Ag Deposited on p-GaSb(100)

Low resistance ohmic contacts essential for fabrication of GaAs or GaSb devices can be viewed also in other systems as Ag-In-Ge/n-GaAs, In-Ag, Ge-Au-Ag, In-Ge-Ag on both n and p-type GaAs, systems where this requirement is satisfied. The undoped p-type, LEC GaSb(100), wafers were prepared prior Ag vacuum deposition by a cleaning procedure in organic solvents followed by a chemical etching in same sequence as n-GaSb(100) i.e.

HF:H2O (DIW) for 15 sec and HCl:H2O (DIW) for 15 sec. The Ag thin film was vacuum deposited at a medium pressure of p ~ 5.6•10^-5 torr, from a Mo boat, at a current I ~ 70 A for a deposition time t ~40 sec. The deposited Ag film was subjected to an annealing procedure at T = 440 ⁰C, for t ~10 min in low vacuum p ~ 6•10^-2 torr. It was stated [29]

the contact resistance is highly dependent on the carrier concentration. Generally, the carrier tunneling through finite potential contact barrier is dominant in current flow, governing the contact resistance, even at room temperature. The Ag contact resistivity, i.e. ρ ~ 10^-5 Ωcm² is one order of magnitude worse for AuGeNi/n-GaSb, and the experimental procedure for obtaining Ag ohmic contacts is dedicated to p-doped layers as

p- doped (Zn) GaSb obtained by thermal diffusion or p-doped (Si⁺) GaSb obtained by ion implantation. The contact resistance of Ag deposited on p-GaSb(100) had a I-V characteristic as presented in Fig. 5.14.

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

Fig. 5.14. I-V characteristics of Ag ohmic contact on p-GaSb(100), S(contact area) = 4 mm² ,R = 10 Ω.

5.4.4. Pd Chemically Deposited on n-GaAs(100) and p-GaAs(100)

Palladium chemical deposition on n-GaAs is an alternative technique for contact deposition, besides vacuum deposition or RF-sputtering. Palladium on GaAs is stated [30]

to be a class of a reactive interface initially obtained by Pd sublimation upon cleaned GaAs substrate, an interface in which the unoccupied intrinsic surface semiconductor states are apparently unaffected by metallization. Regarding the properties of Pd thin films the general aspects of the kinetics for palladium chemical deposition reaction on n and p-type GaAs is coupled with the adhesion properties of deposited films in the sense that adhesion characteristic of as-deposited Pd film is compared with characteristics of usual ohmic metallic contacts on GaAs as Au, Ag or Au-Ge obtained by vacuum deposition.

Palladium films were deposited on n-(100) GaAs (Te doped, c ~10¹⁸ cm^-3) and p-GaAs (Si doped, c ~10¹⁸ cm^-3) substrates from an aqueous solution of PdCl2 at different concentrations and reaction times. Before Pd deposition the GaAs wafers were subjected to a cleaning procedure in organic solvents but GaAs wafers were not chemically etched.

It is worth to mention that the Pd film adherence is strongly dependent on the cleaning procedure which should therefore be made as stringent as possible. For Pd deposition the starting solution was: PdCl2: H2O (1.2 g/l) in HCl (37 %):(40 ml) and for other concentrations the dilution method was used [31]. The formation of a Pd-GaAs compound is activated by maintaining the bath at 50 ⁰C and the mechanism of chemical reactions

that conduced to the nucleation and growth of metallic palladium, starts with the activator solution that contains HCl in excess in order to dissolve PdCl2 by complexing it to form PdCl42-. The deposition of metallic Pd from this species occurs according to the reaction [32] and Pd deposition on GaAs wafer is uniform indicating that the semiconductor surface has a high density of nucleation sites where the Pd grains are generally randomly oriented.

