drain current (I ds) as a function of gate voltage (V gs) for the In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs p-HEMT.
linearity and a wide operation region. Defining the ‘width’ of the broad region as the difference between two I ds values corresponding to 90% of maximum G m [1], we obtained an operational drain current latitude of 250 mA/mm, as shown in Fig. 4. The I ds versus V ds characteristic, as shown in Fig. 5, exhibits a sharp pinch-off behaviour. Due to the high etch selectivity of the InGaP/GaAs layer, good control during gate recessing was achieved, and the deviation of the threshold voltage was only ⬃0.1 V. This clearly demonstrates the significant advantage of having the InGaP etchstop Schottky layer. The peak current density (I ds) obtained
was 360 mA/mm. This value is similar in magnitude to that reported by Yang et al. [12], as well as to that of the Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs p-HEMT. Geiger et al. [4] have reported InGaP/InGaAs heterostructure field effect transistor (HFET) devices with higher current density, but this was achieved with quarter micron gate length dimension and the use of a doped channel in the device structure. Hence our results indicate that molecular beam epitaxial growth of the In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs material system using the valved phosphorus cracker cell is clearly viable for applications in high electron mobility transistors.
Fig. 4. Plot of transconductance (G m) as a function of drain current (I ds) for the In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs p-HEMT.
S.F. Yoon et al. / Microelectronics Journal 30 (1999) 23–28
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Fig. 5. Plot of drain current (I ds) as a function of source–drain voltage (V ds) for various gate voltages (V gs) for the In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs p-HEMT.
4. Conclusions In conclusion, we report in this paper the successful fabrication of a In 0.48Ga 0.52P/In 0.20Ga 0.80As/GaAs pseudomorphic HEMT grown by solid source molecular beam epitaxy using a valved phosphorus cracker cell for the InGaP Schottky layer and spacer. A relatively high transconductance of 267 mS/mm and drain current density of 360 mA/mm, respectively, were obtained. The gate–drain breakdown voltage of 33 V is more than doubled the value in a conventional Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs pHEMT of almost identical 2DEG concentration and layer thickness. These characteristics are possible because of the use of the InGaP Schottky layer and spacer which eliminate problems associated with the use of Al in a conventional Al 0.30Ga 0.70As/In 0.20Ga 0.80As/GaAs device. The high etch selectivity between InGaP and GaAs simplifies the HEMT fabrication process, hence enabling good control of the DC characteristics of the device. Our results have shown that the In 0.48Ga 0.52P/ In 0.20Ga 0.80As/GaAs material system grown by solid source MBE using the valved phosphorus cracker cell is clearly a viable technology for p-HEMT device applications.
References [1] Y.S. Lin, T.P. Sun, S.S. Lu, Ga 0.51In 0.49P/In 0.15Ga 0.85As/GaAs pseudomorphic doped-channel FET with high-current density and high-breakdown voltage, IEEE Electron Device Letters 18 (4) (1997) 150–152. [2] Y.S. Lin, S.S. Lu, Y. J Wang, High-performance Ga 0.51In 0.49P/GaAs airbridge gate MISFET’s grown by gas-source MBE, IEEE Transactions on Electron Devices 44 (6) (1997) 921–928.
[3] M. Tam, M. King, C.S. Wu, M. Sanna, H. Edwards, R. McGlothin, Impact of gate recess profile on GaAs microwave device performance using AFM, GaAs MANTECH (1997) 18–21. [4] D. Geiger, E. Mittermeier, J. Dickmann, C. Geng, R. Winterhof, F. Scholz, E. Kohn, InGaP/InGaAs HFET with high current density and high cut-off frequencies, IEEE Electron Device Letters 16 (6) (1995) 259–261. [5] G.B. Stringfellow, The importance of lattice mismatch in the growth of Ga xIn 1¹xP epitaxial crystals, J. Appl. Phys. 43 (8) (1972) 3455–3460. [6] Y. Ohba, M. Ishikawa, H. Sugawara, M. Yamamoto, T. Nakanisi, Growth of high-quality InGaAlP epilayers by MOCVD using methyl metalorganics and their application to visible semiconductor lasers, Journal of Crystal Growth 77 (1986) 374–379. [7] C.H. Yan, D.Z. Sun, H.X. Guo, X.B. Li, S.R. Zu, Y.H. Huang, Y.P. Zheng, M.Y. Kong, High-quality InGaP and InGaP/InAlP multiple quantum well grown by gas-source molecular beam epitaxy, Journal of Crystal Growth 136 (1994) 306–309. [8] Ph. Maurel, J.C. Garcia, Ph. Regreny, J.P. Hirtz, E. Vassilakis, A. Parent, M. Baldy, C. Carriere, Room temperature 600 mW CW output power per facet from single GaInAs/GaAs/GaInP large area laser diode grown by CBE, Electronics Letters 29 (1993) 91–93. [9] T. Shitara, K. Eberl, Electronic properties of InGaP grown by solidsource molecular-beam epitaxy with a GaP decomposition source, Appl. Phys. Lett. 65 (1994) 356–358. [10] S. Loualiche, A. Ginudi, A. Le Corre, D. Lecrosnier, C. Vaudry, L. Henry, C. Guillemot, Low-temperature DC characteristics of pseudomorphic Ga 0.18In 0.82P/InP/Ga 0.47In 0.53As HEMT, IEEE Electronic Device Letters 11 (4) (1990) 153–155. [11] L.W. Laih, S.Y. Cheng, W.C. Wang, P.H. Lin, J.Y. Chen, W.C. Liu, W. Lin, High-performance InGaP/InGaAs/GaAs step-compositioned doped-channel field-effect transistor (SCDCFET), Electronics Letters 33 (1) (1997) 98–99. [12] Y.F. Yang, C.C. Hsu, E.S. Yang, Integeration of GaInP/GaAs heterojunction bipolar transistor and high electron mobility transistors, IEEE Electron Device Letters 17 (7) (1996) 363–365. [13] M. Missous, A.A. Azta, A. Sandhu, InGaP/InGaAs/GaAs high electron mobility transistor structure grown by solid source molecular beam epitaxy using GaP as phosphorus source, Jpn. J. Appl. Phys. 36 (6a) (1997) 647–649.
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S.F. Yoon et al. / Microelectronics Journal 30 (1999) 23–28
[14] S.F. Yoon, K.W. Mah, H.Q. Zheng, P.H. Zhang, Effects of substrate temperature on the properties of In 0.48Ga 0.52P grown by molecular beam epitaxy using a valved phosphorus cracker cell, Journal of Crystal Growth (1998) in press.
[15] S.R. Bahl, J.A. del Alamo, A new drain-current injection technique for the measurement of off-state breakdown voltage in FETs, IEEE Transactions on Electron Devices 40 (8) (1993) 1558–1560.