GaAs surface passivation using in-situ oxide deposition

surface science ELSEVIER

Applied Surface Science 104/105 (1996) 441-447

GaAs surface passivation using in-situ oxide deposition M. Passlack 1, M. Hong *, R.L. Opila, J.P. Mannaerts, J.R. Kwo A T& T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974-0636, USA

Received 28 June 1995; accepted 14 November 1995

Abstract

In-situ deposition of Ga203, SiO 2, and MgO films on clean, atomically ordered (100) GaAs surfaces grown by molecular beam epitaxy using a multiple-chamber ultra high vacuum system has been investigated. Using this technique, direct bonding of oxide molecules to GaAs surface atoms revealing intrinsic oxide-GaAs interface properties has been achieved. The GaAs surface reconstruction prior to deposition as observed by reflection high energy electron diffraction and the chemical shift of the interfacial As 3d core level acquired by X-ray photoelectron spectroscopy depth profiling are clearly correlated. AsxO ~. or elemental As were not detectable at in-situ fabricated oxide-GaAs interfaces. In sharp contrast to SiO 2- and MgO-GaAs interfaces which are characterized by a Fermi level intrinsically pinned at midgap, Ga203-GaAs interfaces exhibit unique intrinsic electronic properties including an interface state density and recombination velocity in the mid 10 l° cm -~" eV- t range and of 4500 cm/s, respectively.

1. Introduction

Since the emergence of GaAs technology, the development of GaAs electronic and optoelectronic devices has been hampered by the lack of dielectric films providing low interface state density [1]. Recent efforts have been mainly focused on a variety of dry and wet surface treatments prior to deposition of dielectric films [2-6]. Interface state densities are still high, typically around 1012 cm -2 eV -1 or above. Apparently, these techniques inadequately ad-

* Corresponding author. I Motorola Inc., Phoenix Corporate Research Laboratories. 2100 E. Elliot Road, Tempe, AZ 85284, USA. Tel.: + 1-6024134962: fax: + 1-602-4135934: e-mail: [email protected].

dress major sources of interface states such as surface exposure and defects [7-11] as well as surface nonstoichiometry [ 12]. An interface state density comparable to that typically obtained in the SiO2-Si system of 101° cm 2 requires a defect density of less than one defect per l05 surface atoms (the GaAs (100) surface has 1.6 × 10 ~5 atoms/cm2). Therefore, prior to deposition (i) extremely low GaAs surface exposure to impurity gases and (ii) preservation of surface stoichiometry are prerequisites. The effects of exposing a clean GaAs surface to impurity gases, in particular to oxygen, are well established; an oxygen exposure of e.g., 10 langmuir (1 L = 10 6 Torr s) results in a surface impurity coverage of 10 -5 to 10-3% of a monolayer or l0 s to 101° surface impurities/cm 2 depending on sticking coefficient [7,8,13]. An oxygen exposure > 10 4 L causes strong Fermi level pinning [7]. Other species such as metals and carbon

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M. Passlaek et al. / A p p l i e d SurCi~ce Science 1 0 4 / 1 0 5 (I996) 441 447

442

give rise to similar effects [7,14,15]. Subsequently, molecules or atoms of insulating materials intentionally deposited on a clean surface must provide a local interfacial atomic structure such that no gap states are induced in GaAs. Thermodynamic stability requires the exclusion of chemical reactions between GaAs surface atoms and the deposited species. This paper investigates intrinsic insulator-GaAs interface properties obtained by in-situ oxide deposition on clean and atomically ordered (100) GaAs surfaces.

[1101

[~Tol

Fig. 2. RHEED picture taken after completion of GaAs growth (t,,) and at start of oxide deposition (t~). The identical RHEED picture shows a (2 × 4) As stabilized surface and indicates preservation of surface stoichiometry.

2. Sample preparation The samples were fabricated using a multiplechamber ultra-high vacuum (UHV) system described in Ref. [16]. Fig. 1 shows the pressure regime after completion of GaAs growth at time t~ (solid line) and the GaAs surface exposure occurring prior to oxide deposition for t~ < t < t~ (dashed line) for in-situ fabrication of oxide-GaAs structures. The sample fabrication comprised 1.5 /xm thick GaAs n-type (1.6× 1016 cm -3) or p-type (4.4× 1016 c m - 3 ) layers grown at 650°C on n + or p+ doped (100) GaAs substrate, respectively, in a solid-source III-V chamber at a background pressure of 2 × 10- t Torr. Subsequently, the wafer with an As-stabilized (2 × 4) surface was transferred (background pressure

l0

6

. . . .

