JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.16, NO.5, OCTOBER, 2016 ISSN(Print) 1598-1657 http://dx.doi.org/10.5573/JSTS.2016.16.5.695 ISSN(Online) 2233-4866

Manuscript received Apr. 5, 2016; accepted Jul. 3, 2016 1 Institute of Applied Physics, Military University of Technology, 2 Kaliskiego Str., 00-908 Warsaw, Poland 2 Vigo System S.A., 129/133 Poznańska Str., 05-850 Ożarów Mazowiecki, Poland E-mail : djalal.benyahia@wat.edu.pl

p-Type Doping of GaSb by Beryllium Grown on GaAs (001) Substrate by Molecular Beam Epitaxy

Djalal Benyahia1,*, Łukasz Kubiszyn2, Krystian Michalczewski1, Artur Kębłowski2,

Piotr Martyniuk1, Józef Piotrowski2, and Antoni Rogalski1

Abstract—Be-doped GaSb layers were grown on highly mismatched semi-insulating GaAs substrate (001) with 2° offcut towards <110> at low growth temperature, by molecular beam epitaxy (MBE). The influence of Be doping on the crystallographic quality, surface morphology, and electrical properties, was assessed by X-ray diffraction, Nomarski microscopy, and Hall effect measurements, respectively. Be impurities are well behaved acceptors with hole concentrations as high as 9×1017 cm-3. In addition, the reduction of GaSb lattice parameter with Be doping was studied. Index Terms—Molecular beam epitaxy, GaSb, doping, high resolution X-ray diffraction, semiconducting III-V materials

I. INTRODUCTION

Antimonide-based semiconductors have potential applications in a wide range of electronic and optoelectronic devices because of their exceptional band alignments, and small effective mass, in addition to extremely high electron mobility [1-3]. For that reason, there has been much attentiveness and huge progress done in the material growth and device invention of antimonide semiconductor heterostructures, such as field effect transistors [4], semiconductor lasers [5], and

infrared detectors [6]. Although up-to-date technical advancements have allowed high quality lattice matched GaSb epitaxy on native substrate, GaAs substrate are likeable for many applications. This is due to: GaAs is low cost, has suitable thermal properties and establishes outstanding n and p ohmic contact. Moreover, transparency of GaAs substrates over a wide 0.9 to ~20µm spectral range enables backside illumination of optoelectronic devices and use of monolithic optical immersion technology resulting in dramatic improvements of performance of these devices [7]. Nevertheless, there is a high lattice mismatch between GaSb and GaAs substrate of 7.8%, which leads to real problems for the growth of good devices.

Nominally undoped GaSb is p-type due to native defects [8]. Silicon is frequently used for n-type doping of III-V semiconductors grown by MBE. The group IV atoms as Si, Ge, and Sn are known as amphoteric dopants of III-V compounds; they have the possibility to be integrated on group III lattice sites as donors, or on group V as lattice sites as acceptors. Alternatively, the group VI elements S, Se, and Te are used. The p-type dopants are C, Ge, Be. This latter is often used in the MBE growth of III-V materials. Furthermore, it is demonstrated that the presence of dopant atoms in epitaxial atoms changes its electrical and crystallographic properties [9-12]. It was shown by Sankowska et al. that the lattice constant of GaSb layer grown on GaSb substrate decreases by the Be doping and unintentional As dopant. It was found that the main influence on the GaSb lattice parameter with high Be doping is the beryllium size effect, while the As size effect is the main influence on the GaSb lattice parameter with low Be

696 DJALAL BENYAHIA et al : p-TYPE DOPING OF GASB BY BERYLLIUM GROWN ON GAAS (001) SUBSTRATE BY …

doping. Also, the change of the strain in the InAs/GaSb superlattice by introducing the Be dopant was demonstrated. Jenichen et al. [13] found that high density of misfit dislocation in AlAs/GaAs Distributed Brag Reflectors (DBRs) can be generated by Be doping. The X-ray measurements of these structure presented a broadening of the satellite peaks, this confirms that the strain was changed by the presence of Be doping. This heterostructure was doped also with carbon, but there was no broadening of satellite peaks.

In this paper, we report on the first results of p-type doping of GaSb by beryllium grown on GaAs substrates by molecular beam epitaxy. The effect of Be doping on the electrical and crystallographic properties of GaSb epilayer grown at low growth temperature was evaluated.