PdCl42-+ 2e^- = Pd + 4Cl^- (E⁰ = 0.62 V). (5.3) In this mechanism, the required electrons must be supplied by the substrate, and in the case of GaAs the following reactions may proceed from right to left [32]:

Ga³⁺ + 3e^- = Ga (E⁰ = -0.54 V), (5.4) HAsO2 + 3H⁺ + 3e^- = As + 2H2O (E⁰ = 0.248 V), (5.5) H3AsO4 + 5H⁺ + 5e^- = As + 4H2O (E⁰ = 0.377 V). (5.6) The E⁰ values indicate that both Ga and As may be oxidized by PdCl42-. Palladium will deposit at the cathodic sites, while substrate oxidation (dissolution) will take place at the anodic sites and chemical deposition involves the nucleation and growth as in a conventional electrodeposition processes. The aspect of uniformly distribution of the very small Pd grains observed on GaAs is indicating that nucleation is intensively promoted GaAs substrate. The palladium films thickness has been established using an interferometric method, with a Linnik type microscope at λ = 540 nm. The measurements performed in a rectangular geometry defined by a photoresist mask for dividing unprotected areas have been compared as order of magnitude with the ones obtained using a α-step profiler. In this regard, Pd film thickness for reaction time in (20-100) sec range, in conditions: c = 1.2 g/l T = 313 K has a linear dependence as related to the exposed mechanism. Is to be remarked that for longer reaction times the chemical deposition act is accompanied by the removing of Pd film due to the lack of adherence. Pd film thickness dependence of PdCl2 solution concentration is presented in Fig. 5.15 (a) for n-GaAs and Fig. 5.15(b) for p-GaAs in the deposition conditions: reaction temperature: T1 = 313 K and T2 = 323 K; reaction time: t= 60 sec in connections with corresponding errors for thickness measurement. The general trend is the linear dependence with PdCl2 solution concentration with a noticeable difference as regarding the electric character of the substrate, in the sense that is observed a more rapidly deposition rate reaction on n-GaAs relative to p-GaAs. A possible explanation is that the n-GaAs substrate has an excess of free electrons, that can be used in order to neutralize Pd²⁺ ions from the chloride solution, so the deposition has the condition to increase with concentration. On the other hand on the p-GaAs substrate there exists a lack of free electrons and according to reaction (5.3) Pd²⁺ions neutralization is more efficient at low concentration of chloride solution, so the general trend for Pd film thickness is a decreasing one with the observation that palladium films thickness dependence on solution concentration is much weaker than in the case of Au chemical deposition on GaAs [33].

Fig. 5.15. (a) Pd film thickness vs. solution concentration for n-GaAs; (b) Pd films thickness vs. solution concentration for p-GaAs.

The metal films adhesion on semiconductor substrate is one of critical factors for ohmic or Schottky contacts in the sense that with an inadequate adhesion, the coating may detach prematurely and consequently cause the mechanical failure of the contact. In this regard, it is important to evaluate the adhesion characteristic between the deposited metal and the substrate in a quantitative way [34]. Is to be said that the critical load measurement in scratch test have been used in assessing adhesion in film/substrate systems [35]. Palladium thin film adhesion measurement on GaAs has been performed using a mechanical method [36] that consists in the action of a vertical force at the interface film/substrate. As it was observed [35], the critical loads are strongly affected by various parameters such as scratching speed, indenture tip radius, and film thickness. In the home-made analyzing apparatus, the vertical force applied at the interface Pd/GaAs should have a maximum value in point of the local removing of the film for a horizontal motion of the point in which the load is applied. The vertical force is applied on a rounded point of sapphire characterized by a curvature radius r. This force produces a plastic deformation in GaAs substrate in a circle contact of radius a. The deformation depends on: substrate hardness (H), vertical applied load (W) and circle contact (a = (W/π H)^1/2). In horizontal motion for rounded point it appeared a shearing force F that is tangent at deformed surface and is depending on the normal pressure P (equal to hardness H) by the relation (5.7) where the mechanical action is presented in Fig. 5.16 according to literature [36]:

. (5.7)

This relation allowed to compute the shearing force F as a function of the vertical load W, where the experimental dependence for vertical load W(g) versus palladium thickness d(nm) curve is presented in Fig. 5.17, and as can be observed there exists a linear decreasing for thin film adhesion as a function of the Pd deposited thickness. This aspect can be explained by the existence of an internal strain in the deposited layer and we

annealing, in the sense that as the film is thinner, its adherence on substrate is improved (e.g. for the first time the ‘scotch tape test’ experimentally showed this behavior).

Fig. 5.16. Forces in lip indentation produced by rounded point.

