Surfaces

Surface properties arising from the physical and electronic structure, as well as surface preparation of wafers fabricated from bulk p-Ga203 crystals, are crucial for device operation and epitaxial growth. Surfaces may have different orientations with different terminations, may differently interact with an environment in which they operate, and may have different electronic structure. For epitaxial growth, the surfaces should be free from a damaged layer that is typically formed during wafer polishing.

Surface Properties

Theoretical study of the physical and electronic structures of the (100), (010), (001), and (101) faces of p-Ga203 was first performed by Bermudez [277]. The lowest energy plane is the easiest cleavage plane (100), which is non-polar and composed of two terminations named A and В (Fig. 4.35). The А-surface terminates in rows of O(II) with back-bonded two Ga(I) atoms, wherein between each O(II) row, there is a row of Ga(II) atoms. O(II) and Ga(II) atoms are singly unsaturated (missing a single nearest neighbor), while 0(1), О (III), and Ga(I) in the surface plane are fully coordinated. The В-surface is terminated in rows of Ga(II) and O(III) atoms, each singlyunsaturated. Ga(I) and 0(1) atoms at the surface are fully coordinated, and there is no O(II) atoms at the surface plane. The (010) surface has only one termination containing all five atom types. All surface atoms are singly unsaturated except Ga(II), which is doubly unsaturated. The (001) surface is also non-polar and has two terminations as in the case of the (100) surface. Here, the А-surface terminates in rows of singly unsaturated 0(1) atoms and back-bonded Ga(II) atoms. The 0(1) rows are separated by rows of singly unsaturated Ga(I) atoms. Ga(II) and O(II) atoms in the surface plane are fully coordinated, and there are no O(II) atoms in the plane. The В-surface terminates in rows of doubly unsaturated 0(III) atoms back-bonded to Ga(I) and Ga(II) atoms, and separated by rows of doubly unsaturated Ga(II) atoms. Ga(I) and O(II) atoms in the surface plane are fully coordinated, and there are no 0(1) atoms in the surface plane.

(3-Ga0 structure viewed along the (010) direction. The labels "A"

Figure 4.35 (3-Ga203 structure viewed along the (010) direction. The labels "A"

and "B" indicate the different (100) and (001) surface terminations. 0(1), 0(11), and 0(111) are inequivalent oxygen sites, while Ga(l) and Ga(ll) are inequivalent Ga sites. The box indicates the unit cell.

The presence of the two terminations A and В of the (100) surface that does not exhibit any reconstruction was experimentally confirmed by Lovejoy et al. [190] with the use of scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED), as well as by Busch et al. [278] with the fast atom diffraction (H, H2, He). The main termination of the (100) face is В-termination, while the contribution of the А-termination is below 5%. The dominating terraces arising from the termination have a height of 5.94 A, and others of 4.4 and 1.5 A. The terraces with the dominating height that corresponds to a half unit cell (a/2]sin(p] were also observed on epi-ready (lOO)-oriented wafers by Wagner et al. [279].

Another important surface property is the occurrence of upward/ downward band bending, which may hinder ohmic/Schottky contact formation. Lovejoy et al. [190] experimentally demonstrated (hard X-ray photoemission spectroscopy, HXPS) the presence of upward band bending (electron depletion layer] by about 0.5 eV of the air-cleaved or vacuum-annealed (100) surface. The upward band bending could be a result of accumulation of negatively charged defects at the surface, either Ga vacancies or interstitial, or adsorbed oxygen [280]. The upward band bending has been confirmed by Navarro-Quezada et al. [184, 281] after applying X-ray photoemission spectroscopy (XPS) to as-cleaved and next annealed in UHV (lOO)-oriented samples. The values of upward band bending of 0.28 and 0.5 eV were found for as-cleaved and UHV-annealed samples, respectively. The upward band bending makes the surface insulating, while the bulk remains semiconducting. This has been experimentally proved by annealing bulk crystals by Galazka et al. [226] and Oshima et al. [282].

Furthermore, Navarro-Quezada et al. [281] showed that (100]-oriented wafers, after chemo-mechanical polishing (CMP], are contaminated with carbon, which can be removed by annealing in UHV (<10'10 mbar] at 800°C for 30 min. The presence of carbon on the (100] surface may affect growth kinetics of p-Ga203 epi-layers and affect their electrical properties. Indeed, Wagner et al. [279] showed the negative impact of carbon (as C02] from Ga precursor tri-methyl-gallium (TMG] on the growth kinetics of homoepitaxial layers.

Surface Preparation

After cutting a bulk crystal into wafers, they are polished typically in the CMP process. For instance, Huang et al. [283] studied (100] surfaces of EFG-grown crystals after mechanical polishing (MP] and

CMP and concluded that very smooth surfaces can be obtained after the CMP process utilizing an alkaline slurry, where an important contribution was OH" ions that react with the p-Ga203 (100) surface.

The surface of the polished wafers has a damaged layer, which should be removed prior epitaxial growth. This can be done by wet or dry etching and/or by annealing. The wafer surface after such post-polishing treatment is epi-ready.

OhiraandArai [284] studied anumberofagentsand temperatures for wet etching of (100) and (OOl)-oriented and CMP polished wafers fabricated from OFZ-grown bulk crystals. Among several solutions, only 47% HF at RT and 60.5% HN03 at 120°C were found to be effective with the etching rate of 31.3-58.7 and 86.6 nm/h, respectively. p-Ga203 seems to be resistant to other acid and basic solutions at RT. With the use of HF, the dissolution of Ga from Ga203 linearly increases with etching time and with HF concentration in the solution. The etching rate of the (100) was found to be almost twice higher as compared to the (001) plane. In the case of Sn-doped samples, the etching rate decreases with Sn-doping concentration.

