GaAs pseudomorphic high electron mobility transistor structures

PERGAMON

Solid State Communications 112 (1999) 661±664

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Improved transport properties of InxGa12xP/In0.2Ga0.8As/GaAs pseudomorphic high electron mobility transistor structures H.Q. Zheng*, S.F. Yoon, K. Radhakrishnan, G.I. Ng School of Electrical and Electronic Engineering (Block S1), Nanyang Technological University, Nanyang Avenue, Singapore, Singapore 639798 Received 26 July 1999; received in revised form 1 September 1999; accepted 2 September 1999 by H. Akai

Abstract A strained In0.40Ga0.60P/In0.2Ga0.8As/GaAs pseudomorphic high electron mobility transistor structure (PHEMT) was proposed to improve the electron transport properties. The structures were grown by the solid source molecular beam epitaxy (SSMBE) technique. With the incorporation of a strained In0.40Ga0.60P barrier layer and a GaAs smoothing layer, higher Hall mobility was achieved, indicating that better electron distribution was formed in the proposed structure. Photoluminescence (PL) measurements veri®ed that the incorporation of a strained barrier and a smoothing layer into the PHEMT structure modi®es the electron distribution so that most of the electrons are distributed in the In0.2Ga0.8As channel, resulting in high electron mobility. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Semiconductors; A. Surfaces and interfaces; B. Epitaxy; D. Electronic transport; E. Luminescence

1. Introduction InxGa12xP/In0.2Ga0.8As/GaAs pseudomorphic high electron mobility transistors (PHEMTs) have attracted great attention for their potential in the high power and high frequency applications [1,2]. Compared to the more matured AlGaAs/InGaAs/GaAs PHEMT system, this material system has many advantages. It can have a higher breakdown voltage because of the wide bandgap of InxGa12xP of 1.96 eV at x ˆ 0:52 (lattice matched to GaAs) [3], higher Schottky barrier height [4], and greater con®nement of holes due to the high DEC/DEV ratio of 0.6 [5], which limits the gate leakage current generated by impact ionization in the channel. It also provides a technological advantage in yield because of the high selective etching of InGaP with respect to GaAs [6]. It could enhance the reliability because the InGaP layer is less susceptible to oxidation compared to AlGaAs. However, the mobilities of the In0.48Ga0.52P/ In0.2Ga0.8As/GaAs structures are generally lower compared to AlGaAs/InGaAs PHEMT structures regardless of the * Corresponding author. Tel.: 165-7904528; fax: 165-793-3318. E-mail address: [email protected] (H.Q. Zheng)

growth techniques used, such as solid-source molecular beam epitaxy (SSMBE) [7], gas-source molecular beam epitaxy (GSMBE) [8] and metal organic chemical vapor deposition (MOCVD) [9]. The low mobility was caused by the In0.48Ga0.52P/In0.2Ga0.8As interface due to As/P interdiffusion driven by the chemical bond energy difference between GaP and GaAs [10], and In surface segregation and desorption [11,12]. Recently, Schuler et al. [8] have con®rmed this hypothesis in a study of interface quality and electron transfer of InGaP on GaAs. They provided a solution to improve the electron transfer in these structures by inserting a few AlInP monolayers at the interface. Although the insertion of six monolayers of AlInp signi®cantly improved the electron transfer and transport properties, AlInP is not a good choice to replace AlGaAs in electronic devices, if the purpose is to have an entirely aluminium-free device. In this letter, we report an alternative way to improve the electron transfer and transport properties without using Al in these structures. In our proposed structures, instead of using lattice-matched In0.48Ga0.52P layers, the combination of a GaAs smoothing layer and strained InxGa12xP layers was adopted.

0038-1098/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(99)00418-4

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H.Q. Zheng et al. / Solid State Communications 112 (1999) 661±664

Fig. 1. InxGa12xP/In0.2Ga0.8As/GaAs PHEMT layer structures studied in this work.

2. Experiments The layer structures studied were grown on (100) GaAs substrates in a SSMBE system equipped with a valved phosphorus cracker cell and a valved arsenic cracker cell. Ph2 and As4 were used as group V sources. Prior to growth, oxide desorption was carried out under arsenic ¯ux at a beam equivalent pressure (BEPAs) of 5 £ 1026 Torr: The process of surface oxide desorption involved slowly ramping up the substrate temperature at a rate of 408C/min until the re¯ection high-energy electron diffraction (RHEED) pattern showed clear 2 £ 4 and 4 £ 2 surface reconstruction. This surface reconstruction transformation was adopted as a means for calibrating the substrate temperature (Ts) which was set to be 5908C measured using an infrared pyrometer of appropriate wavelength sensitivity and temperature range. The indium compositions of the InxGa12xP layer were measured by using double-axis X-ray diffraction (XRD) measurements in conjunction with ¯ux adjustments. The growth condition optimization for the InxGa12xP layers has been previously reported [3]. Fig. 1 shows the layer structures studied in this work. The growth was initiated by a Ê GaAs buffer layer grown at 6008C, followed by a 5000 A 90 s growth interruption to reduce the substrate temperature