Fig. 5.17. Dependence of vertical rupture load versus Pd film thickness.

A comparison regarding the adhesion of chemical deposited Pd films on GaAs, with results concerning the shearing force for metallization of GaAs substrate in case of vacuum deposited metal films and alloys as: Au/GaAs, Ag/GaAs, Au-Ge/GaAs is presented in Table 5.1, with indications related to film nature, substrate type, film thickness, computed shearing force, deposition conditions and including annealing procedure (usually at T = 763 K in low vacuum: (10^-2-10^-3) torr).

As can be observed, the calculated shearing force F for chemical deposited Pd/n- GaAs has the same order of magnitude as the shearing force F in the case of vacuum as-deposited Au/n-GaAs and Ag/n-GaAs films i.e. without annealing. An important increasing of the computed shearing force F is putted into evidence in the case of vacuum deposited Au/n-GaAs and Au-Ge/n-GaAs films that were subjected to an annealing procedure, due to the reactivity of metals at annealing temperature. The adhesion data presented in

350 355 360 365 370 375

0.0 0.5 1.0 1.5 2.0

Room temperature

Vertical applied load W(g)

Thickness d(nm )

Fig. 5.17 are related to a Pd film which thickness is in the experimental limit of 260 ± 20 nm, and the measured shearing force F in case of Pd/n-GaAs and Pd/p-GaAs are in the same magnitude limit. The computed shearing force F in the frame of Eq. (5.7) is related to GaAs hardness i.e. HB = 750 ± 40 Kg/mm². As related to the ohmic character of the chemical deposited Pd/GaAs films, there were experimentally recorded the series resistance for solar cells of AlGaAs/GaAs devices with large active area of (0.5-1) cm². The series resistance were in the same (5-15) m range, as the results obtained in a quasi-classical technology followed by our group, that involved a vacuum deposition coupled with an electrochemical method to thicken the Ag-Zn/GaAs ohmic contact [37].

Palladium chemical deposition can be viewed as an alternative technique to obtain contacts on GaAs, and the comparative results concerning the mechanically recorded adhesion data on different metal thin films deposited on this semiconductor suggested a similar behavior in the sense that as the deposited films are thinner, they have a large bonding to substrate and as a consequence a better adherence.

Table 5.1. Comparative study for the adhesion of thin films deposited on GaAs substrate.

Film Substrate

GaAs Thickness d(nm) Shearing force

F (× 10⁷ N/m²) Remarks

5.4.5. InGeNi Deposited on GaAs(SI)(110) and n-GaSb(100)

InGeNi contacting scheme solution brings together advantages for low contact resistance on III-V semiconductor compounds i.e.: formation of a new heavily doped intermediate semiconductor layer at the interface and the existence of a low barrier height at contact metals [4]. In this metallization system i.e. InGeNi/GaAs(SI)(110) it was stated [38] the Ge role is to diffuse in the semiconductor layer providing donors to GaAs, Ni part is to bring a thermal stability, surface smoothness in other words good mechanical properties and regarding In behavior, it is worth to mention that at elevated temperatures (T > 300°C) a ternary compound namely InxGa1-xAs is developed the effect being the appearance of a low energy barrier height at GaAs/metal interface.

In different metallization systems, the contact morphology is affected by: i) Diffusion processes that appeared during deposition and thermal annealing; ii) Chemical reactions through layer; iii) Presence of contaminants and of other chemical compounds presents detected after etching treatments. Due to this problems, in order to overcome some of shortcomings trails imposed by the classical metallic deposition, it was proposed [38] a new way solution for InGeNi namely in-situ metallic evaporation on a clean GaAs(SI) (Semi-Insulating)wafer obtained by cleavage in high vacuum prior to contact deposition procedure. The GaAs(100) wafer used was a semi-insulating type semiconductor with a