Dry etching of (100), (010), and (201)-oriented p-Ga203 samples with the use of reactive ion etching (RIE) and inductively coupled plasma (ICP) was investigated by Hogan etal. [285]. Among different etching agents (BCl3, CF4/02, BCl3/SF6, and BCl3/02), the highest etching rate was achieved by BC13. Both (010) and (201) planes revealed similar etching rates, while (100) showed lower etching rate. The highest etching rate was 19 and 33 nm/min for the RIE and ICP techniques, wherein the latter produced a lower surface roughness. However, Yang et al. [286] concluded that dry etching of (201)-oriented p-Ga203 in BCl3/Ar by ICP may affect properties of Schottky diodes with Ni/Au contacts.

Thermal stability of the surface morphology of Sn-doped, (OlO)-oriented samples obtained from OFZ-grown bulk crystals was studied in detail by Togashi et al. [287]. Prior annealing, the samples were polished (CMP), etched in H2S04 + H202 + deionized water (1:4:1) at RT for 5 min, rinsed in deionized water, and dried in flowing N2. Annealing was performed in an RF furnace within a horizontal quartz tube with the samples placed on a pyrolytic barium nitride. The annealing experiments were carried out at temperatures 250 to 1450°C with flowing N2 during heating up to a predefined temperature, and next the samples were treated at the predefined temperature for 60 min in a flowing gas mixture of H2 + N2. After the treatment at the predefined temperature, the gas was switched to N2 during cooling down to RT. The total gas flow was 10000 seem.

When the samples were annealed in N2, there was no change in surface morphology after annealing at temperatures below 1150°C (Fig. 4.36a). Annealing at 1150°C or above revealed stripes parallel to the [001] direction, the number of which increased with temperature (Fig. 4.36b-e). These stripes are related to the decomposition of p-Ga203.

NDIC (Nomarski differential interference contrast) images o

Figure 4.36 NDIC (Nomarski differential interference contrast) images of 3-Ga203(010) surfaces after 60 min of heat treatment in N2 flow at temperatures of (a) 1050, (b) 1150, (c) 1250, (d) 1350, and (e) 14S0°C. Reprinted with permission from Ref. [287], Copyright 2015, The Japan Society of Applied Physics.

Annealing in the atmosphere containing H2 (H2/(H2 + N2) = 0.75 or H2/N2 = 3:1) at different temperatures showed no changes in the surface morphology after annealing at 250°C (Fig. 4.37a). However, a sample annealed at 350°C showed first signs of decomposition in the form of grains (Fig. 4.37b), which increased in size with annealing temperature (Fig. 4.37c-e). The samples that were annealed at 450°C and above revealed the formation of metallic Ga in the form of droplets on the surface.

NDIC images of p-Ga0(010) surfaces after 60 min of heat treatment in a mixed flow of H and N

Figure 4.37 NDIC images of p-Ga203(010) surfaces after 60 min of heat treatment in a mixed flow of H2 and N2 (H2/N2 = 3:1) at temperatures of (a) 250, (b) 350, (c) 450, (d) 550, and (e) 650°C. Reprinted with permission from Ref. [287], Copyright 2015, The Japan Society of Applied Physics.

The decomposition rate increases with increasing the H2 content in a similar way as with temperature at a constant H2/(H2 + N2) ratio.

The Czochralski-grown crystals showed thermal stability (10 h annealing time) up to about 1300°C in oxidizing atmosphere (02, air), up to about 1200°C in a neutral atmosphere (N2, Ar), and up to about 600°C in a reducing atmosphere 5%H2 + 95%Ar (H2/Ar = 1:19) [8]. Here, thermal stability was indicated as a measurable mass loss during annealing and surface deterioration due to decomposition.

Conclusion

Surfaces (orientation, termination, band bending, cleanliness, stability) play a crucial role in epitaxy and device fabrication. Their preparation is, therefore, an important step in a technological chain. (100) and (001) surfaces of p-Ga203 are amixture of so-called Aand В terminations with a domination of the latter (>95%). For polishing, the CMP process was found effective. Different wet and dry etching agents can be used for epi-ready surface preparation of (3-Ga203, e.g., solutions of HF or HN03, and BC13. Thermal surface stability depends on atmosphere type, annealing temperature, and annealing time. In an oxidizing atmosphere, the crystals are substantially stable up to about 1300°C, in a neutral atmosphere up to 1150-1200°C, while in a reducing atmosphere up to 600°C depending on hydrogen content in the atmosphere and annealing time as conditions governing the surface decomposition. These basic surface properties are listed in Table 4.8.

Table 4.8 Basic surface features of bulk P-Ga203 single crystals

Feature

Description or value

Reference

Surface termination

A (<5%) + В (>95%) for (100) and (001) surfaces

277,190, 278

Surface band bending

Upward for (100)

190, 280, 184, 281

Effective surface etchants

47% HF at RT, 60.5% HN03 at 120°C, BC13 (dry)

284, 285

Surface thermal stability in an oxidizing atmosphere (02) [°C]

130069

8

Surface thermal stability in a neutral atmosphere (N2) [°C]

115000/120069

287/8

Surface thermal stability in a reducing atmosphere (H2) [°C]

35069/60060

287/8

Annealing time = MlO h; Ml h; Ml h (H2/N2 = 3:1); «10 h (H2/Ar = 1:19)

 
Source
< Prev   CONTENTS   Source   Next >