A quantum well δ-doped GaAs FET fabricated by low-pressure metal organic chemical vapor deposition

Solid-State Electronics Vol. 34, No. 6, pp. 649-653, 1991

0038-1101/,91 $3.00+ 0.00 Copyright © 1991PergamonPress plc

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A Q U A N T U M WELL f - D O P E D GaAs FET FABRICATED BY LOW-PRESSURE METAL ORGANIC CHEMICAL VAPOR DEPOSITION W. C. Hsu, W. LIN and C. WANG Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China (Received 26 September 1990; in revised form 30 November 1990)

Abstraet--A quantum well tS-doped n-type GaAs layer with Ga source open has been grown successfully by the low-pressure metal organic chemical vapor deposition (LP-MOCVD). The measured capacitancevoltage profile shows that a sheet-doping concentration up to 5 x 1012 c m -2 for the 6-doped GaAs layer can be easily achieved. The full-width at half-maximum (FWHM) is quite narrow. From the Hall measurement, the electron mobility increases inversely proportional to the 6-doping concentration. An enhanced mobiity can be obtained more than 2300 and 4300cm2/Vs with doping concentration of 5.0 x l0 t8cm -3 at 300 and 77 K, respectively. Based on this technique, a quantum well 6-doped GaAs FET has been fabricated and demonstrated. With a gate geometry of 5 x 250/~m2 and doping concentration of 5.9 x l018c m -3, the estimated transconductance of the tS-doped FET is 64 mS/ram. Since there is an undoped GaAs layer grown on the top of the 6-doped sheet, the breakdown voltage can be increased significantly(> 17 V). Furthermore, the saturation current density can be obtained higher than I l0 mA/mm.

1. INTRODUCTION In 1980, a high electron mobility transistor (HEMTs) was fabricated successfully using molecular beam epitaxy (MBE) by Mimura et al.[1]. In this structure, free carriers were allowed to be spatially separated from parent donors, so that the mobility was very high due to the existence of a two-dimensional electron gas (2DEG). However, since the free carriers were supplied from the upper doped layer, the breakdown voltage of the gate could not be increased. Furthermore, the traps in the interface and the DX center in the AIGaAs layer degrade both the mobility and the device performance. To overcome the disadvantages mentioned above, a 8-doped field effect transistor grown by MBE was investigated[2]. Comparing with the MESFET structure, the ~-doped FET has the merits of higher mobility and transconductance. In addition, the undoped layer grown on the top of ~-doped sheet, elevated its breakdown voltage higher than that of HEMTs. Recently, Hobson et al.[3] reported the growth of zinc 6-doped GaAs by the atmospheric pressure organometallic vapor phase epitaxy (OMVPE). Conventionally, during the growth of the 6-doped GaAs layer, the Ga source is shut off, and the AsH3 and Sill4 source left open. As a result, Si atoms replaced some Ga sites in a single sublattice plane and a highly localized doping sheet was thus formed. However, the electron density and the channel current provided by the ~-doped GaAs layer were still limited. In this paper, the method of low pressure metal organic chemical vapor deposition (LP-MOCVD) is employed to grow a 6-doped layer using Si as the n-type

dopant, while leaving the Ga source open. This approach is different from that used by Hobson et a/.[3] and others[2, 4--6] in which the Ga source was interrupted during the growth of the 6-doped layer. The relationships between the 6-doping concentration and the mobility for this structure are also discussed. The electron mobility of the 6-doped GaAs is significantly higher than that of the bulk GaAs. To demonstrate the phenomena of the enhanced mobility and the existence of the 2DEG layer, bulk GaAs crystals with different concentrations were grown for comparison. From the capacitance-voltage measurements, the sharp doping concentration profile and narrow full-width at half-maximum (FWHM) of -doped GaAs are easily shown. Finally, the 6-doped FET and the conventional MESFET made by the LP-MOCVD system are investigated.

2. EXPERIMENTS

In this experiment, all the samples were grown on (100) oriented Cr-doped GaAs substrates by an LPMOCVD system. All the layers were grown at 725°C with chamber pressure 70 torr. Furthermore, triethylgallium (TEG) and AsH 3 were used as the Ga and As sources, respectively. The V/III ratio was 34. The growth rate was set at 300 ,~/min, The growth conditions for the 6-doped GaAs are described as follows. First, a 0.6/~m undoped GaAs was grown on the GaAs substrate. Then, the silane source (Sill4, 500 ppm in H 2 base) was turned on for l0 s, while keeping the Ga source open to grow the 6-doped n-type GaAs layer. In order to obtain different

649

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W. C. Hsu et al.