Ê In0.2Ga0.8As channel was grown. A to 5208C. Then a 140 A Ê GaAs smoothing layer was incorporated in some of the 10 A structures. A growth interruption of 60 s was applied between the InGaAs layer and the spacer layer. The spacer Ê Al0.3Ga0.7As layer or a 30 A Ê layer was either a 30 A InxGa12xP layer. A Si planar doping layer of 6 £ 1012 cm22 was deposited after the spacer layer. The Ê -thick InxGa12xP Schottky layer is found to be: (1) a 220 A Ê of InxGa12xP grown on top of a 100 A Ê layer; or (2) a 120 A Ê Al0.3Ga0.7As layer. of Al0.3Ga0.7As layer; or (3) a 220 A Ê of Si-doped Finally, the structure was capped with 450 A 18 23 GaAs …6 £ 10 cm †: The carrier concentrations and mobilities of the twodimensional electron gas (2DEG) were measured using the van der Pauw con®guration at room temperature and 77 K. Before the Hall measurement, the Si-doped GaAs cap layer was removed by a selective etchant of citric acid/hydrogen peroxide/ammonium hydroxide [13]. The photoluminescence (PL) measurements were carried out at 5 K. The samples were mounted in a closed-cycle He cryostat and excited at near-normal incidence to the plane of the sample using a 514 nm argon laser. The PL spectra were detected using a liquid nitrogen cooled germanium detector with a 0.6 m grating spectrometer used in association with a conventional lock-in technique. 3. Results and discussion Table 1 summarizes the Hall results for the PHEMT structures. The referenced 2DEG mobility of an Al0.3Ga0.7As/In0.20Ga0.80As/GaAs PHEMT is 6310 cm 2/ (V s), with a sheet carrier concentration of 2:3 £ 1012 cm22 : The 2DEG mobility decreases and concentration increases as more InGaP layers replace the AlGaAs layers and close to the InGaAs channel (samples #1±#4). The 2DEG mobility decreases to a low value of 1700 cm 2/ (V s), while the concentration increases to 3:3 £ 1012 cm22 for the In0.48Ga0.52P/In0.20Ga0.80As/GaAs PHEMT structure (#4). These mobility data are comparable if not better to those obtained in similar structures grown by SSMBE, GSBME and MOCVD [7±9]. The low mobility and high concentration can be explained in terms of the

Table 1 2DEG sheet carrier concentrations, n2DEG (10 12 cm 22) and mobilities, m (cm 2/(V s)), for the PHEMT structures studied No.

Ê) Schottky layer (A

Ê) Spacer layer (A

n2DEG (300 K)

m (300 K)

n2DEG (77 K)

m (77 K)

#1 #2

Al0.3Ga0.7As 300 In0.48Ga0.52P 120/ Al0.3Ga0.7As 100 In 0.48Ga0.52P 220 In0.48Ga0.52P 220 In0.40Ga0.60P 220 In0.48Ga0.52P 220 In0.40Ga0.60P 220

Al0.3Ga0.7As 30 Al0.3Ga0.7As 30

2.27 2.27

6310 6100

1.69 1.96

24 000 20 800

Al0.3Ga0.7As 30 In0.48Ga0.52P 30 In0.40Ga0.60P 30 In0.48Ga0.52P 30/GaAs 10 In0.40Ga0.60P 30/GaAs 10

3.01 3.30 2.38 2.02 1.90

2330 1700 2620 3310 4280

2.88 3.06 2.27 1.91 1.81

2680 1790 4060 6080 10 800

#3 #4 #5 #6 #7

H.Q. Zheng et al. / Solid State Communications 112 (1999) 661±664

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Fig. 2. 5 K PL spectrum of the InxGa12xP/In0.2Ga0.8As PHEMT structures: (a) the solid lines represent the spectrum of the lattice-matched In0.48Ga0.52P/In0.2Ga0.8As PHEMT structure (sample #4); and (b) the dotted lines represent the spectrum of the strained In0.40Ga0.60P/In0.2Ga0.8As PHEMT structure (sample #7). The arrow indicates the transition associated with the parasitic quaternary well.