I

. . . .

I

. . . .

I

. . . .

I

i

. . . .

= 6 × 10 j~ Torr) from the MBE growth chamber into an oxide deposition chamber (1 × 10 -~° Tow). Note that the typical GaAs surface exposure (predominantly oxygen) prior to oxide deposition (t~ < t
10

In-Situ Oxide - (100) GaAs i0 -7

~"

Start Deposition (ts) "

Cool

"11

-500

. , ,

~. 4 ¢1

,~

",

Down

(8 rain)

tc 10

6

Heating to T s (13 mini Transfer (4 min) ',,

10 -9

=

2 ~

~ I

0

. . . .

i

. . . .

t ..................

500 1000 Time (s)

3. Sample characterization 3.1. Structural surface and interface properties

Turn on e-Beam(tt) --

10"8

10.10

8 "~

I.~,

1500

,

,

,

0

2000

Fig. 1. Pressure (solid line) and surface exposure (dashed line) measured between completion of GaAs epitaxial growth (t c) and start of oxide deposition (t~).

GaAs surface and oxide-GaAs interface properties were investigated using reflection high energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS), respectively. The GaAs surface was characterized in situ by RHEED at time of (i) completion of GaAs growth (t~), (ii) reaching the substrate temperature it, for oxide deposition (t~), (iii) opening the shutter for oxide deposition (t~), (iv) completion of the first oxide monolayer, and (v) completion of oxide deposition. RHEED pictures taken from a (100) GaAs

M. Passlack et al. / Applied Surface Science 104/105 (1996) 441-447

[110]

[1]0]

Fig. 3. RHEED picture taken prior to turn on of e-beam (t c) and at the start of oxide deposition (t~). The RHEED picture shows a (4 × 6) reconstructed surface and indicates preservation of surface stoichiometry.

surface after completion of GaAs epitaxial growth (t c) and at start of oxide deposition at a substrate temperature T~ of 350°C (t~) are identical, showing a (2 X 4) reconstructed, As stabilized surface (Fig. 2). This indicates preservation of an atomically ordered, stoichiometric GaAs surface prior to oxide deposition (t c < t < t~). The estimated As coverage of a ( 2 x 4 ) reconstructed surface is > 6 0 % [18,19]. Lower As coverage is typically obtained when the wafer is heated to temperatures above 500°C prior to oxide deposition. This is demonstrated in Fig. 3

443

showing a (4 X 6) GaAs surface reconstruction with an estimated As coverage of = 30% [18] at a substrate temperature ~ = 600°C (time tt). Note that the subsequent surface exposure to impurity gases of less than 10 L (t, < t < t~) does not affect the RHEED pattern and the (4 x 6) GaAs surface reconstruction is completely preserved prior to opening the shutter for oxide deposition (t~). Thus, prior to oxide deposition, (i) the surface stoichiometry is maintained as observed by RHEED and (ii) extremely low GaAs surface exposure of typically less than 10 L has been accomplished. Therefore, the estimated GaAs surface impurity coverage is = 10-4% of a monolayer or = 109 cm -2 assuming a typical initial oxygen sticking coefficient of 10 - 6 . As 3d and Ga 3d core level spectra were acquired from XPS depth profiles on fabricated oxide-GaAs structures. A Perkin Elmer 5600 series XPS spectrometer equipped with a monochromatic A1 K a X-ray source was used. The photon energy was 1486.6 eV. Depth profiling was done in situ in an UHV chamber (background pressure = 5 X 10-~° Torr) by Ar sputtering using an ion gun at 4 keV. The XPS spectra were acquired using an energy step width of 0.2 eV.