II. EXPERIMENT

Be-doped GaSb layers has been grown on (001)-oriented semi-insulating GaAs substrates with 2º offcut towards <110> in a RIBER COMPACT 21-DZ solid-source molecular beam epitaxy (MBE) system. The growth details of GaSb layers have been reported elsewhere [14]. We have used As4 and Sb2 as group-V elements. The substrate temperature was measured by the manipulator thermocouple calibrated from the GaAs deoxidization temperature. Four GaSb layers were grown at a growth temperature of 385 ºC with Sb/Ga flux ratio of 5: one undoped GaSb epilayer, and three Be-doped GaSb epilayers. The Be effusion cell temperature was 700 ºC, 760 ºC and 830 ºC. The GaSb epilayers thickness was 2µm. Beside, the growth rate was 0.6 ML/s. The growth process was monitored by in-situ reflection high-energy electron diffraction (RHEED).

The crystallographic properties of the samples were assessed by high-resolution X-ray diffractometer of PANalytical X’Pert. We have used the Cu Kα1 radiation (λ ≈ 1.5406 Å) originating from a line focus. A four bounce, Ge (004) hybrid monochromator was utilized to monochromatize the X-ray beam. The measurements were made in both ω and 2θ-ω directions. Hall measurement using Van der Pauw method was used to evaluate electrical characteristics. This measurement has been performed by using ECOPIA Hall effect measurement system. This latter gives the opportunity to measure the electrical parameters between 80K and room

temperature by a chosen step (here, we used 5K as a step). Having cooled to liquid Nitrogen temperature, this system measures Hall parameters at the initial temperature, then, it is heated to the next temperature, and measure again, and so, until room temperature. While surface properties were characterized by Nomarski optical microscopy and high resolution optical profilometry.

III. RESULTS AND DISCUSSION

When the GaSb growth started, the RHEED pattern switched from (4×2), characteristic of GaAs layer, to sparkling spots, which means that the growth was changed from a two-dimensional (2D) growth to a three-dimensional (3D) one. This consequence confirms that GaSb growth is a Volmer-Weber growth mode [15]. A clear (1×3) RHEED pattern was seen, after few minutes, as it is shown in Fig. 1, pointing a flat surface of GaSb layer.

Be does not seem to manifest surface segregation in GaSb. In fact, the growth of GaSb under Sb stabilized conditions indicates (1×3) RHEED pattern both in the absence and in the presence of Be, which confirms the result of Longenbach et al. [16], they found out that dopant segregation is not a problem in GaSb.

Fig. 2 illustrates the surface morphology of Be-doped GaSb characterized by Nomarski microscopy. Shiny mirror-like Be-doped GaSb layers were obtained under optimized growth temperature and Sb/Ga flux ratio for the four samples. In addition, the same surface quality was obtained at 250 °C to 540 °C temperature range [14]. Not mirror-like surface were acquired after the growth of GaSb outside this range of temperature [Fig. 2(b)].

Fig. 3(a) represents the (004) diffraction curve of Be-

(a) (b)

Fig. 1. Be-doped GaSb layer RHEED pattern (a) (× 1) pattern,(b) (× 3) pattern. The Be cell temperature is 830°C.

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doped GaSb layer in 2θ- ω direction. It can be seen that there are two peaks: GaAs substrate peak and GaSb layer peak. The separation, which can be noticed from Fig.

3(b), between these two peaks represents the strain between the GaAs substrate and the GaSb layer. As shown in Fig. 4, this separation decreases from 19201 arcsec to 19183 arcsec when the temperature of Be cell increases from 700 °C to 830 °C (Table 1). While, the lattice constant of doped GaSb layer decreased as the Be cell temperature increases, as can be seen in Table 1. This suggests that the GaSb buffer layer becomes more strained. The present formulas have been used:

2=nλ d sinθ (1)

2 2 2

2 2

1 + +=

hkl

h k l d a

(2)

where n is a positive integer, λ is the wavelength of Cu Kα1 radiation, θ is the scattering angle, dhkl is the spacing between two lattice plans which miller indices is (hkl), and ‘a’ is the lattice constant of GaSb layer.

