Optical and transport properties of InSb thin films grown on GaAs by metalorganic chemical vapor deposition

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Optical and transport properties of InSb thin films grown on GaAs by

metalorganic chemical vapor deposition

Tzuen-Rong Yang

a

, Yukun Cheng

a

, Jyun-Bi Wang

b

, Zhe Chuan Feng

b,

*

a

Department of Physics, National Taiwan Normal University, Taipei, 116 Taiwan, ROC

b

Graduate Institute of Electro-Optical Engineering and Department of Electrical Engineering, College of Electrical Engineering, National Taiwan University, Taipei, 106-17 Taiwan, ROC

Available online 19 August 2005

Abstract

Optical and transport properties of InSb thin films grown on GaAs by metalorganic chemical vapor deposition (MOCVD) have been investigated by far-infrared (FIR) reflectance spectroscopy. The lattice vibration behaviors of a series of MOCVD InSb/GaAs(100) materials grown under different growth conditions were studied. Effects of III – V source ratios on the films crystalline quality were examined. Two additional weak modes in the wavenumber regions of 210 – 240 cm 1were observed and they appeared more prominent at low temperatures. Interference fringe effects modify the FIR reflectance band of the GaAs substrate, which are related to the uniformity of film thickness and crystalline perfection. The dielectric constant, phonon modes and other optical parameters, as well as transport properties including carrier concentration, mobility, effective mass were calculated theoretically and compared with experimental results. The obtained distribution values of the InSb LO phonon mode frequency, line width, relative integrated intensity ratio between the forbidden and defect-related TO phonon and the allowed LO mode are adopted as figures of merit for the quality of the InSb films. The electrical transport properties of carrier concentration, mobility, and effective mass as well as the dielectric constant of these films have been determined by optical method non-destructively.

Keywords: Metalorganic chemical vapor deposition (MOCVD); Characterization; InSb; GaAs; Fourier transfer infrared spectroscopy

1. Introduction

InSb has important applications in infrared, optical,

microwave, and millimeter-wave devices [1 – 4]. It is a

narrow gap semiconductor with a low effective mass and possesses the highest electron mobility among III – V com-pound semiconductors, which makes it very suitable for

high-speed devices, magnetic sensors and so on[2,3].

InSb-based thin films are especially promising in the fabrication of long wavelength infrared detectors, and wavelengths greater than 12 Am can be detected with these compounds based

photodiodes at 77 K [5]. For the preparation of InSb thin

layers on GaAs substrate, it is required to have special technological treatments to overcome the large lattice

mismatch of ¨ 14.6% between InSb and GaAs and to ensure their growth quality with specified electrical as well as

optical properties of the layers [3,4]. There have been a

number of studies on this material concerning the growth and optical characterization of InSb based compound

semi-conductors [6 – 8]. Efforts of the growth of these materials

on GaAs substrate have been explored by various growth

technique, such as molecular beam epitaxy (MBE) [3,4,9 –

12], liquid phase epitaxy (LPE) [13], magnetron sputter

epitaxy[14], metalorganic vapor phase epitaxy (MOVPE) or

metalorganic chemical deposition (MOCVD) [15 – 17]. But

there lacks penetrating investigation on transport and lattice behaviors of these epitaxial materials.

Metalorganic vapor chemical deposition (MOCVD) technology has been shown a good technology to produce large size InSb thin film materials on GaAs substrate for

industrial infrared and automobile applications [15 – 18].

* Corresponding author. Tel.: +886 2 3366 3543; fax: +886 2 2367 7467. E-mail address: zcfeng@cc.ee.ntu.edu.tw (Z. Chuan Feng).

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In this paper, far-infrared (FIR) reflectance spectroscopy has been employed to study the lattice vibration behavior for a series of MOCVD-grown InSb films on GaAs substrates. From these MOCVD films, the effects of III – V source ratios on the film crystalline quality have been investigated.

2. Experimental

The experimental MOCVD InSb epitaxial films were grown on commercial GaAs (100) substrates. Trimethyin-dium (TMIn) and Tris-dimethylamino-antimony (TDMASb) were used as In and Sb sources, respectively. The indium and antimony bubblers were operated at 429 and 323 Torr, respectively. Flows through the indium and anti-mony bubblers were varied from 130 sccm to 270 sccm

and the carrier H2gas flow were in the range of 320 – 775

sccm, respectively. Growth rates investigated were between 0.65 and 1.2 Am/h with an optimum growth rate of around 0.95 Am/h. Growth temperature was monitored using a single wavelength low tempera-ture pyrometer. Samples N01-08 were prepared at a III – V ratio of between 4.2 and 4.5. Runs at these III – V values typically showed indium droplets on the surface of the film after growth. A second set of growth runs starting with run sample N09 was done using an III – V ratio of 6.2. These growths have resulted in surfaces with excellent morphology that were typically free of indium droplets. For all epitaxial InSb samples, a variety of characterization measurements have been performed, including surface morphology, Hall measurements, RF sheet resistivity, scanning electron microscopy (SEM), etc.