In - Situ G a 2 0 3 - GaAs As - Stabilized C(2x4) Ts = 360 ° C tox = 26.0 nm As - 3d

Ga - 3d

i ASGaAs

~-

26 48

44

40

36 28

24

20

16

Binding Energy (eV)

Fig. 4. Ga and As 3d core levels of a Ga203-GaAs interface measured by XPS. Energy positions and full line width at half-maximum (FWHM) of GaGa~o3, GammAsand ASGaAspeaks are 21.3 _+ 0.1, 19.2 + 0.1, and 41.2 + 0.1 eV, respectively, and 1.80, 1.34, and 1.60 eV. respectively. The As 3d level exhibits a shift of 0.44 ± 0.2 eV at the interface.

coverage (%)

0 360 400 550 500

deposition temperature (°C)

substrate

Oxide

Ga20 ~ Ga20 ~ SiO 2 Ga203 Ga203

type

19.2 19.2 _+0.1 19.2_+0.1 19.3 + 0.1 19.2+0.1 19.2_+0.1

GaAs bulk

Ga 3d

+ 1.2 + (2.0 + 0.2) +(2.1 _+0.2) -+(2.0+0.2) +(2.0_+0.2)

Ga203 bulk shift

Observed binding energy (eV)

a (4 × 2) surface reconstruction was obtained by heating the sample to 620°C for a few seconds.

(published binding energies [7-9,21-23]) (2 × 4) > 60 (2×4) > 60 (2 × 4) > 60 (4×6) = 30 (4×2l ~ < 20

estimated As

surface

renconstruction

GaAs

Table l Observed binding energies of Ga and As 3d core levels

--+(0.36_+0.2) ---

- -

interface shift

41.2_+0.1 41.2+0.1 41.2+0.1 41.2+0.1 41.2+_0.1

4 1 . 2

GaAs bulk

As 3d

+(0.42+0.2) +(0.42+0.2) +(0.41 +0.2) +(0.24_+0.2) 0___0.2

- -

interface shift

"-1

4~

4~

t_n

2.

,-..,

2-

.e:

4~ 4~

M. Passlack et al. / Applied Surface Science 104/105 (1996) 441-447

Fig. 4 and Fig. 5 show typical depth profiles of Ga and As 3d core levels of in-situ fabricated G a 2 0 3 - G a A s structures as a function of depth, at depths near the G a 2 0 3 - G a A s interface. These spectra are shown for structures fabricated using a (2 × 4) As and a (4 × 2) Ga stabilized GaAs surface with an estimated As surface coverage prior to deposition of > 60% and < 20%, respectively [18,19]. AospUttering rate of 7.4 A / c y c l e (Fig. 4) and 1.5 A/cycle (Fig. 5) provided a depth resolution considerabl~¢ better than an estimated escape depth of ---25 A [20]. In the following, (i) the binding energy (BE) of Ga 3d levels in bulk Ga203 and GaAs, (ii) the BE of the As 3d level in bulk GaAs, and (iii) the interfacial chemical shifts of As and Ga 3d peaks will be discussed. Table 1 compares Ga and As 3d BEs of Gaca,o ~, GaGaAs and ASGaAs peaks to previously reported results [7-9,21-23]. The BEs of GaGaA~, and ASoaA~ are identical to standard XPS lines reported earlier (19.2 and 41.2 eV, respectively). The chemical shift of the Ga 3d peak in our bulk G a 2 0 3 films is 0.8 _+ 0.2 eV larger than a shift of 1.2 eV typically obtained from XPS surface analysis during the initial stage of GaAs oxidation [8,9,21,24]. The 2.0 _+ 0.2 eV shift reported here is based on bulk values; as sputtering proceeds the peak appears to

445

gradually shift from the bulk G a 2 0 3 to the GaAs on a length scale consistent with the electron escape depth. The intermediate peak can easily be fitted as a sum of two components; band bending is not excluded, but is not necessary to explain these results. Furthermore, oxide-GaAs structures with different band bending of -~ 0 eV for Ga203-GaAs and ---0.7 eV for SiO2-GaAs (see discussion of electronic interface properties) exhibit a virtually identical interfacial As 3d core level shift. Previously reported smaller shifts of Ga 3d level during initial oxidation of GaAs is complicated by the fact that these shifts in the Ga 3d level may represent G a suboxide formation a n d / o r formation of As oxides [9,10]. The chemical shift of the interfacial As 3d core levels acquired by XPS depth profiling are clearly correlated to As surface coverage prior to deposition and thus, to surface reconstruction (Table 1). The chemical shift is 0.44 + 0.2 eV for an As stabilized surface ( > 60% As coverage), however, no shift is detectable at a Ga stabilized surface ( < 20% As coverage). The increase in As 3d binding energy can be explained by (i) an increasing A s / G a atomic surface ratio and/or (ii) the existence of A s - O bonds. No quantitative analysis is possible at this

In - Situ Ga203- GaAs G a . Stabilized C(4x2) "Is= 500°C tox [] 25.3 nm

Ga - 3d

As - 3d &¢-

.