As follows from Fig. 4, the crystal quality was found to vary enormously with respect to the Be effusion cell temperature. The full width at half maximum (FWHM) of GaSb peak measured in ω direction increases as the Be doping increases. This is due to the incorporation of Be atoms into the GaSb lattice, which affects the crystal

(a) (b)

Fig. 2. The Nomarski optical microscopy pictures with a magnification of 1000 of 2 μm thick Be-doped GaSb layer grown at 385 °C with Be cell temperature 700 °C (a) and undoped GaSb layer grown at not optimized growth parameter (b).

(a)

(b)

Fig. 3. (004) high-resolution x-ray diffraction curve of Be-doped GaSb layer in 2θ-ω direction (a) and a zoom in the GaSb peaks (b).

Fig. 4. FWHM of Be-doped GaSb epilayers and the separation between the GaAs substrate peak and GaSb layer peak versus Be cell temperature. All the samples have a thickness of 2 μm. Table 1. Structural and Electrical properties of Be-doped GaSb layers at room temperature

Be cell temp, °C undoped 700 °C 760 °C 830 °C Separation, arcsec 19209.33 19201.32 19192.68 19183.32

Lattice const, Å 6.09752 6.09734 6.09712 6.09688 Conc, cm-3 7.4 × 1016 7.7 × 1016 1.3 × 1017 9.2 × 1017 Mobility, cm2V-1s-1 632 507 545 386

698 DJALAL BENYAHIA et al : p-TYPE DOPING OF GASB BY BERYLLIUM GROWN ON GAAS (001) SUBSTRATE BY …

quality of the GaSb layers. The hole concentration and mobility of undoped GaSb

layer and Be-doped GaSb layers, are shown in Table 1. It can be seen that the undoped GaSb is unintentionally p-type doped, which is consistent with the results of Anayama et al. [10]. When the Be cell temperature is 700 °C, the hole concentration is 7.76×1016 cm-3, which is almost the same value as that for undoped GaSb layer (7.43×1016 cm-3), therefore, we can consider that the doping is too weak when the Be cell temperature is 700 °C.

The hole concentration and the Be vapor pressure as a function of reciprocal Be cell temperature are shown in Fig. 5. The values of p for GaSb layers range from 8×1016 cm-3 to 1×1018 cm-3. At moderate Be source temperature, p exhibits an Arrhenius dependence on temperature. The activation energy of beryllium is 55 meV, it has been calculated from the slope of the characteristic ln (Na) as a function of (1/kT), where k is Boltzmann constant and Na is the carrier concentration. The solid line in Fig. 5 represents the Be vapor pressure, plotted from the Eq. (3) [17]:

( ) 1log 8.042 17020 0.4440 logp atm T T-- -= ´ ´ (3) The fact that the slope of vapor pressure curve matches

well the 1/T dependence of the hole concentration is significant and suggests that, the Be doping level is merely proportional to the Be atoms arrival rate.

Fig. 6 shows the dependence of hole concentration on the temperature. The hole concentration is practically

constant for high doping (Be cell temperature is 830 °C). However, for undoped GaSb and low doped GaSb, the

hole concentration decreases at low temperatures. This is due probably to the native defects in GaSb, which have higher ionization energy in comparison to Be atoms.

On the other hand, the hole mobility as a function of temperature is plotted in Fig. 7. Below room temperature, the hole mobility varies virtually as T 1.23, it increases with decreasing temperature, until 120 K is reached, then drops off, which is consistent with the results of Leifer et al. [18], who found that the mobility follows in this temperature range the T 1.5 law. Above the temperature of the maximum (120K) in Fig. 7, The dependence between the hole mobility and temperature is practically linear, Leifer et al. found the hole mobility obeys T -0.87 law. While temperature increases, phonon concentration increases, and causes increasing scattering which lowers

Fig. 5. The hole concentration and Be vapor pressure versus reciprocal Be cell temperature of Be-doped GaSb temperature.

Fig. 6. The hole concentration of undoped GaSb layer and Be-doped GaSb layers versus the temperature.

Fig. 7. The hole mobility of undoped GaSb layer and Be-doped GaSb layers versus the temperature.

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the carrier mobility more and more at higher temperature. Below 120K, the mobility lowering is governed by the impurities scattering (charged and neutral impurities), imperfections and space charge barriers in the material [18].