resistivity ρ > 10⁷ Ωcm, for metallic deposition there have been considered In, Ge and Ni ohmic contact and Al or Ti Schottky contact with specification that all the elements were of high purity e.g. 4N for Ti and Ni and 5N for Ge and that together GaAs(SI) wafers were provided on the market. The cleavage of GaAs substrate was realized in a home-made device in high vacuum conditions with an alignment of cleaved edge perpendicular to the evaporation sight. InGeNi were simultaneously deposited from a W boat in medium pressure conditions p ~5•10^-5 torr, and then the contact was subjected to annealing procedure for t = 5 min. at T = 430-450 °C in low vacuum (p ~6•10^-2 torr). Al or Ti Schottky contacts were sequential deposited and another annealing procedure was applied i.e. T = (370-400) °C t = 5 min. for Al, and T = (430-450) °C T = 5 min. for Ti. In deposition technique the quantities of each metal were calculated for a desired thickness of contact layer i.e. InGeNi:130 nm (60 nm In, 60 nm Ge and 10 nm Ni) and for Ti or Al layers was 200 nm. Evaluation of electrical characteristics was performed on InGeNi/GaAs/Ti and InGeNi/GaAs/Al device type detectors of S = 3×3 mm² with two deposited opposite edges.

The characteristics of InGeNi/GaAs contact were investigated by an XPS controlled Ar⁺ ion etching depth profiling in the as exposed analyzing conditions in case AuGeNi. The etching rate was estimated at 20 Å/min, and a number of 12 etching sequences were necessary until the metal/semiconductor interface was revealed. The XPS measurement of as-deposited InGeNi surface revealed the presence of with adventitious carbon and oxygen, and Ge, In, Ni, As and Ga. The total quantity of O and C exceeds 60 % of atoms of the surface. The relative concentration of these elements are presented in Fig. 5.18. in some conditions regarding quantitative analysis namely: i) XPS signal intensity arises from a depth of ~10 nm, for this distance the calculation method assumes a chemical uniformity, ii) 10 nm is also the thickness of the surface layer removed after each etching session. In this regard, the computed concentrations values for the detected elements, are not rigorously true.

Fig. 5.18. Relative atomic concentrations of In, Ge, Ni, Ga and As in the layer.

The XPS recorded spectra putted into evidence a pronounced diffusion of Ga with a benefic effect since Ge atoms will find unoccupied vacancies in the GaAs bulk. During experiment, it was observed that carbon signal does not any longer occur in the deep layers of the metallic deposition, while oxygen concentration will follow a gradient, decreasing towards the interface. The oxygen is almost completely present as gallium oxide (Ga2O3)

and apparently the presence of gallium in the layer induce oxygen diffusion and most likely it happens during the thermal annealing. On the other hand, on the surface un-exposed to Ar⁺ ion etching, there exist considerable amounts of other elements oxides.

Regarding As signal, it was found [38] that As is present in a metallic state with a concentration of 38.7 %, while As quantities in As2O3 and As2O5 are 39.1 and 22.2 % respectively. Germanium is present in oxide no less than 40 % of Ge amount, the rest being found in a metallic state. Indium is oxidized in a proportion of 30 %. Regarding Ni signal is to be remarked that only Ni behaves as a metal during entire experiment. The O1s XPS spectrum indicates both metallic oxides, the C-O and –OH chemical bonds and also molecular absorbed oxygen and after the first two etching sessions, only the gallium associated oxide remains visible in the spectra. Indium has a stable behavior and regarding Ge oxides their XPS signal disappear immediately after first etching step. In order to investigate the formation of InxGa1-xAs type compounds, in XPS measurement was considered In4d spectral line. From literature [39-41], is observed that the binding energy (BE) of In4d^5/2 spectral line increases as x increases and reaches 17.4 eV for a pure InAs compound, in this frame, in Fig. 5.19 are presented the experimental [38] measured binding energies corresponding to In4d^5/2 after each etching session. As can be observed, towards the interface it was found the compound InxGa1-xAs with x value very close to 1, and in these conditions is expected that reduction of Schottky height to practically 0 eV, a result that facilitates a smooth current flow through the contact.

Fig. 5.19. Binding energies (BE) of In 4d^5/2 spectral line within metallic layer.