The far-infrared reflectance spectra were measured at

near normal incident in far-infrared range, 60 – 500 cm 1,

by a BRUKER IFS 120HR Fourier transform infrared (FTIR) spectrometer at different temperatures between 80 and 300 K with the spectral resolution better than 1

cm 1. Mercury-Arc lamp was used for infrared light

source. A mirror-like gold plate was mounted next to the samples on the cold finger of cryogenic. The absolute reflectivity value of sample was determined by compa-rison with the gold mirror. An APD cryogenic system was employed to the temperature-dependent measure-ments. The temperature of finger tip inside the cryogenic

was controlled by a Lake-Shore 331 temperature con-troller with a temperature stability of 0.5 K or better.

3. Results and discussion

3.1. Experimental far-infrared spectra of MOCVD films III – V source ratio has an important influence on the

resulting epitaxial InSb films. Behet et al. [15]studied the

dependence of InSb growth rate on III – V ratio in low-pressure (20 Pa) plasma MOVPE with triethylantimony (TESb) as Sb precursor. In this work, a series of InSb thin films were grown on GaAs under different III – V ratio conditions. The experimental values of III – V ratio and resulted sample surface Normaski microscopy and

thick-ness uniformity are given in Table 1. Samples N01-N08

were grown with III – V ratio of 4.1 – 5.1. InSb films prepared at these III – V values typically showed indium droplets on the surface of the films after growth. Sample N09 was made using a III – V ratio of 6.2. This growth with a high III – V ratio had resulted in surfaces with excellent morphology that typically free of indium droplets. Hall measurements, sheet resistivity, SEM, and mid-infrared spectrum measurement for thickness showed a high thickness uniformity distribution, high mobility of the films which are very good for fast speed electron device applications.

Table 1

III – V ratio, surface morphology, thickness uniformity of samples

Sample No N01 N02 N03 N04 N05 N06 N07 N08 N09

III – V ratio 5.069 4.906 4.546 4.641 4.546 4.106 4.528 4.448 6.20

Surface morphology In-rich/ droplet Sb-rich/ big lots In-rich/ droplet Good/ less drop Good/ no drop In-rich/ droplet Good/ no-drop Sb-rich/ less drop Good/ no drop Thickness uniformity (%) 6.06 6.16 7.81 6 6.88 2.59 1.69 3.29 4.76 100 150 200 250 300 350 400 6.20 4.641 18370 10085 35140 40311 14268 20630 11300 10731 6911 µ (cm2/V-s) : V/III ratio : 6.59 5.56 13.7 14.4 9.12 14.4 2.85 2.71 1.52 n (x 1016) : 4.448 4.528 4.106 4.546 4.546 4.906 5.069 N09 N08 N07 N06 N05 N04 N03 N02 N01 Reflectivity (a.u.) wavenumber (cm-1₎ 80K InSb/GaAs MOCVD

Fig. 1. FIR reflectance spectra of nine InSb/GaAs samples, measured at 80 K.

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A series of MOCVD-grown epitaxial InSb films on GaAs substrates were studied by FTIR method in the FIR region. The measured spectra of these samples (80 K) were

shown inFig. 1. The vibration modes of all these samples in

these spectra are shown around 180 cm 1and between 270

and 300 cm 1. Mode 183 cm 1is assigned to the InSb TO

phonon mode, and modes around 272 – 295 cm 1are due to

GaAs phonon modes from substrate.

3.2. Theoretical simulation results for MOCVD InSb/GaAs The dielectric function of a semiconductor or layered

structure,((x), dependent on frequency x, can be described

as[19,20]: e¼ eVþ X j Sjx2TOj x2 TOj x2 icjx x 2 p x x þ ic_p ð1Þ

where(” is the dielectric function in high frequency limit,

xTOj and cTOj are the jth transverse optical (TO) mode

frequency and damping factor, xpand cpare the free carrier

plasma frequency and damping constant, respectively. The carrier concentration, n, and mobility, l, can be obtained from:

x2_p¼ ne

2

m4eVe0

and c_p¼ e

m4l; ð2Þ

where e is the electron charge, m* is the electron effective

mass and (0is the vacuum dielectric constant. Based upon

these theoretical equations, simulation can be performed to fit

to all experimental FIR spectra in the long wavelength limit. A multi-layers (InSb + GaAs two layers) fitting technique was applied in theses theoretical calculation. Contributions of inner shell dielectric value and a set of Lorentzian vibrators as well as carrier contribution and also the contribution from GaAs

substrate are included. A typical fitted datum is shown inFig.