.....

Ag/Au-Ge

Au

Source

Gate

Drain

' ~ ~ i Und_op_ed__G~0A_~_ - :--

Undoped GaAs

0,6 p.m

Buffer layer

Table 1. The comparisonsof electron mobilityand doping concentrationbetweensampleA, B and bulk GaAs Concentration Mobility (1018 cm- 3) (cm2/Vs) 300 K 300 K 77 K Sample A 5.9 2015.7 3989.6 Sample B 5.0 2326.8 4394.5 Bulk 3.5 1876.4 2104.8

Ag/Au-Ge

Quantumwell 6 - d o50A ped layer

5 x 250 pm 2, The distance between source and drain electrodes was 30pm. Figure 1 shows the h-doped GaAs FET structure studied.

/

3. R E S U L T S

AND

DISCUSSIONS

Cr - doped S.I. GaAs substrate

/

Fig. 1. The f-doped GaAs FET structure. doping concentrations, the flow rates of Sill 4 source were controlled in the range of 15-100cm3./min. Once finishing the growth of the h-doped layer, the Sill4 source was shut off instantly, and a 600 ~, undoped GaAs layer, acting as a cap-layer, was then grown. The doping profile was obtained by capacitance-voltage measurement. The electron mobility was measured by Hall measurement at 300 and 77 K respectively. To compare the electron mobility, the bulk GaAs with different doping concentrations were grown. Finally, the h-FET with h-doping concentration 5.9 x 10TMcm -3 (sample A) and 5.0 x 10TMcm -3 (sample B), and a conventional MESFET (sample C) were all processed by the standard photolithography and lift-off techniques. Alloyed Au-Ge metal was used for source and drain ohmic contacts and then Ag was evaporated to reduce the bonding resistance. The Au was used as the Schottky contact for the gate. The doping concentration and thickness of the active layer for sample C are 1.8 x 10]7cm -3 and 0.16pro, respectively. Due to the restriction of laboratory facility, the gate geometry was only

Figure 2 illustrates the doping profile of sample A. It is obvious that the peak concentration of h-doped GaAs sheet is 5.9 x 1018cm -3, and the full-width at half-maximum (FWHM) is about 50ik, which is comparable to the width of a quantum well. The peak concentration of sample B is 5.0 x 10JScm 3. The 3D concentration was calculated via No = (Ns) 3/2. During the growth of the h-doped layer, since the Ga source was not interrupted, the h-doping concentration is dependent on the flow rate of the silane source. The higher the Sill4 flow rate, the higher the peak concentration will be. Table 1 lists the electron mobility at 300 and 77 K and the h-doped concentrations between sample A, sample B, and bulk GaAs for comparison. Although the h-doping concentrations of sample A and B are higher, their mobilities are higher than that of bulk GaAs. In addition, for the electron mobility at 77 K, both sample A and sample B are almost enhanced to about twice the value of 300 K. The enhanced mobility is clearly attributed to the drastic decrease of the impurity scattering. Moreover, a 2D electron gas (2DEG) is believed formed in the well of the h-doped layer. The Hall measurements of 6-doped and bulk GaAs with different concentrations are shown in Fig. 3. The solid line in this figure stands for the empirical representation of electron mobility in n-type GaAs at 300 K obtained /

A ~L cO I L ~E o k

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1012 om-2 E 0

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o

1017

I

I

I

I

200 400 600 800 1000120014001600

I

D i s t a n c e (A)

Fig. 2. The C - V doping profile of sample A (the 6-doping concentration is 5.9 x 10Is cm-3).

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1018

at 300K

~

1019

0"arrier c o n c e n t r a t i o n

1020 (cm 3)

Fig. 3. The Hall mobility vs concentration at 300 and 77 K.

5 mA/div [G

50p. A / d i v ~5 V/day

q

[i

I

I

=

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Fig. 4. The gate-drain 1 - V characteristic of sample A (the ~-doping concentration is 5.9 x l0 ~8cm-3). (a)

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/ /

J

I

•

1V/div

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(b)

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Fig. 5. The 1 V characteristics between source and drain of sample A (a) and sample B (b). 651

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5mA/div Ia

> 0 I c0

>0 t~

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(

=

2V/div

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(b) 5 mA/div

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>O

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I

•

1V/div

VDS

Fig. 6. (a) I V characteristic of sample A. (b) I V characteristic of sample B.