formation of two kinds of quaternary layers between the InGaAs channel and the InxGa12xP spacer layers, which act as an additional parasitic well (In0.48Ga0.52PxAS12x) and barrier (In0.2Ga0.8AsxP12x). The electrons have lower mobility in the parasitic quaternary well due to the fact that the electrons are nearer to the d-doped layer and the possible imperfect quaternary interface scattering, while the quaternary In0.2Ga0.8AsxP12x barrier layer enhances the localization of electrons in the parasitic well [8]. This explains why high 2DEG concentration and low mobility are always measured in the lattice-matched In0.48Ga0.52P/In0.2Ga0.8As PHEMT structure. With the incorporation of a GaAs smoothing layer and strained In0.40Ga0.60P layers into the PHEMT structures (samples #5±#7 in Table 1), the effects of the undesired quaternary layers formed at the In0.2Ga0.8As and In0.40Ga0.60P interface are much suppressed. The effect of strained In0.40Ga0.60P layers is that the quaternary well formed (In0.40Ga0.60PxAs12x) will be shallower, which results in fewer electrons staying in this low mobility channel. The effect of a GaAs smoothing layer is that the probability for the formation of the quaternary In0.2Ga0.8AsxP12x barrier layer will be greatly reduced. In addition, the strained In0.40Ga0.60P barrier enhances DEc and con®nes the electrons to the higher mobility channel [14]. This explains why higher 2DEG mobility of 4280 cm 2/(V s) with lower electron concentration of 1:90 £ 1012 cm22 was obtained for sample #7 compared to sample #4. Low-temperature PL was used to investigate the layer structures. Fig. 2 shows the 5 K PL spectrum of the InxGa12xP/In0.2Ga0.8As PHEMT structures. The solid lines represent the spectrum of sample #4, while the dotted

lines represent that of the sample #7. The two spectrums are almost identical at the high energy side, and consist of two peaks located at about 1.27 and 1.34 eV. These two peaks are associated with the transitions from the second conduction-band level to the ®rst heavy-hole level in the valence band (e2±hh1), and the ®rst conduction-band level to the ®rst heavy-hole level in the valence band (e1± hh1), respectively [15]. At the low-energy side, there is a weak peak located at about 1.23 eV for sample #4, as indicated by the arrow in the ®gure, while there is no peak at this low energy region for sample #7. It is unlikely for this low energy peak to be associated with the impurities or defects, as it is not found in the similar structure sample #7. Schuler et al. [8] performed charge transfer calculations using a selfconsistent one-dimensional SchroÈdinger±Poisson solver and they attributed the low-energy peak to the transition from the parasitic quaternary In0.48Ga0.52PxAs12x well formed between the In0.2Ga0.8AsxP12x barrier and the In0.48Ga0.52P barrier. Their room temperature PL measurement revealed that the quaternary well peak position is about 1.20 eV, which is consistent to our PL peak position of 1.23 eV at 5 K, however, we could not observe this peak at room temperature. The low intensity of this peak is probably due to the fact that the parasitic quaternary well and barrier layers are thinner and the localization effect of the electrons is thereby reduced. The thinner parasitic quaternary layers are most likely the result of the different growth conditions of the SSMBE and GSMBE. For sample #4, the quaternary well (In0.48Ga0.52PxAs12x) is relatively deep, together with the quaternary (In0.2Ga0.8AsxP12x) barrier, resulting in the localization of electrons in the parasitic

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well. For example #7, the combination of a strained barrier together with a GaAs smoothing layer produces a shallower In0.40Ga0.60PxAs12x well which may just be located next to the In0.2Ga0.8As channel and serves as a sub-channel. This explains why there is no PL peak in the low energy side for sample #7. The PL results con®rmed that the incorporation of a strained barrier and a smoothing layer into the PHEMT structure modi®es the electron distribution so that most of the electrons are distributed in the In0.2Ga0.8As channel, thus resulting in higher electron mobility. Higher mobility is expected if greater strain of InxGa12xP layers was used in the structure. But great care must be taken to ensure that the strained layer thickness is within the critical thickness limit. 4. Conclusions In conclusion, In0.48Ga0.52P/In0.20Ga0.80As/GaAs PHEMT structure suffers from low mobility caused by the parasitic quaternary layers formed between the In0.48Ga0.52P barrier and the In0.20Ga0.80As channel. A strained In0.40Ga0.60P/ In0.20Ga0.80As/GaAs PHEMT structure was proposed to improve the electron transport properties in the channel. Different InxGa12xP/In0.20Ga0.80As/GaAs PHEMT structures were successfully grown by SSMBE. Higher 2DEG mobility of 4280 cm 2/(V s) with electron concentration of 1:90 £ 1012 cm22 was achieved in the proposed strained PHEMT structure. PL measurement at 5 K revealed that better electron distribution could be obtained in the strained PHEMT structures which ensure high mobility. Acknowledgements The authors would like to thank W. Shi and Adele Kam for their helpful discussions and assistance. This work is

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