2. The mode of around 180 cm 1by least square fit is assigned

as InSb TO mode and the mode around 270– 300 cm 1was

fitted with different InSb film thickness and assigned as GaAs

TO mode in 270 cm 1at 300 K. All of the phonon modes

assignments of these samples were calculated and fitted by dielectric response model. The fitted optical parameters of all

these InSb films at 300 K were given inTable 2.

3.3. The temperature dependence of FIR spectra on InSb/ GaAs

The variation of temperature can cause the above modes blue-shifted. The temperature-dependent FIR spectra of all these samples have been measured and well fitted (not shown here). The results showed that the InSb TO mode

has about 3 cm 1 blue shift from temperature at 300 K

down to 80 K, and that the GaAs TO mode from the

substrate has about 1 – 2 cm 1 blue shift mostly. The

wavenumber values of TO modes of InSb and GaAs of all these samples are corresponding to those calculated according to the Kramers – Kroning (K – K) relation of reflection spectrum.

3.4. Carrier concentration and mobility

Carrier concentration and sheet resistance are critical for device applications. The values of carrier concentration and mobility are dependent on sample growing conditions. The fitted values of carrier concentration in the present InSb/

GaAs samples are varied between 1016 and 1017 cm 3 at

300 K as shown inTable 2. These values are about the same

order as that from Hall measurement with a very slight difference. The sample with higher carrier concentration has higher absolute reflectance intensity in the long wavelength

range as shown inFig. 1. The higher reflection intensity in

the long wavelength range is due to the plasma effect and also due to that InSb has a very narrow band gap which

Fig. 2. Experimental (solid line) and theoretical (dash line) FIR reflectance spectra of InSb/GaAs, sample N01 at 300 K.

Table 2

Samples optical parameters were fitted by dielectric response model in the long wavelength limit Sample III – V ratio (8 xTO (cm 1) Strength S Damping G (cm 1) Carrier concentration (1016 cm 3) Mobility (cm2/V s) Effective mass Thickness (Am) N01 5.069 17.88 179.3 2.5 1.3 1.52 6911 0.0129 1.98 N02 4.906 17.88 178.7 2.4 3.3 2.71 10730 0.0170 1.43 N03 4.546 17.88 178.4 2.5 1.6 2.85 11300 0.0173 1.55 N04 4.641 17.88 179.5 3.2 4.7 1.44 20630 0.0152 1.09 N05 4.546 17.88 180.5 2.4 1.5 9.12 14268 0.0186 1.57 N06 4.106 17.88 180.1 1.9 4.0 1.44 40311 0.0176 0.66 N07 4.528 17.88 178.8 2.0 3.1 1.37 35140 0.0177 0.61 N08 4.448 17.88 178.3 2.0 2.5 5.56 10085 0.0135 0.91 N09 6.20 17.88 180.5 2.5 4.2 6.59 18370 0.0166 1.41

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enhances the plasma effect as long as the carrier concen-tration become higher.

3.5. Extra modes due to interface reaction

The large lattice mismatch (¨ 14.6%) between the InSb Film and GaAs substrate is accompanied by a high density of dislocations and defects between the interface of the film

that can deteriorate the quality of thin film[21]. Because of

the longer penetration depth in the long wavelength range of

FIR spectra, inFig. 3and insert, we can clearly see the third

and fourth small vibration temperature-dependent modes

appear in around 218 cm 1 and 228 cm 1. These modes

were due to the interface reaction between film and substrate and observed obviously in the lower temperature of 80 – 150

K as indicated in the three upper curves in Fig. 3. These

vibration modes were assigned as InAs and GaSb TO modes

[22]. Also we see these modes having blue shift of measured

spectra as temperature decreases. 3.6. InSb film thickness

The determination of InSb epitaxial film thickness and distribution over the entire layer is critical in the epilayer growth, especially in the large wafer diameter production. A scanning electron microscope (SEM) was usually employed

to measure the InSb film thickness [16,18]. But, it is

destructive and not convenient for industrial production. The reflectance fringes do not appear in the ultraviolet and visible range from the InSb/GaAs heterostructure. However, the reflectance fringes from InSb/GaAs can be observed in

the mid-near infrared range [17]. We have found that it is

easy to observe the interference fringes in far-infrared range

of 60 – 500 cm 1 or 20 – 160 Am. In this study, we have

obtained the InSb film thickness by experimental datum fitting. The contribution of InSb film absorption to its dielectric value in the long wavelength range limit was calculated with respect to different film thicknesses, and the obtained values were used to fit the experimental datum.