GaGaAs

,.-,_

200 ~e e o

150 48

44

40

36 28

24

20

16

Binding Energy (eV)

Fig. 5. Ga and As 3d core levels of a Ga,O3-GaAs interface measured by XPS. Energy positions and FWHM of Gaoa2o~, Ga~aA~and ASGaAspeaks are 21.2 + 0.1 19.2 5: 0.1, and 41.2 + 0.1 eV. respectively,and 1.83, 1.39, and 1.63 eV, respectively.No chemical shift of the As 3d peak is detectable at the interface.

446

M. Passlack et al. / Applied Surface Science 104/105 (1996) 441-447 102 n-type (I00) GaAs / / ~ A I G a A s - G a A s ~

101 100

8

10-1

.~ 102

M

~

O

2 GaAs

! f.

10-3 )~0= 514.5 nm Io = 580 W/cm2 104 , • . . . . . . . . . . . . . . , , , 750 800 850 900 950 Luminescence Wavelength (nm)

Fig. 6. Measured PL spectra of G a 2 0 3 - , Al045Ga05~As-. SiO 2 - . and M g O - G a A s structures as well as of a corresponding bare surface. The deposition temperatures for Ga203 are 620, 360, and 550°C from the highest to the lowest measured spectrum, respectively. The other results were typically obtained for 7", = 690°C (A10.45Gao.~sAs), and 0°C < T~ < 500°C (SiO 2, MgO).

time and further work is required to clarify the specific bonding mechanisms of oxide molecules on a clean and atomically ordered (100) GaAs surface.

3.2. Electronic interface properties Electronic interface properties including interface recombination velocity and interface state density have been investigated by steady-state photoluminescence (PL) and capacitance-voltage ( C - V ) measurements, respectively. Although the results will be briefly outlined in the following, the reader is referred to other reports for details [25,26]. Typical PL spectra of GaeO3-, A10.45Ga0.55As-, SiO2-, and M g O - G a A s structures as well as of a corresponding bare surface (Fig. 6) have been measured using an argon ion laser (A o = 514.5 nm). While Ga203-GaAs structures exhibit PL spectra comparable to that of excellent A10.45Ga0.55 As-GaAs interfaces, SiO z- and M g O - G a A s structures show PL similar to that of a bare surface. The interface recombination velocity S has been derived from the nonlinear dependence of the steady state PL on incident light intensity I o measured for 20 < I o < 5000 W / c m 2. In the case of Ga203-GaAs struc-

tures, the best fit to the measured data using a model solving Poisson's and current continuity equations self-consistently (see for example [27]), has been obtained for 4500 < S < 7000 c m / s [26]. The interface state density Dit has been determined by quasistatic and high frequency C - V measurements. A midgap interface state density in the mid 101° cm -2 eV -1 range has been inferred for Ga203-GaAs structures [25] using the quasi-static/high frequency technique [28]. For SiO 2- and MgO-GaAs structures, interface state densities and recombination velocities of = 5 × 1013 cm -2 eV -1 and = 107 c m / s have been inferred, respectively. The results clearly indicate fundamental differences in electronic interface properties.

4. Conclusions The analysis of structural and electronic interface properties of Ga203, SiO2, and MgO films deposited in-situ on clean and atomically ordered (I00) GaAs surfaces allows the following conclusions. (i) Chemical reaction products, in particular As203 (44.6 eV) and As205 (45.7 eV) are not detectable at in-situ fabricated oxide-GaAs interfaces. (ii) Consequently, thermodynamic stability is obtained as predicted by thermodynamic phase diagrams [29]. In particular, the chemical reaction As203 + 2 GaAs ~ Ga203 + 4 As (AG = - 6 2 kcal/mol) resulting As formation and degradation of electronic interface properties [2,12] is excluded. (iii) Interfacial As 3d and Ga 3d lines are essentially identical for all considered in-situ fabricated oxide-GaAs structures. (iv) Electronic interface properties, however, are fundamentally different. While Ga203-GaAs interfaces exhibit unique intrinsic electronic properties with interface state densities in the mid 10 ~° cm -2 eV -l range, SiO 2and MgO-GaAs interfaces are characterized by an intrinsically pinned Fermi level at midgap.

Acknowledgements M.P. gratefully acknowledges support by the Deutsche Forschungsgemeinschaft.

M. Passlack et al. / Applied Surface Science 104 / 105 (1996) 441-447

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