A further reduction of the contact resistance is attributed to Ge in the following sense: Ge concentration is high at the surface but then begins to reduce and after the first etching Ge is found exclusively in an alloyed metallic state. Towards the interface, near to Ge3d spectrum line it appears a peak at a higher binding energy that could be attributed to Ge2O3, however this peak is correlated to a loss peak of Ga. The Ge signal is present in XPS analysis even after the last Ar⁺ ion etching session, that is why the assuming that there is an intermediate semiconductor layer heavily n-doped right above the GaAs bulk is correct. The formation of GeAs2 compound, that could induce higher contact resistance, has been avoided due to maintenance of annealing temperature bellow 450 °C. The binding energy of As3d peak is related to stable metallic state, with BE = 41.2 ± 0.1 eV and a FWHM (Full Width at Half Maximum) around 1.65 eV. Generally, Ga/As ratio tends to grow towards the interface, due to a preference of As atoms sputtering by Ar⁺ ions as it was experimentally putted into evidence [42, 43].

Regarding I-V characteristics is to be mentioned that two different types of Schottky device were investigated, namely. InGeNi/GaAs(SI)/Ti and InGeNi/GaAs(SI)/Al. Due the characteristics of semi-insulating GaAs is correct to assume that the thermionic emission is the conduction mechanism. In thermionic emission conduction mechanism of Schottky diodes, in order to extract the valuable electrical information i.e. ideality factor, Schottky barrier height (ФB), series resistance (RS), it was applied the method proposed by Cheung [44]. This method is specifically suitable for diodes with high series resistance and allows to determinate the values of n, ФB and RS from a single I-V measurement where are

For experimental interpretation the in-put data are Schottky diodes surface area Richardson constant; in this experiment it was used a common value of Richardson constant for GaAs i.e. A^* = 8.6 A/cm²K² is used and due to a logarithmic relation between ФB and A^*, there are small quantitative modifications relative to a large interval of A^*values. Since the GaAs(SI) substrate is highly resistive, it had to be taken into account that the suppositions of theories and extrapolations applied to normal semiconductor materials are not necessary applicable in case of semi-insulating materials [45], e.g. it is accepted that the depletion region width in case of reverse bias is almost direct proportional with the bias voltage, rather than with its square root; and even in forward I-V characteristics, it is not implied a constant RS, but a rather RS = RS(I). In this regard, due to Schottky barriers on GaAs(SI) substrate the Cheung functions, have been plotted at small bias voltages i.e. maximum 3 V and assuming a constant RS, as can be observed in Fig. 5.20(a, b) in case of Al/GaAs(SI)/InGeNi:

Fig. 5.20. Cheung method applied to Al/GaAs(SI)/InGeNi device (a) dV/dln(I) = f(I) and (b) H(I).

From Fig. 5.20(a) an ideality factor of n = 1.5 and a series resistance of RS = 340 MΩ were determined and with these considerations from Fig. 5.20(b) it was measured the barrier height, i.e. H = 0.83 eV, an usual value for GaAs Schottky contacts. In case of Ti/GaAs/InGeNi Schottky device, the ideality factor is n = 2.1, the series resistance is RS = 1.16 GΩ and the barrier height is found to be H = 0.85 eV. It is worth to mention that based on these experimental data it was computed a resistivity ρ = (1.5∙10⁷-4∙10⁷) Ωcm for GaAs semiconductor substrate, a value in concordance with manufacturer specifications. The height barrier values indicate the absence of an interfacial layer and the fact that the barrier chemical inhomogeneity has dominated the non-ideality behavior of Schottky diode. In case of Ti/GaAs/InGeNi is expected for Ti to induce the formation of a less ideal junction due to the appearance of a TiAs compound which affect the

From Fig. 5.20(a) an ideality factor of n = 1.5 and a series resistance of RS = 340 MΩ were determined and with these considerations from Fig. 5.20(b) it was measured the barrier height, i.e. H = 0.83 eV, an usual value for GaAs Schottky contacts. In case of Ti/GaAs/InGeNi Schottky device, the ideality factor is n = 2.1, the series resistance is RS = 1.16 GΩ and the barrier height is found to be H = 0.85 eV. It is worth to mention that based on these experimental data it was computed a resistivity ρ = (1.5∙10⁷-4∙10⁷) Ωcm for GaAs semiconductor substrate, a value in concordance with manufacturer specifications. The height barrier values indicate the absence of an interfacial layer and the fact that the barrier chemical inhomogeneity has dominated the non-ideality behavior of Schottky diode. In case of Ti/GaAs/InGeNi is expected for Ti to induce the formation of a less ideal junction due to the appearance of a TiAs compound which affect the