by Hilsum[7]. The dot line represents the bulk GaAs grown by LP-MOCVD system. Comparing with the predictions from the Hilsum relation and the bulk n-GaAs, the 6-doped GaAs has the higher mobilities. Furthermore, the electron mobilities of f-doped GaAs obtained in this work are comparable to those of 6-doped GaAs grown by molecular beam epitaxy (MBE)[2,8] in which the Ga source was interrupted during the growth of the 6-doped GaAs layer. The measured gate~lrain current-voltage characteristic of sample A is shown in Fig, 4. As can be seen, a low leakage current ( < 15 #A) and a high breakdown voltage ( > 17 V for sample A and >23 V for sample B) are obtained. This is due to the undoped GaAs layer grown on the f-doped GaAs layer. Obviously, the breakdown voltage obtained for the gate of the f-doped FET is higher than that of

conventional MESFET (V,D= 10V for sample C). Therefore the f-doped FET has the great potential for the application of power devices. Figure 5 shows the current-voltage characteristic between the source and drain of sample A and B, respectively. Due to the high 6-doping concentration, the source~drain resistance (RDs) of sample A (1 l0 f~) is lower than that of sample B (130 £~). Figure 6 shows the experimental current-voltage characteristics of sample A and sample B, respectively. Both of them are depletion mode. The maximum output drain saturation current per gate width for samples A and B are I 12 and 96 mA/mm, respectively. In addition, the maximum transconductance of sample A is 64 mS/mm, which is higher than that of sample B (48 mS/mm). These higher drain-current density and transconductance values of sample A are

Quantum well 6-doped GaAs FET (a)

of transconductance of samples A and B are much flatter than that of conventional MESFET (sample C). Such a 6-FET can yield a reduction on the third-harmonic distortion and thus perform well as a linear amplifier. Thus the 6-doped FET's exhibit high performance and flexibility for device applications.

¢D

a

140

• Sample A o Sample B x Sample C VDS - 5 V

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¢1) "U

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e0 4. CONCLUSIONS

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A p p l i e d g a t e v o l t a g e V G (V)

(b) E o~ o oa

70 -

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I -0.S

I -1.0

-,, I x,,, I -1.5 -2.0

A quantum well 6-doped GaAs FET with the features of high saturation current density, enhanced electron mobility, high breakdown voltage and transconductance was fabricated and demonstrated in this paper. This enhanced mobility is comparable to the results by Schubert and Sasa et a1.[2,8,9]. With doping concentration over 5.0 × 10~8cm -3, the mobility at 77 K is almost twice the mobility at 300 K. Moreover, in the 6-doped FET structure, higher transconductance and saturation current density can be obtained from higher 6-doping concentration. Since an undoped GaAs layer was grown on the 6-doped GaAs layer, the gate breakdown voltage can be higher than that for the conventional MESFET and can be applicable for power FETs. Also, due to the flatter variation of transconductance, the -FET is useful in the designs for the linear amplifier. Meanwhile, if the gate length can be impoved to the 1/am region, a power FET with superior microwave properties will be expected. Acknowledgement--This work was supported in part by the

I -2.5

National Science Council of Republic of China under contract no. NSC 78-0417-E006-07.

A p p l i e d g a t e v o l t a g e V G (V)

Fig. 7. (a) The drain saturation current density vs gate voltage. (b) The normalized transconductance vs gate voltage. based on the lower channel resistance (RDs). It is also noted that the gate lengths of all the samples are 5/am. If the gate length can be scaled down, much higher transconductance and drain current density should be expected. The influence of applied gate voltage VC on the drain saturation current density (IDs) and the normalized transconductance (gm) are shown in Fig. 7. It is known that the los and gm are decreased monotonically with the applied gate voltage. Obviously, the variation trend

REFERENCES

1. T. Mimura, S. Hiyamizu, T. Fujii and K. Nanbu, Jpn J. Appl. Phys. 19, L225 (1980). 2. E. F. Schubert, A. Fisher and K. Ploog, 1EEE Trans. on Electron Devices 33, 625 (1986). 3. W. S. Hobson, S. J. Pearton and E. F. Schubert, Appl. Phys. Lett. 15, 1546 (1989). 4. K. Ploog, J. Crystal Growth 81, 304 (1987). 5. E. F. Schubert, J. Vac. Sci. Technol. A8(3), 2980 (1990). 6. T. Makimoto, N. Kobayashi and Y. Horikoshi, J. Appl. Phys. 63, 5023 (1988). 7. C. Hilsum, Electron Lett. I0, 259 (1974). 8. E. F. Schubert, J. E. Cunningham and W. T. Tsang, Solid-St. Commun. 63, 591 (1987). 9. S. Sasa, S. Muto, K. Kondo, H. Ishikawa and S. Hiyamizu, Jpn J. Appl. Phys. 24, L602 (1985).