Fig. 8 shows the electrical conductivity of undoped and Be doped GaSb layers. It can be seen that the conductivity increases as the Be cell temperature increases. On the other hand, the conductivity has nearly the same trend as the mobility discussed before. The maximum of the conductivity is at 170K. For instance, a conductivity as high as 70 S/cm has been obtained for a Be cell temperature of 830 °C. Owing to Be doping, carrier concentration increases, which leads to the mitigation of the resistivity, therefore, an increasing of the conductivity.

IV. SUMMARY AND CONCLUSIONS

Be-doped GaSb epilayers were grown on GaAs substrates. Hall effect measurement, X-ray diffractometry and optical microscopy were used to evaluate the effect of Be doping on the electrical properties, crystalographic quality and surface characterization of Be-doped GaSb layers, respectively.

The crystalline quality degrades when the Be doping level increases. Beside, the strain increases between GaSb layer and GaAs substrate. Moreover, GaSb lattice constant decreases when the Be doping increases due to Be size effect.

The undoped and Be-doped GaSb epilayers exhibit p-

type conductivity over 80 to 300K temperature range. The highest hole concentration ~ 1018 cm-3 was obtained with a Be cell temperature at 830 °C.

ACKNOWLEDGMENTS

This paper has been completed with the financial support of the Polish National Science Centre, Project: UMO-2015/17/B/ST5/01753.

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Djalal Benyahia was born in Mila, Algeria, on 1989. He received the B.S., degree in the Polytechnic Military School, Algiers, Algeria in 2013, and M.S., degree in telecommu- nication in the Military University of Technology, Warsaw, Poland, in 2015.

His research interests the growth and characterization of the group III-V semiconductors, bulk materials, type-II superlattices (InAs/GaSb), and infrared detectors by molecular beam epitaxy.

Łukasz Kubiszyn was born in Rzeszów, Poland, on 1990. He received M.S. degree in Warsaw University of Technology in technical physics in 2014. He joined at Vigo System S.A., where he has been working in the area of the

growth and investigation of group III-V materials, including: bulk materials, type-II superlattices and infrared detectors.

Krystian Michalczewski is a researcher at the Institute of Applied Physics, Military University of Technology (MUT) in Warsaw, Poland. He received the M.S. degree in chemical technology from the Department of Chemistry, Warsaw

University of Technology in 2015. His specialization is the growth of group III-V semiconductors, bulk materials, type-II superlattices (InAs/GaSb), wet etching and passivation.

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Artur Kębłowski was born in Warsaw, Poland, on 1983. He received M.S. degree in automation and robotics from the Faculty of Production Engineering, Warsaw University of Technology, in 2008. Since 2014, he is the head of the

Epitaxy Laboratory in VIGO System S.A. His research interests the morphology of CdTe deposited onto GaAs substrate, engineering of the chemical vapor deposition of HgCdTe material.

Piotr Martyniuk is an assistant professor at the Institute of Applied Physics, Military University of Technology (MUT) in Warsaw, Poland. He received the M.S. degree in technical physics at MUT (2001), PhD in electronics at Warsaw

University of Technology (2008), Poland and D.S. in electronics in MUT (2015). He is currently the head of the Institute of Applied Physics at MUT. The main subject of his investigation is HgCdTe, InAsSb and T2SLs IR detectors.

Józef Piotrowski is a researcher and development manager at Vigo System S.A., Warsaw, Poland, and a scientific advisor at the Military Institute of Armament Technology, Zielonka. Prof. Piotrowski proposed numerous concepts and practical

solutions related to uncooled detection. He developed more advanced IR devices (Dember, magnetoconcentra- tion, and photovoltaic) based on HgCdTe and other material systems (HgZnTe, HgMnTe, InAsSb, InGaAs).

Antoni Rogalski has made pioneering contributions in theory, design, and technology of different types of IR detector. He is currently an ordinary member at PAN (Polish Academy of Sciences), and a Pro- fessor with the Institute of Applied

Physics, Military University of Technology, Warsaw, Poland. Prof. Rogalski is an active member of the international Technical Community. He is a Chair and Co-Chair, an Organizer, and a member of Scientific Committees of many national and international conferences on optoelectronic devices and material science.