The results of fitted film thickness were shown inTable 2.

3.7. The splitting of GaAs TO mode

FromFigs. 1, 3 and 4), an interesting phenomenon can be found, i.e., the splitting of the GaAs TO mode in the FIR

240 260 280 300 320 340 360 N09: 1.42µm N08: 0.91µm N07: 0.61µm N06: 0.66µm N05: 1.57µm N04: 1.09µm N03: 1.55µm N02: 1.43µm N01: 1.98µm Thin Thick Film Thickness ( µ m) 300K InSb/GaAs Relative Reflectivity wavenumber (cm-1₎

Fig. 4. FIR reflectance spectra, between 240 and 330 cm 1, of nine InSb/ GaAs samples, listed in the order of the film thickness. The TO mode in around 270 cm 1is well known as the GaAs mode which is contributed from the film substrate. The film thickness by fitted values is shown in the figure. We use the GaAs TO modes (¨ 270 cm 1at 300 K) instead of two modes to well fit the splitting of this GaAs mode.

Fig. 5. FIR spectrum and fits of MOCVD InSb/GaAs, N06, at 300 K. The mode at 180 cm 1is assigned for the InSb TO phonon mode, and the mode around 270 – 290 cm 1 is assigned for the GaAs phonon mode from substrate. The split of this GaAs mode is assuming due to the contribution of absorption of different InSb film thickness.

100 150 200 250 300 350 400 450 300K 250K 200K 150K 100K T: 80K NO6 InSb/GaAs MOCVD reflectivity wavenumber (cm-1₎ 210 225 240 100K 80K

Fig. 3. FIR reflectance spectra of InSb/GaAs sample N06 at 80 – 300 K with the spectral line shifted up as the temperature increases. The InSb TO mode has about 3 cm 1blue shift from temperature at 300 K down to 80 K. The GaAs mode from the substrate has about 1 – 2 cm 1 blue shift from temperature at 300 K down to 80 K. The insert shows two weak modes appeared at low temperature of 80 – 100 K.

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spectra from some InSb/GaAs samples but not in some other

samples. InFig. 3, the TO mode in around 270 cm 1is well

known as GaAs mode which is contributed from the film substrate. The splitting of this GaAs mode could clearly be seen in the FIR spectrum and can be explained due to the variation of the InSb film thickness. We use the GaAs TO

mode (¨ 270 cm 1, 300 K) instead of two modes with

varying the thickness of InSb film to fit all these FIR spectra

in this range. Fig. 5 shows such an example with the

experimental spectrum and its fitting. The variation of InSb film thickness from our dielectric response fitting is found to be exactly the factor contributed to this mode splitting as

observed inFigs. 4 and 5.

4. Conclusion

We had performed the optical characterization on a series of InSb thin films grown on GaAs by MOCVD under different growth conditions by way of far-infrared spectro-scopy and theoretical simulation. Various mode parameters, e.g., the TO mode frequency, carrier concentration, mobility, etc., were obtained from these InSb/GaAs materials. All the specimens were analysed by a dielectric response model. It was found that the extra modes appear in FIR spectra due to the interface reaction between the film and substrate. Interesting splitting of the GaAs transverse optical phonon mode was observed and explained using the dielectric response model to fit the FIR reflectance spectrum by varying the InSb film thickness. Effects of III – V source ratios were successfully studied by FIR spectroscopy and the optimized growth parameters have been obtained. The InSb film grown with an III – V ratio of 4.1 possesses the highest mobility and nearly lowest carrier concentration. The results of this study show that MOCVD technology is capable to produce high quality far-infrared InSb epitaxial materials, and the FTIR method is a very useful tool for non-destructive characterization of large size wafers in industrial mass production.

Acknowledgement

We acknowledge the support, materials preparation and help in this work from Drs. N. Schumaker, R. Stall, I.

Ferguson, and C. Beckham. The work at National Taiwan University was supported by funds from National Science Council of Republic of China, NSC 93-2218-E-002-011 and 93-2215-E-002-035.

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